John Broskie's Guide to Tube Circuit Analysis & Design

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Welcome to over 50 articles on amplifiers, tube-based preamps, crossovers, headphone amplifiers, single-ended amplifiers, push-pull amplifiers, Circlotron circuit design, hybrid amplifiers, cascode circuits, White cathode followers, grounded-cathode amplifiers, tube series regulators and shunt regulators, the Aikido amplifier, tranformer coupling, DACs and tubes — and hundreds of vacuum tube circuits that use 6SN7s to 300Bs, all explained in careful detail.

07 December 2014 post



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Tr-PS-3: +/-12Vdc Power Supply
The Tr-PS-3 is a regulated bipolar power supply that puts out +/-12Vdc @ 750mA. It uses an LM317 for the positive output and an LM337 for the negative output. The PCB-mount power transformer is the same one used in the Tr-PS-1, a MagneTek FS24-1500...

Stereo-Mute-Mono Switch
Here is something that I have wanted for myself for the longest time; its a rotary switch for phono playback that presents three positions: mono, mute, and stereo....

PS-10: All-in-One Tube Power Supply
The PS-10 is a new All-in-One power-supply kit for tube lovers. It offers both a high-voltage B+ and a regulated 6.3Vdc heater power source. It is an All-in-One due to it holding everything, including the power transformer...

Tr-PS-1: Regulated Low-Voltage PS
Brother to the Tr-PS-2, the Tr-PS-1 is a low-voltage, regulated 12Vdc (or 9Vdc or 12.6Vdc) power supply, perfect for powering heaters or replacing the wallwart power supply that powers DACs and other small audio gear. The Tr-PS-1 holds its own power transformer...

Tr-PS-2: High-Voltage Power Supply
Power supplies, particularly high-voltage power supplies, are often the big stumbling block to a successful completion of a tube-based audio project. Getting a PCB and all the capacitors, diodes, and resistors needed to populate the PCB is only half the problem. The other half is getting the power transformer. Well, with the new Tr-PS-2 power supply both problems are solved at once, as the Tr-PS-2 holds its own power transformer...

Cathode-Coupled Amplifier
I have been writing about the cathode-coupled amplifier for a long time now and many have asked for a PCB based on this circuit. Well, the wait is over. Back in blog number 245, I revealed a clever cathode-coupled-amplifier variation that greatly enhanced the circuit's PSRR...

PS-SS
Brace yourself something completely different: the PS-SS, a new GlassWare power supply kit, isn't for tube fanciers, as it's intended use is in solid-state power amplifiers....

PS-15 Power Supply Kit
The PS-15 is a new GlassWare power supply kit for tube fanciers. What another power supply? Yes, indeed, as it fills a gap. The PS-15 was designed for those who need a high-voltage power supply with two sets of regulated low-voltage power supplies (usually for powering heaters). Why would anyone need two low-voltage regulators? More than you might imagine....

New Octal Aikido Cathode Follower
The noval Aikido cathode follower (ACF) PCB now has a brother: the octal ACF PCB. Why? Some just prefer the sound from octal tubes, such as the famous 6SN7, so its absence was felt, myself included...

PS-18 Bipolar Power Supply Kit
Recently, I have been experimenting with cathode-coupled circuits that required a high-voltage, bipolar power supply, which is why had the PS-18 PCBs made. The boards are small, being only 4in by 4in, but pack quite a wallop due to the large-valued capacitors used. Two RC-based pi filters smooth the bipolar DC outputs....

New CCDA Octal PCB
The octal complement to the noval CCDA is here. The PCB is almost identical to its noval brother, save for the octal tubes and being 1/10 of an inch shorter. It holds two sets of output coupling capacitors and two B+ RC filters, one per channel...

New Select-5 Signal Selector Switch
I have a new signal selector switch, the Select-5, which—in spite of what its name seems to imply—allows you select between four input signals and four signal grounds....

Tube CAD, Se Amp CAD, Audio Gadgets
For those of you who still have old computers running Windows XP (32-bit) or any other Windows 32-bit OS, I have setup the download availability of my old old standards: Tube CAD, SE Amp CAD, and Audio Gadgets. The downloads are at the GlassWare-Yahoo store and the price is only $9.95 for each program. So many have asked that I had to do it...

New Products
What is that in the image above? LOOK, it's a bird, it's a plane, NO IT'S SUPERMAN, no wait, it is a very fuzzy photo of the new Universal Octal PCB and ceramic socket, which includes the black plastic 2in hole trim and four sets of aluminum, hex standoffs and screws and rubber O-rings, but—alas—not the NOS Sylvania 6SN7...

Cute ACF-2 Project
My family face the same problem every Christmas: What do you get the man who has everything but commonsense and taste? Quite a big problem that. Well, I decided to give myself a stocking stuffer. I have been eager to try one of my 6in by 6in PCBs in a Hammond extruded aluminum enclosure. I have used this type of box many times before, but never with so wide a PCB. Below is the result...

Cornell Dubilier 1kV 1µF
Polypropylene Film Capacitor

Always on the outlook for good-sounding coupling capacitors, I recently bought pairs of capacitors from Jantzen Audio (their Superior Z-Cap series), Audyn-Cap (their Plus series), several Japanese polypropylene capacitors, and Cornell Dubilier's 940C series 1µF/1kV capacitors. Every capacitor has its positive and negative attributes. The problem we humans face is that we can readily hear commissions, but fail to spot omissions....

The Return of the SRPP+
Due to popular demand, the SRPP+ is back, minus the power supplies. The new PCB is 6in by 5in and, like all GlassWare PCBs, is crazy overbuilt, with extra thick double-sided PCB material and heavy 2-ounce copper traces. Each channel gets its own large power supply RC filter, whose voltage rating can be a low 250Vdc, perfect for high idle current applications, such as headphone amplifiers; or, 400Vdc, perfect for line-amplifier use...

New Unbalancer PCB
After many a season returns the Unbalancer. This will be welcome news to many audiophiles, particularly for those who need to convert their DAC's balanced output to an unbalanced output signal, as the Unbalancer can readily receive its balanced signal. If a stand-alone DAC already holds unbalanced outputs, why should I bother using its balanced outputs? The answer is that the balanced outputs are usually taken earlier in the circuitry chain, which allows us to avoid having our delicate signal passing through anymore OpAmps....

New GlassWare Octal Aikido PCB & Kit
This new Aikido stereo PCB is a brother of the Noval Aikido. In fact the PCB is the same size, 6in by 6in. And like the Noval Aikido, the Octal Aikido PCB holds a large RC power-supply filter in each channel, but no power supply. The reason behind this is that you are free to whichever power supply you wish: tube rectification form some, regulated solid-state high-voltage for others...

PS-17
The PS-17 is a new power-supply RC filter. It is not a complete power supply, as it requires an external power supply to establish the B+ voltage. In other words, it is an add-on for an existing power supply. It could, however, be used with a GlassWare Rectifier-1, which would create complete high-voltage power supply. This PCB uses five cascading RC filters to provide suitably well-filtered B+ voltage for class-A tube circuits...

GlassWare Tilt Control
In my previous post, I promised the unveiling of the new GlassWare Tilt Control. well, here it is. This is a simple affair: a small PCB (1.4in by 2.8in), a stereo, shorting, five-position rotary switch, four capacitors and eight resistors. It is a passive design, which entails a -3dB insertion losses and whose center frequency is 500Hz and its nominal input impedance is 100k...

CCDA Noval Stereo PCBs
Almost square, being 6 inches wide and 5.6 inches tall, the new CDDA noval PCB does not hold a raw DC power supply, much like the new Aikido Noval PCB. Note the pattern: no more All-in-One designs...

New Aikido 12Vac PCB
Why yet another Aikido PCB? A very good question that deserves a very good answer. I know that many customers are using four 6DJ8 tubes with the Aikido LV PCB and a 24Vdc power supply, as they write me to tell me about how great their Aikido sounds. Well, here is the thing: I have tried using 6DJ8 with 24Vdc and they sound so much worse than the 6GM8 tubes in the same circuit that I would never recommend the pairing. Yes, yes, I know that their setups my indeed sound much better than their previous solid-state or tube line stage. The solution is more B+ voltage. But how do we retain the advantages of low-voltage and still get a decent B+ voltage? Well, the Aikido 12Vac is my solution, as the key advantage of the Aikido 12Vac is that it can be operated from a lowly wallwart power supply, yet develop a fairly high-voltage B+ for the triodes...

Aikido Noval Stereo PCBs
The new noval Aikido stereo board is square not rectangular and does not hold a raw DC power supply, neither for the B+ nor for the heaters. No more All-in-One, in other words. Why no power supply portion? Simplest answer is that because no one size fits all. Some want to use an external power supply, some want fully-regulated high-voltage power supplies, some want many cascading RC filters and no regulation, some want only solid-state rectifiers, some want only vacuum-tube rectifiers, some want DC on the heaters, but no regulation, some want AC on the heaters... but all want the Aikido circuit...

A3-Mini Stepped Attenuator
The standard A3 36-position stepped attenuator is 9 inches long, whereas this new A3 Mini is only 5.8 inches wide, but is otherwise identical. Where the resistors lay flat against the standard A3 PCB, the resistors stand perpendicular to the PCB on the new A3 Mini attenuator....

Aikido Cascode
The Aikido Cascode amplifier is a compound circuit consisting of a cascode input stage amplifier and an Aikido cathode follower output stage buffer. ... In a nutshell, the cascode circuit offers two huge features: gain, lots of gain; and low, very low input capacitance; thus, it delivers high gain and wide bandwidth....

Octal CCDA Rev. A
The Noval CCDA has a brother. When I laid out the new version of the noval CCDA, I also brought the Octal CCDA PCB into Rev. A. The PCB is now 7 by 6 inches (1 inch longer than the old PCB), which allowed me to add an improved B+ and heater power supplies and to add a second set of output coupling capacitors....

CCDA Noval Rev A.
Last year, I laid out a new version of the noval CCDA PCB, bringing it into Rev. A. The PCB is now 7 by 6 inches (1 inch longer than the old PCB), which allowed me to add an improved B+ power supply and to add a second set of output coupling capacitors....

PS-12 New Bipolar Low-Voltage Regulator
I have been playing with OpAmps lately and I needed a small bipolar low-voltage regulator that would fit in small boxes that were too small for my big B-PS-1....

Trim-1: 11-Position Stepped Attenuator
The Trim-1 single-channel, stepped attenuator offers 11 positions and can viewed as high-quality replacement for a potentiometer....

A-5 Stereo Stepped Attenuator
Amazing, just amazing. This stereo attenuator offers 36 volume settings, but only holds two 6-position rotary switches and 24 resistors. This cleverly designed stepped attenuator exploits both the series-attenuator and the shunt-attenuator configurations to yield the best compromise between flexibility, performance, and cost....

Select-4
One of the best sellers at the GlassWare-Yahoo store is the Select-2 three-input signal selector switch and PCB. I love this signal selector, as it switches the grounds along with the signal hots...On the other hand, most audio gear is wired with all the input and output RCA jack grounds tied together at the rear panel by a long strip of bus wire. In other words, you don't get to choose how to handle the ground connections, as the descission has been made for you. In such a setup, the following selector swith can come in handy, as it only switches right and left chanel hots. Moreover, the Select-4 allows up to six input signal sources to be used....

Select-Phase Switch
The following item has been available, but I haven't made a big deal about it, so many do not know that it exists. With a balanced system, we can easily flip the phase of the signal with the GlassWare Select-Phase switch and PCB. I get a lot of e-mail asking if I believe in phase. In other words, do I believe that we can hear the difference that phase reversal makes? I do....

New LV-Regulator
The LV-Regulator uses a simple RC filter (1 ohm & 10kµF) as a pre-filter before the LDO regulator and holds bypass capacitors for all the electrolytic capacitors and a 4.7µF/400V polypropylene shunting capacitor at the output. The 1-ohm resistor is a 4W device, so the maximum current output is 2A....

PS-7 High-Voltage Power Supply
This new power supply board is only 2 by 3 inches big and holds a very simple circuit. Yet the PS-7 can pack a wallop, as it can be used to make a power supply of up to 400Vdc and it holds two 47µF/450V capacitors; a lower voltage version holds two 220µF/200V capacitors, for a power supply of up to 170Vdc; and still another version holds two 1kµF/63V capacitors, for a power supply of up to 50Vdc....

Mono 66-Position Balanced Stepped Attenuator
Good news for the those audiophiles who run balanced systems. The BM-1 is a new stepped attenuator that I have designed. It uses 24 resistors and two rotary switches: one 11-position, 1-pole, shorting switch; one 6-position, 2-pole, shorting switch. The configuration is interesting, as it consists of a shunt stepped attenuator cascading into two series stepped attenuators, one for each phase....

New Mono Step Attenuator
I have a new stepped attenuator design to offer the DIY audio world, the M1 mono stepped attenuator; and I just love it. The M1 stepped attenuator combines both series and ladder stepped attenuators into a single functional attenuator. The ladder attenuator's job is to provide six fine steps of attenuation, each step being -1dB; the series attenuator, eleven coarse steps of attenuation, each step being -6dB. The result is that 66 attenuation settings are possible. In other words, we can set the attenuation to any value from 0dB to -65dB in -1dB decrements....

PS-9 New Heater Power Supply
Over the years, I have hard-wired together the following circuit many times, as I often have used solid-state rectification in my own tube power amplifiers and I didn't want the 5Vac rectifier winding to go to waste. The circuit is a simple voltage doubler that converts the 5Vac into 12Vdc for the power amplifier's frontend circuitry...

New Balanced Step Attenuator
Like its unbalanced predecessor, this stepped attenuator offers 36 steps of attenuation for two channels of signal. The center rotary switch controls both channels and presents coarse decrements, while the two flanking switches afford fine volume decrements for each channel....

PS-6
The PS-6 power supply offers a low-voltage bipolar output (up to +/-50Vdc), suitable for solid-state power amplifier or OpAmps; in addition, the PS-6 power supply holds a voltage quintupler that yields a single high-voltage B+ output voltage roughly equal to five times the rail voltage....

Aikido All-in-One LSA/HPA Rev A
I ran out of this PCB a while ago. I then made a few improvements to the PCB, so it is now in Rev. A and in stock at the the GlassWare/Yahoo store. I tested the new board by building a tube-based headphone amplifier, this time with 6N1P and 6H30 tubes. I am running the output tubes hard and I am using a higher B+ voltage (+240Vdc) than last time (200Vdc). It sounds impressive. The bass is rock solid and it slams. I swear I can feel the bass notes in my chest. The music comes out alive and forceful. On the other hand, my octal equivalent sounds more mellow, in a radiant, glowing way, which is altogether beguiling. It isn't sloppy by any means; it's just more yang than yin. These two HPAs prove that no one HPA can ever be perfect, as different music requires different amplification....

Aikido LV PCB
The old 24V Aikido PCB was a hit. I sold many PCBs and kits and I have gotten great comments from users. (I still listen to headphones with it and it sounds amazingly good as a line-stage amplifier.) The 24V Aikido, however, faced a problem: no 6GM8/6N27P/ECC86 tubes; well, at least no moderately-priced tubes. Not too long ago, the tube was selling for $5, or less; today, they go for closer to $25 to $30; next year, $40 to $50?. Moreover, when a supply of a trendy tube grows small, two things happen: obviously, the price goes up and, all too often, the quality goes down. Diminished quality? ...

PS-3: High-Voltage
Power Supply & Heater Regulator

High voltage power supplies are a pain. One thing that I love about OpAmp circuits is being able to hook up two 9-volt batteries and run, with just a single bypass capacitor across the positive and negative power supply pins—no lethal voltages and no heater power supply to worry about. Low-voltage power supplies are a breeze....

PS-4 Tube Power Supply
On this small four by four inch, extra thick (0.094), 2oz-copper traces, USA-made PCB resides both a simple high-voltage power supply and a low-voltage power supply and low-voltage regulator, with each finding its own raw power supply, including the rectifiers and power-supply reservoir capacitors. The low-voltage regulator is meant to power the tube heaters; the high-voltage power supply, the rest of the tube circuit...

Octal All-in-One LSA/HPA
I have received many requests for an octal All-in-One LSA/HPA PCB like the 9-pin version that weds a PS-1 solid-state, high- and low-voltage regulator to a 9-pin Aikido stage on one PCB. But I decided that since octal tubes seem more retro than 9-pin tubes and the Janus regulator is a pure-tube design that even uses a tube rectifier, why not wed a Janus regulator to the Aikido stage instead?...

Tube Clock
Tube clock? Is it a piece of artwork, worthy of window display in exclusive art galleries? Or is it the long-awaited analog replacement to the famed Tice digital clock? Or is it a chronometer of exceptional precision? Or is a timepiece of uncanny beauty, suitable for adorning walls of Beverly Hills mansions and Manhattan penthouses? Or is it something like a badge of affiliation to an almost cult-like adoration of vacuum tubes? Well, it’s all of the above. It is also my latest kit offering....

Now for something completely different...
I have created small, 4 by 6 inch PCB that holds a low-voltage bipolar regulated power supply for solid-state use. Why? I have been experimenting with OpAmp circuits lately and I needed a killer low-voltage, bipolar power supply....

New and Slightly Improved
The PS-1 regulator PCBs were part of the shipment; but now the boards are in revision B. I made the PCB half an inch taller, which allowed a fatter heatsink to be used. In general, a fat, short heatsink is better than a tall, skinny heatsink. The intrinsic thermal resistance of the metal is effectively placed in parallel in a fat heatsink; in series, a tall heatsink. The upshot is that the PS-1 regulator now holds the same Aavid Thermalloy 529802B02500G heatsink that the Janus and H-PS-1 regulators use. This chubby heatsink boasts a thermal resistance of only 3.7, based on a 75°C rise in natural convection....

New Signal and Capacitor Selector Switches
I have created small, 1.4 by 2 inch, PCBs that hold a single rotary switch and nothing else, other than termination pads for hookup lead. The selector switch assembly accepts three stereo inputs, with both the hot and grounds of each signal source to be selected. So if a signal source, say a CD player, is not selected, neither its outputs or grounds make any connection to the line-stage amplifier...

PS-1 Solid-State Regulator Kit
Finally, after many a tease and far too many false starts, arrives the new PS-1 regulator. The PCB is only four by six inches, yet it holds an all-solid-state two regulated power supplies, a high-voltage regulator for the tube B+ and a low-voltage regulator for the tube heaters. Each voltage regulator also finds its own raw power supply, holding the all the rectifiers and power-supply reservoir capacitors required for feeding each regulator its raw DC voltage. In other words, except for the power transformer(s), the PS-1 PCB holds all that is needed to make a superb regulated power supply for tube-circuits....

Janus Regulator PCB, Rev A
The revised Janus regulator PCB features a one inch increase in height and now all four high-voltage electrolytic capacitors hold bypass capacitors...

At long last: the Aikido Phono Preamp
This Aikido phono preamp uses passive equalization, rather than active, feedback-based equalization. By cutting the highs and boosting the bass, the phono stage’s inverse RIAA equalization of the LP’s RIAA equalization curve returns the signal to flat. The passive equalization network sits in between two Aikido gain stages...
      13 Jul 2007

After Many a Month,
Returns the TCJ Stepped Attenuator

Reloaded and ready for action, it's back; but it's not the same—it's much better. First of all, the PCBs are meant to hold resistors on both sides; the switch spacing is now 3” instead of 2.5 inches; and, as a result, the PCBs are now shorter, 1.4 inches tall, and a tad longer, 9 inches long. Why? Now the attenuator will fit within a 1U rack-mount enclosure. Second, the TCJ stepped attenuator now offers many more positions, a total of 66 steps with 1dB resolution, as the center switch now presents 11 positions, rather than the old 6 positions. Third, and most importantly, the old open-frame rotary switches have been replaced by Elma switches. Swiss-made, gold-heavy, precisely-designed and exquisitely-made, Elma rotary switches are justly famous as the gold-standard in switches. And like all things golden, they are obscenely expensive. But when only the best will do…

Janus Shunt Regulator
The feedforward shunt regulator only looks forward, creating a counter noise signal to null the original power-supply-induced noise. Unfortunately, it is blind to what develops on the other side of the series resistor. In contrast, the feedback-based shunt regulator sees only the disturbance on the output side of the series resistor. Now, what would happen if we wed the two approaches together?
     24 Jun 2007

Three-Switch Stepped Attenuators
If you don’t know what the attenuator is all about, you didn’t follow the link to the GlassWare Yahoo! Store. The attenuator is a hybrid design that uses both series and ladder attenuators and three rotary switches to yield 36 positions of attenuation in -2dB decrements. In the first six positions, the attenuator is just a ladder attenuator, with no more than two resistors in the signal path; thereafter, the attenuator uses both a ladder and series attenuator configurations, with never more than eight resistors in the signal path. With -2dB decrements, a maximum of -70dB of attenuation is accomplished. ...
       26 Mar 2006

Printed Circuit Boards for the Aikido Amplifier
Dear Readers, I’ve got good news and bad news. First the good news: the rumored Aikido printed circuit boards do exist, and they are beautiful. They look fabulous and feel solid in the hand. They are extra thick, 0.093" (inserting and pulling tubes from their sockets won’t bend or break this board), double-sided, with plated 2oz copper traces, clean silkscreen and solder mask. (The comment was made repeatedly that they look “military grade,” as if their intended use was inside a spy satellite, not a line stage.)...
       18 Mar 2006

 

Basic Topologies
“Fundamentals are the building blocks of fun,” so said Mikhail Baryshnikov— rightly so, in my opinion. In electronics, the building blocks are the simple, basic, fundamental circuit configurations from which elaborate electronic are built, such as the cathode follower and grounded-cathode amplifier...

Augmented Tube Unity-Gain Buffers
Although amplifiers abound, dominating our efforts, unity-gain buffers are also an essential part of electronics. They can be as simple as a cathode follower or as complex we are willing to imagine and create....

Augmented SRPP Buffers
The SRPP circuit is usually used as an amplifier, not a unity-gain buffer. When used as an amplifier, many falsely imagine that it offers a low output impedance, which it seldom does. For example, an SRPP based on two 6DJ8 triodes offers an output impedance of about 1500 ohms; in contrast, a 6DJ8-based cathode follower will present an output impedance of about 100 ohms....

Plate Followers
The plate follower (AKA anode follower) is a cool circuit that is seldom used, alas. It is an inverting unity-gain buffer. (Actually, it can provide either signal reduction or signal gain, depending on the ratio between resistors R1 & R2.) ...
    07 Dec 2014

OpAmp-Based Input Stages
IC OpAmps make an electrical engineer's job easier to do, as they do the work of a hundred discrete components, yet are no bigger and small spider and vastly smaller than a 1µF PIO coupling capacitor. And while the first OpAmps, such as the LM741, sounded just dreadful, the newer OpAmps, such as the OPA637, sound quite good. For us tube-loving folk, however, one big problem presents itself when dealing with OpAmps in a tube projects: their low maximum power-supply voltage limits, seldom more than 36Vdc from negative power-supply pin to positive pin...
    30 Nov 2014

Batteries & Tubes
In my last post, while on the topic of grounded-grid amplifiers, I showed a battery being used to bias a triode's grid to a voltage below ground. ... The punch-line, the bottom-line, the end-of-the-day conclusion is that if a battery is used not to deliver power but only provide a fixed voltage drop, so that the battery is not used by the circuit but by the passing of time itself, then the battery is neither expensive nor in need of repeated replacement...

Batteries and Cathode Followers
It's not just grounded-grid and grounded-cathode amplifiers that can benefit from the addition of a battery, as the cathode follower can be energized by the battery's inclusion...

Batteries and Cathode-Coupled Amplifiers
The cathode-coupled amplifier offers many pluses: low input capacitance and no phase inversion and a high-impedance feedback port. Unfortunately, the problem of dissimilar cathode-to-plate voltages, however, is the one big difficulty to overcome....

Other Uses for Batteries
Long ago, John Atwood inspired me building a power-supply tickler or poker. What's that? He built in a small box a simple circuit that held a high-voltage, power MOSFET, an oscillator, and a battery....
   19 Nov 2014

Tr-PS-3: +/-12Vdc Power Supply
The Tr-PS-3 is a regulated bipolar power supply that puts out +/-12Vdc @ 750mA. It uses an LM317 for the positive output and an LM337 for the negative output. The PCB-mount power transformer is the same one used in the Tr-PS-1, a MagneTek FS24-1500...

Stereo-Mute-Mono Switch
Here is something that I have wanted for myself for the longest time; its a rotary switch for phono playback that presents three positions: mono, mute, and stereo....

The Real Grounded-Grid Amplifier
If there is a "real" grounded-grid amplifier, is there then an "unreal" grounded-grid amplifier? Yes, indeed, which is a nuisance and headache for someone like me. How so? I often receive e-mails that ask about the grounded-grid amplifier and I must figure out if they really mean the circuit shown below, which uses one triode, or the cathode-coupled amplifier that Bruce Rozenblit designed, which uses three triodes....

Aikido Grounded-Grid Amplifiers
Long-time readers will recognize the following topology, as I have shown it many times before. The idea is a simple one: we use the grounded-grid amplifier's grid as an inverting input and feed it a sampling of the power-supply noise, so we can create a null at the grounded-grid amplifier's output...
    09 Nov 2014

Post RMAF Ponderings
I have been digesting the last RMAF. Digest? In what sense do I digest? The word "digest" can mean several different things; for example, to convert food into simpler chemical compounds to be assimilated by the body; or to absorb or assimilate mentally; or to organize into a systematic arrangement, usually by summarizing or classifying; or, lastly, to endure or bear patiently. Well, with the exception of the first definition, I'd say that I have been doing quite a bit of each with my memories of the 2014 RMAF...
     25 Oct 2014

RMAF 2014
Okay, I'll admit it: I was tempted. I was enticed by the thought that I would write about an RMAF that didn't actually occur, but one that I had wished had happened...

Personalities
Last year, I got to meet David Fletcher (cofounder of Sumiko, SOTA, and Pacific Microsonics), which was a treat, as David super knowledgeable and funny, a great combination. This time, I got to meet Frank Van Alstine, who has been making audio gear for over three decades now...

RMAF Trends
While I didn't see any truly new audio products, no force-field loudspeakers for example, I did note emerging and continuing trends. DSP (digital signal processing), whose aim is the improvement of digital signals through mathematical manipulation, grows in extent and application...

Most Innovative Product
While most sales reps will claim that the new champagne-colored faceplate on their audio gear constitutes a true advance in the science of home-audio reproduction, truly groundbreaking, pioneering designs were rare, if nonexistent. The exception came from Red Wine Audio, the company famous for its excellent battery-powered audio gear...

Schiit Audio
One stop that I must make at every RMAF is the Schiit table in the CanJam room. Why? No one offers a better deal than they do. Let me put it this way: if they were located not in the USA, but Europe, and if they spent a tad bit more on fancy boxes, all of their products would cost three times more and still be amazing bargains....
   15 Oct 2014

Pre-RMAF Ramblings
The Rocky Mountain Audio Festl looms before me. The RMAF is, as they like to put it, "the largest consumer audio and home entertainment show in the United States." A big audio show, in other words. Why it wasn't instead named "RMAS, the Rocky Mountain Audio Show" is a good question...

FETs and Aikido
My recent posts on hybrid power amplifiers, the ones that held a cascade of technologies: FET, vacuum tube, and then MOSFET or transistor output stages, prompted more interest in FETs than I expected. One reader asked if a FET-based buffer stage could be added to his existing Aikido line-stage amplifier....
    08 Oct 2014

New Lower Shipping Cost!
Shipping is a pain. For example, it used to be both cheap and fast to mail to Canada; today, it is neither... Well, things have changed. The US Post Office has a new flat-rate, plastic, padded envelope...

Parafeed Amplifiers
I am often asked why I don't write about parafeed amplifiers or some other popular topology. Is it because I don't like the parafeed topology?...

Parafeed and Plate Feedback
I was asked if the partial-feedback and Ultra-Empathic feedback techniques could be used on a parafeed power amplifier. The quick answer is yes...
27 Sep 2014

PS-10: All-in-One Tube Power Supply
The PS-10 is a new All-in-One power-supply kit for tube lovers. It offers both a high-voltage B+ and a regulated 6.3Vdc heater power source. It is an All-in-One due to it holding everything, including the power transformer...

More on the SRPP Topology
... Well, the gap that I endeavor to fill is the one that exists between what many tube lovers believe an SRPP to be and what it can do and the reality of what an SRPP circuit actually is and what it can in fact do...

SRPP Tube Output Stage
Why use an SRPP output stage in a tube power amplifier? Many love the idea of forgoing the need for a phase splitter, making the SRPP output stage seem quite compelling. This, too, is not a non-gratuitous SRPP design, as the SRPP output stage has a low-impedance load to drive, the output transformer's primary impedance...

No-Signal Transformer SRPP Amplifier
The previous designs made use of an inter-stage or signal transformer to relay the input signal to the bottom output tube, while decoupling the DC voltages and power-supply noises. No one will deny that nothing beats a good signal transformer for these two tasks. The problem is finding one and not paying $$$...

SRPP² Headphone Amplifiers
In post number 293, I revealed a non-gratuitous SRPP headphone amplifier. This headphone amplifier belongs to the SRPP² category, as it consists of cascading SRPP stages...

TCJ Remake of RCA Phono Stage
In general, the original RCA phono stage isn't very good, as it isn't particularly flat or suitable for modern use. On the other hand, it does use passive equalization, which is cool; and it is very simple, which is why it is so popular...
    21 Sept 2014

Odds 'N Ends
I don't have the time for one of my usual long posts, so here is hodgepodge of topics (in the sense of a heterogeneous mixture, not a confused or disorderly mass of stuff; well, at least that is my hope)....

A Challenge
In my last post, while on the topic where the input and output were placed within a topology made all the difference, I stated that a non-inverting amplifier could be made out of the famous diamond transistor topology....

Push-Pull Ultra-Empathic Amplifiers
We started the topic of Ultra-Empathic amplifier with a push-pull design, so let's return to push-pull, but a tad differently....
   07 Sep 2014

Tr-PS-1: Regulated Low-Voltage PS
Brother to the Tr-PS-2, the Tr-PS-1 is a low-voltage, regulated 12Vdc (or 9Vdc or 12.6Vdc) power supply, perfect for powering heaters or replacing the wallwart power supply that powers DACs and other small audio gear. The Tr-PS-1 holds its own power transformer...

Ultra-Empathic Configuration
In the previous post, we saw the resurrection and modification of an old topology, known as the partial feedback amplifier.... In a nut shell, the partial feedback technique applied some, hence the name "partial," negative feedback across the output tube(s), thereby reducing both distortion and the output impedance from the output tube(s)....

What Would John Do
I was about to sign off, which would be a mistake. I often end posts because I am called away or my fingers give out or... The problem with that method is that most readers assume what is last is what I deem the best, which it often isn't. So, what follows is what I would be mostly likely to build, if I had the time....
    03 Sep 2014

Tr-PS-2: High-Voltage Power Supply
Power supplies, particularly high-voltage power supplies, are often the big stumbling block to a successful completion of a tube-based audio project. Getting a PCB and all the capacitors, diodes, and resistors needed to populate the PCB is only half the problem. The other half is getting the power transformer. Well, with the new Tr-PS-2 power supply both problems are solved at once, as the Tr-PS-2 holds its own power transformer...

OpAmps (Operational Amplifiers) & Tubes
After writing a new post, I am often surprised, which is something—after fifteen years of posting—I find surprising. Here is an example: my recent posts have shown how OpAmps could be used within a hybrid power amplifier; and as I have covered OpAmps many times before, my expectation was that little interest would be generated by these little, insect-like, eight-pin devices. I was wrong....
     31 Aug 2014

FET-Triode-MOSFET Hybrid Amplifier
This is makes the third—but not likely the last post—on the topic of designing a hybrid power amplifier that uses three different electronic device types: the FET, triode, and MOSFET...

Conventional-Differential-Amplifier Hybrid
Two P-channel FETs are used in the conventional differential amplifier input stage. (No Bastode arrangement obtains, as the FET's drain—not their sources—connect to the transistor emitters.) The two MJE350 PNP transistor cascode the FETs, which both shields the FETs from high voltage and improves their performance....

Penta-Polar Power Supply
One reader thought my initial three-technology hybrid design, which used a FET-input OpAmp such as the OPA637 cascading into an Aikido gain stage that drove a MOSFET-based output stage, as good as anyone could hope to get, as the FET input stage within the OPA637 comes pre matched and optimized. Perhaps, he is right....
    18 Aug 2014

Cathode-Coupled Amplifier
I have been writing about the cathode-coupled amplifier for a long time now and many have asked for a PCB based on this circuit. Well, the wait is over. Back in blog number 245, I revealed a clever cathode-coupled-amplifier variation that greatly enhanced the circuit's PSRR...

More 3-Technologies Hybrid
Remember from my last post, long-time reader, Kerry, asked, "Why not use FETs in the input stage of a power amplifier...?" My answer sidestepped his question, offering an easier solution: use an OpAmp and Aikido gain stage with a MOSFET-based output stage. While this alternate suggestion offers much to like, its glibness bothered me. Well, after more thought, the following design overview came to me....
   09 Aug 2014

Three-Technologies Hybrid Amplifier
A long-time reader, Kerry, made an interesting suggestion in an e-mail: Why not use FETs in the input stage of an amplifier, which are known for their low noise and low distortion; followed by a tube-based driver stage, as tubes are known for their ability to swing cleanly huge voltage swings; then, terminating the signal chain with a MOSFET-based, unity-gain output stage, as these devices are known for offering wide bandwidth and a high input impedance? A good question....
     31 Jul 2014

Blog Number 300!
This is my 300th blog post! Unbelievable. It makes both my typing fingers and head ache just to contemplate the number 300....

Making Snazzy Schematics
I love schematics. I like them so much that I have wanted to pay old ladies to make large cross-stitch renderings of famous tube circuits with which I might adorn my naked walls....

Schematic Rules
Fortunately, unlike English grammar, drawing schematics has few rules. The aim of a good schematic is not the conservation of paper or pixels, not the artistic, avant-garde, Bohemian, far-out, innovative, experimental display of expression of emotions, and not just the technically-correct layout of a circuit's topology—as this is the universal assumption and which, in itself, is not enough—but clarity.
   21 Jul 2014

CCDA Sale
Now that the power-supply kit sale is over, let's have a CCDA line-stage amplifier sale, as I have a healthy supply of these two boards. The CCDA is great beginner project, as only two tubes are needed, which saves on tube cost and the need for a more robust power supply....

More Aikido Active-Tail Phase Splitters
My last post contained several examples of the Aikido active-tail phase splitter, which injects the power-supply noise into the grid of the bottom triode, the active load device, so that the power-supply noise will null at the two outputs....
    13 Jul 2014

Constant-Current-Source Amplifiers
In the last post, the following power amplifier circuit made an appearance. Both the differential input stage and the push-pull, class-A output stage used constant-current sources (CCS) at the common cathodes...

Aikido Active Long-Tail Phase Splitter
Since the topic of phase splitter seem to be a favorite of many readers, we will continue with the Aikido long-tail phase splitter. Here a triode takes the place of the long-tail cathode resistor, acting as an active load...
    04 July 2014

Getting Close to 300
This blog post 297, just three away from post number 300! Sure, I know that many other bloggers have passed their own one-thousand milestone, and that many make daily posts to their blogs, bringing them to the that one-thousand goal in less than three years....

FET-Triode Hybrid
Bastode Line Stage Amplifier

Wow! Where to start? The following hybrid circuit combines the bastode topology with the grounded-grid topology—and throws in a healthy measure of Aikido magic....

Cathode-Driven Phase Splitter
It seems that the topic of phase splitters is a hot one, as it has prompted much e-mail since blog number 294. Apparently, each has his own strong favorite topology....

CCS and Push-Pull Amplifiers
Like the topic of phase splitters, the double-CCS push-pull power amplifier shown in blog 293 has provoked many comments. As always is the case, a few more thousand words could have been summoned to explain this circuits functioning, advantages, and liabilities....
     28 Jun 2014

Power Supply Sale
I am over stocked on four power-supply kits: the Janus Shunt Regulator, the Janus Solo, the PS-4, and the LV-Reg. Thus, I am putting all four on sale...

Unbalanced-Input Circlotron
My last two posts have caused a stir in Tubeland it seems. The topic of driving what is normally seen as a balanced circuit with an unbalanced signal provokes more interest than I expected...

MOSFET-Based SRSFPP Circlotron
I should have stopped at that last tube circuit, but I couldn't stop myself from going on to an SRCFPP Circlotron. First, note the small change in its name: SRSFPP versus SRCFPP. The MOSFET equivalent to the cathode follower is the source follower; thus, the "SF" change...
     22 Jun 2014

PS-SS
Brace yourself something completely different: the PS-SS, a new GlassWare power supply kit, isn't for tube fanciers, as it's intended use is in solid-state power amplifiers....

No-Tail Phase Splitters
In my last post, we saw what looked like a long-tail phase splitter, except there was no tail. Instead of a common cathode resistor or constant-current source, the two cathodes attached to a PNP transistor's emitter, which presented half of the input signal to the cathodes, thereby establishing balanced differential output swings at the two plates....
    17 Jun 2014

Phase Splitters
In short, a phase splitter is a device that accepts a single input waveform and yields two (or more) output waveforms that differ in phase from each other. The word "device" was used because not all phase splitters are electronic circuits; for example, a center-tapped signal transformer can be used as a passive phase splitter....

Bastode Phase Splitters
Our old friend the bastode topology is back, where a P-version of some solid-state device, such as a FET or MOSFET or transistor, has its source or emitter attached to the emitter or source of an N-version of some solid-state device, such as a FET or MOSFET or transistor—or even a tube’s cathode....

Metaphrase Phase Splitter
Make sure your mental seatbelts are still securely fastened, as we can expect some conceptual turbulence ahead. Let's start with an altogether different type of phase splitter, one that places a constant-current source atop the common/shared/conjoint plates...

Unbalanced Push-Pull
Cathode-Follower Power Amplifier

The first step in creating a cathode-follower-based output stage that used an unbalanced input signal is to move the output transformer to the cathodes. The tubes are no longer required to produce voltage gain, only current gain....
     15 Jun 2014

PS-15 Power Supply Kit
The PS-15 is a new GlassWare power supply kit for tube fanciers. What another power supply? Yes, indeed, as it fills a gap. The PS-15 was designed for those who need a high-voltage power supply with two sets of regulated low-voltage power supplies (usually for powering heaters). Why would anyone need two low-voltage regulators? More than you might imagine....

Non Grata Gratuitous SRPP
In my last post, I stipulated that any hybrid design that I created had to abide a fifth rule: No gratuitous—not called for by the circumstances—SRPP stages. Just minutes afterwards, my mind balked at the constraint...

Gratuitous SRPP Circuits
In contrast, a gratuitous SRPP circuit is one that does not require the SRPP's ability to swing heavy push-pull current into a load. The telltale sign is when the output could be taken from the bottom triode's plate with no difference in performance....

Non-Gratuitous SRPP Hybrid Power Amplifier
Returning to a non-gratuitous SRPP hybrid power amplifiers, just what would a non-gratuitous SRPP hybrid power amplifier look like? Was it even possible?...

Non-Gratuitous SRPP Headphone Amplifier
After coming up with the SRPP-based hybrid power amplifier, I began to think about other non-gratuitous SRPP designs. The secret to bypassing "gratuitous" mark of shame is to give the SRPP circuit some actual work to do...
    07 Jun 2014

New Octal Aikido Cathode Follower
The noval Aikido cathode follower (ACF) PCB now has a brother: the octal ACF PCB. Why? Some just prefer the sound from octal tubes, such as the famous 6SN7, so its absence was felt, myself included...

Hybrid Topologies
Just how many possible hybrid topologies are there? Certainly fewer than the Shannon number, the educated guess of the total possible number of chess positions by the American mathematician, "the father of information theory," electronic engineer, and cryptographer Claude Elwood Shannon....
     31 May 2014

DGNNAHA, Once Again
The acronym has been submitted to the federal Acronym Selector Service, and DGNNAHA awaits its approval. What? You don't know what stands DGNNAHA for? It is simple enough; but before I explicate how this word was formed from the initial letters of a series of words, let's recall a familiar experience. Your favorite DIY audio magazine (or web-site) loudly proclaims: A New Hybrid Amplifier. Whereupon, your soul plummets....

What is a Hybrid Amplifier?
A hybrid amplifier is a design of mixed composition, usually mixed technologies, such as vacuum tubes and solid-state devices...

Triodes and MOSFETs in Parallel
In most hybrid designs, one technology passes its signal to another technology, a cascade of signal. But what if both technologies worked in parallel, rather than one after the other? For example, below are three possible arrangements of triode and MOSFET in parallel...

MOSFETs in Parallel vs in Series
Note how two P-channel MOSFETs were used in series in the above hybrid output stage. Why did I use different types and why did I place them in series, rather than in parallel? Good questions. Another good question would be, Why did I use two MOSFETs rather than just one?..
     24 May 2014

Single-Ended Circlotrons
In my last post, blog number 289, I stated that the Circlotron was intrinsically, fundamentally a push-pull topology. I shouldn't have. As an astute reader pointed out—thanks Paul—what I should have said was that, as far as I know, all the commercially available Circlotron power amplifiers and what most be understand as being a Circlotron are push-pull designs, but that a single-ended Circlotron could be devised; in fact, had been devised by me and displayed 14 years ago in December 2000 and recently in blog posts numbers 252and 253. In other words, it is possible to build a single-ended Circlotron...

Zen Power Amplifier
Famously, Nelson Pass asked the interesting question: What is the sound of one MOSFET amplifying? He found the answer in his Zen amplifier, a single-ended design that held a solitary MOSFET loaded by a constant-current source...

Circlotron Meets Zen
Circlotrons have long been made that used solid-state output devices, rather than vacuum tubes. A solid-state Circlotron was patented in 1959 by A. W. Donald III. In 1980, the famous James W. Bongiorno patented the Sumo Nine power amplifier topology. I have shown countless MOSFET-based Circlotron designs, which any google search will quickly reveal...

CCCS ????
About 25 years ago, when I lived in Silicon Valley, I was talking to an agitated friend. Earlier that day, he had a terrible realization, which had left him deeply worried. It had suddenly hit him—no, it was nothing as mundane as a 30-yearold's realization that he, too, was mortal, or that the world was not as peaceful a place as he had supposed. No, what had so troubled him was the dawning comprehension that we would soon run out of all the possible acronyms...
     19 May 2014

More OTL Design
Before we scale any higher up the steep mountainside of OTL design, let’s secure our position and firmly fasten what gains we have made. In the previous posts, we established that we could squeeze more potential output power from the OTL’s output tubes by driving their grid positively. In addition, we recognized that dynamically expanding power-supply rail voltage would yield more power from power triodes, as the greater the cathode-to-plate voltage, the greater the potential current flow....

Supercharged Circlotrons
The other common alternative to the the totem-pole OTL amplifier is the Circlotron circuit. The Circlotron is beloved by many for many reasons, some good and some bad...

Other OTL Supercharging Possibilities
Another consideration is that since my topologies use solid-state devices, could we not gain more by placing the power MOSFETs in parallel with the output tubes, rather than in series with them, as I showed in the many posts devoted to supercharging tubes, such as blog number 256?...
    11 May 2014

Supercharged OTL
We finally arrive. The last three posts, although no doubt interesting and provocative in their own right, were but a preamble to this post—a prolonged throat clearing perhaps, but I prefer to view these three posts as establishing a foundation. I have discovered that when I forgo the building of a foundation for understanding, too many readers cannot catch on, finding it a leap too far...
   03 May 2014

Hard-Core OTL Design
OTL amplifiers bewilder. Unlike the "normal" transformer-coupled, tube-based power amplifiers that use common and readily available output tubes, such as the EL34 and 6550, and simple input and driver circuits, OTL power amplifiers use oddball tubes, such as the 6AS7, 6C33, EL509, and 26HU5...

Coupling-Capacitor Blocking Distortion
What happens if we let the grids travel into positive voltages? First of all, the above formula no longer holds true. Second, the grid forms a forward-biased diode with the cathode, so the grid suddenly no longer presents a near-infinite impedance to the driver circuit, but a relatively-low impedance...

OTL in Class-G
We were able to overcome the limitation imposed by the restriction to negative grid voltages by using DC-coupled cathode followers (and possibly with one of my circuits). Well, what if we were able to overcome the limitation imposed by the fixed B+ voltage?...
    30 Apr 2014

OTL Design
OTL power amplifiers inflame the imagination, as no other tube circuit category can match. They are by necessity massive, monolithic, macho efforts that thrill us with their potential danger of exposed high voltage and heavy current flow, for no output transformer shields us from the scary power supply, as only glowing glass envelopes, a fragile barrier at best, dam up the lethal potential within the chassis....

OTLs and Stealth Feedback Loops
Speaking stealth feedback that sneaks in without causing alarm, I am reminded of what the great American writer Raymond Chandler said of prose style: "My theory has always been that the public will accept style provided you do not call it style either in words or by, as it were, standing off and admiring it." The same holds true of negative feedback; as long as an audiophile doesn't know it's there, he is fine with it...

More Thoughts on the
Sandman Distortion Null Topology

My last post continued the Sandman error-take-off circuit thread. I pointed out how the error-take-off amplifier, the "virtual-ground" amplifier, must get hotter than the main power amplifier, as the error-take-off amplifier's output devices must see the full power-supply rail voltages at the full current swing into the speaker...
     19 Apr 2014

PS-18 Bipolar Power Supply Kit
Recently, I have been experimenting with cathode-coupled circuits that required a high-voltage, bipolar power supply, which is why had the PS-18 PCBs made. The boards are small, being only 4in by 4in, but pack quite a wallop due to the large-valued capacitors used. Two RC-based pi filters smooth the bipolar DC outputs....

More A. M. Sandman Circuits
Almost ten years ago, back in Blog Number 20, I described A. M. Sandman's error-take-off circuit. His ingenious idea was that if an amplifier's distortion was presented to the other end of a loudspeaker, in phase and in equal magnitude, the distortion would fall out of the equation, as there would be no differential distortion signal for the speaker to see and, thus, reproduce....

OTL Power Amplifier Design
Over the last 15 years, I have shown many, many OTL designs. One of my favorites is the following, which appeared in Blog Number 250...
     Mar 22 2014

 

Class-A Wizardry
When we desire to label something as being of the highest order, quality, or degree, surpassing and superior to its competition, we seek out superlatives, such as excellent, exemplary, exceptional, first-class, first-rate, foremost, nonpareil, matchless, outstanding, peerless, pukka, superb, superior, top-grade, topnotch, unmatched, and of course class-A. And when we describe a class-A power amplifier's sonic attributes, we often use those same superlatives. This is something of a coincidence and a small tragedy....

Power-Boosted Class-A Amplifiers
My last post dealt with giving the venerable 300B a power boost. We saw how a transformer-coupled, single-ended amplifier could attach to an impedance-multiplier circuit (IMC), which would augment the amplifier's output wattage by threefold and reflect the desired load impedance to the single 300B...

J. L Hood Class-A Single-Ended Amplifier
John L. Hood described in an article, entitled "'A simple class A amplifier" in the magazine, Wireless World, in April 1969 his simple class-A, solid-state power amplifier...

Quad Current-Dumping Topology
Several readers have commented that the power-boosting idea sure looks like the old Quad current-dumping concept, wherein a small class-A amplifier worked in parallel with a powerful, but dirty class-B amplifier...

A. M. Sandman's Class-S
I have written about Dr. Sandman's wonderfully clever distortion-elimination schemes before, such as in Blog Number 20 and Number 22. (I even received an e-mail from Dr. Sandman, which was a treat for me.) One of his distortion-elimination topologies uses a small class-A amplifier and a class-B amplifier, which he calls class-S....
    19 Mar 2014

New CCDA Octal PCB
The octal complement to the noval CCDA is here. The PCB is almost identical to its noval brother, save for the octal tubes and being 1/10 of an inch shorter. It holds two sets of output coupling capacitors and two B+ RC filters, one per channel...

Aikido Cascade
The Aikido Cascade looks just like the typical cascading of of two grounded-cathode amplifiers, with the added feature of DC coupling between stages, which is neither strikingly new nor uncommon. What is novel is the careful ratios between resistor values, which give rise to a surprising result: a stellar power-supply-rejection ratio...

A Boosted 300B Single-Ended Power Amplifier
Audiophiles love the 300B. I truly like what it does for jazz saxophones. The problems with the tube are that its cathode and heater are one entity, which of course some see as its best attribute, and that they cost so much...
    09 Mar 2014

New Select-5 Signal Selector Switch
I have a new signal selector switch, the Select-5, which—in spite of what its name seems to imply—allows you select between four input signals and four signal grounds....

New House-GND Kit
The wall outlet's third jack connects to the house ground, which is also known as "earth," as the house ground is often created by attaching the wall socket's neutral connection to an 8-foot metal rod buried in the dirt under your house or to the cold-water pipe, assuming that the pipe is made of metal...

A Universal Power Booster
Here's a quick question for you to answer: After reading my last post, are you eager to rush out and buy eight NOS Western Electric 300B tubes so that you can build a tiny, stereo, 1-W-output OTL amplifier? No, how odd...
     02 Mar 2014

Circlotron & Impedance-Multiplier Circuit
Last year, in blog number 255, the tantalizing image shown below was used to show what a hybrid Circlotron might look like in reality, with potted toroidal power transformers and eight output tubes on top, accompanied by massive heatsinks at the sides. A lovely picture, a lovely design concept. My next post, blog number 256, further developed the concept. But as so often happens, I ran out of time before I could finnish what I had intended to write...

Book Review: Tubes and Circuits
I didn't plan on reviewing Bruce's latest effort, but now that I have mentioned his 1W OTL, I feel obligated to do so. I have quite mixed emotions regarding his latest book. This makes it a bit awkward for me...

Rozenblit's 300B OTL
I will not show the entire schematic, as I want those who plan on buying his book to do so. Instead, here is just the output stage...

Somersault OTL
Rather than use an LM317, we use an LM337, which is an adjustable three-pin negative voltage regulator. In addition, we must use two resistors to set the idle current, as that allows for push-pull operation...
     25 Feb 2014

Bose 901 Loudspeakers, Seriously?
No, you haven't entered The Twilight Zone. Nor have I indulged in Colorado's new marijuana frenzy. (In fact, a quick spellcheck revealed that I didn't even know how to correctly spell the word "marijuana." I am not only proud to say that I have led a pot-free existence, but positively smug about it.) Amar Bose died last year, yet his famous 901 speaker persists—as does his company, the celebrity, and the controversy...

A Bit of History
As I remember the story, in the early 1960s, Mr. Bose performed some psychoacoustic experiments with exploding wires: thin-gauge wires that, at the snap of a relay, experienced the high-voltage charge held in a large capacitor, causing the wires to explode, thereby creating nice pulse waves in the air...

The 901 Deconstructed
Well, in a very indirect way, starting with more thoughts about power-booster circuits and corresponding with my cousin about his latest loudspeaker-design adventures, my mind returned to the Bose 901. I wondered what a loudspeaker would sound like if it were set up for two-amplifier use...

Odd Results
Now, if you have had any experience with surround-sound loudspeaker systems, you must have noted at least two odd results...

Power Boosters
Our meandering path now returns us to the Bose 901 and power boosters. I would love to perform the experiment of placing four identical loudspeakers into two-channel stereo configuration, with the second set of speakers facing the back wall and sitting three feet behind the front speakers...

Headphones and Loudspeakers
Working Side by Side

I have mentioned before my preference for listening to headphones in combination with a subwoofer. I discovered this back in the late 1970s, as a college student. I owned Sennheiser HD414 headphones that offered uncommonly un-muffled sound, but little bass response...

Simple, Bread-and-Butter Power Boosting
I know that many will not be persuaded or even tempted by what I have so far written. These audio fundamentalists do not approve of polyamorous loudspeaker arrangements or onanistic headphone listening. Fine, to each his own...
19 Feb 2014

More Sonic Control
I received a few e-mails that asked how to implement the sonic width control on existing tube designs, such as the CCDA and Aikido. Since I already covered this topic with regard to the Aikido, back in blog number 33, I will move on to the CCDA. Adding a sonic width control is easy enough, as single Balance Trim can be used...

Adding a Tilt Control to the CCDA
Ideally, the Tilt Control will see a low-impedance signal source and a high-impedance input from the following audio circuit. The CCDA's second stage is a cathode follower, which will achieve the first requirement, but what about the second?..

Adding a Tilt Control to the Aikido
The Aikido line-stage amplifier, known for its stellar PSRR and low-output impedance, makes a good circuit to drive the Tilt Control. But we could use the same approach as was used in the CCDA example by undoing the Aikido's power-supply-noise null. What!..

Robo Speaker Stands
I caught some grief from those who thought my idea of an active speaker stand ludicrous in the extreme. "Who would need such a thing?" they asked. Well, I would...
   07 Feb 2014

RMAF Aftershocks
Last year's RMAF still reverberates in my mind. While at the show, I was trouble not so much by what I saw, but by what I didn't see. For example, reasonably priced audio equipment. Surely not everyone is in the market for a $50,000 pair of loudspeakers or $80,000 turntables or $4,000 interconnects...

Sonic Controls
Although many are happy with their high-res downloads and fancy DACs, something is still missing. Many DACs allow you to adjust the playback volume, but not balance between channels. Nor do all of them allow phase reversals or tone alteration or narrowing the stereo image or widening it. Yet, all of the above are often needed. So, one new product I would love to see being sold would be a sonic-control palette...

Tube Sonic Control
The following circuit uses only five triodes per channel, and could use only four, if the input stage used a simple plate resistor, rather than the active plate load. Yet, this circuit performs all four functions of offering balance, width, phase, and tilt control. Impressive, no? The totem-pole input stage offers some signal gain and allows stereo blend and balance control. The second stage consists of a split-load phase splitter, whose output allows phase selection and offers a low-impedance balanced signals to drive the tilt control...
     31 Jan 2014

Tube CAD, Se Amp CAD, Audio Gadgets
For those of you who still have old computers running Windows XP (32-bit) or any other Windows 32-bit OS, I have setup the download availability of my old old standards: Tube CAD, SE Amp CAD, and Audio Gadgets. The downloads are at the GlassWare-Yahoo store and the price is only $9.95 for each program. So many have asked that I had to do it...

New Products
What is that in the image above? LOOK, it's a bird, it's a plane, NO IT'S SUPERMAN, no wait, it is a very fuzzy photo of the new Universal Octal PCB and ceramic socket, which includes the black plastic 2in hole trim and four sets of aluminum, hex standoffs and screws and rubber O-rings, but—alas—not the NOS Sylvania 6SN7...

Aikido PP LSA/HPA Project
I covered my plans for this LSA/HPA back in March of 2013, blog number 257. Yes, indeed, it has taken me this long to finnish the project. Why so long a time? The project just fought me every step of the way, which was entirely my fault...
     04 Jan 2014

Happy New Year!
I thought that I might might reach blog number 300 this year, but, alas, no. Just twentyfour more to go. Perhaps next year. I wonder if I can hit blog number 1,000 before dying. If I keep them short, maybe...

PS-9 Back in Stock
The PS-9 is back in stock. Why should you care? If you ever build a tube-based audio project that uses a typical tube power transformer, you might end up with a 5Vac or 6.3Vac windings that you will not use....

Single-Ended Power
Amplifier PSRR Enhancement

Almost fifteen years ago, I revealed a simple technique that I had used to dramatically lower the amount of power-supply noise from a single-ended power amplifiers output. Since then, I have mentioned this Aikido-esque technique here several times...
     31 Dec 2013

Cute ACF-2 Project
My family face the same problem every Christmas: What do you get the man who has everything but commonsense and taste? Quite a big problem that. Well, I decided to give myself a stocking stuffer. I have been eager to try one of my 6in by 6in PCBs in a Hammond extruded aluminum enclosure. I have used this type of box many times before, but never with so wide a PCB. Below is the result...

A Hybrid Single-Ended Amplifier
I cannot leave you without some novel circuit, so here is an idea for a hybrid single-ended power amplifier that hold several fresh touches...
     23 Dec 2013

Cornell Dubilier 1kV 1µF
Polypropylene Film Capacitor

Always on the outlook for good-sounding coupling capacitors, I recently bought pairs of capacitors from Jantzen Audio (their Superior Z-Cap series), Audyn-Cap (their Plus series), several Japanese polypropylene capacitors, and Cornell Dubilier's 940C series 1µF/1kV capacitors. Every capacitor has its positive and negative attributes. The problem we humans face is that we can readily hear commissions, but fail to spot omissions....

SRPP and
Grounded-Cathode-Amplifier Disambiguation

I seemed to have have made a mess of it back in post Number 272. My goal was to point out that the new SRPP+ PCB was more flexible than many might imagine, as the circuit could arranged in at least three ways: as plain-Jane SRPP, as an SRPP+, and as a grounded-cathode amplifier with an active plate load resistance....

Current-Out Circlotron
Back when I was deep into—and relishing—the the task of creating current-out amplifier designs, both tube-based and solid-state-based efforts, a gap in the array was formed by the current-out Circlotron, which eluded me. Well, I moved on to other electronic concerns; but then, about two months ago, I was spurred on again by an e-mail correspondence with Zygmunt Jerzyński of hiend-audio.com. He is convinced that current-out, rather than voltage out, is the way to go. He might be right....

Designing a Current-Out Circlotron
How we approach a design problem often makes all the difference. As Albert Einstein and Leopold Infeld put it...

Tube Current-Out Circlotron
I know that many are bit dizzy, if not nauseated, from seeing so many solid-state schematics. Well, relief is on the way. Before jumping into it, let's pause and consider the task before us. We want a high-output impedance, which seems an easy task for a vacuum tube. We also require a high flow of current, however, something tubes are not good at, when compared to transistors and MOSFETs....
     31 Oct 2013

RMAF 2013
The Rocky Mountain Audio Festival took place in Denver once again. I made the hour-and-half drive on Friday and Saturday, but I could not summon the will to do so again on Sunday. Don't get me wrong here; I had a great time...

The Equipment
Speaking of loudspeakers, I have to say that I did hear some very good sounding systems at the RMAF. Unfortunately, all of them cost a fortune...

Best Bargain
The best audio bargain was truly inexpensive, at only $119. First let me say that I love everything about Schiit—their products, their prices, their sonics, everything but their name...

I Have Got the Blues
Back in 2005, after my trip to the CES, I wrote about the the modern obsession with blue LEDs. I hate them still. Rather spew new invective, I will just quote myself from blog number 28...

RMAF Conclusions
As a teenager, I was disheartened to discover that although H. L. Mencken had died in the year of my birth, he had died too late for me to be his reincarnation...
    16 Oct 2013

Circlotron Gaps
"Once more unto the breach, dear friends, once more; or close the wall up with our English dead!" The "breach" that Shakespeare refers to is an aperture, cleft, division, gap, hole, opening, puncture, rent, separation, or split in a wall. The breach that I have been trying to fill up, since 1999, is that of understanding the Circlotron circuit. I have covered an amazing array of topologies, single-ended Circlotrons and solid-state Circlotron, hybrid and inverted Circlotrons, choke-isolated and transformer-coupled Circlotrons. With over twenty posts on this topology, what is there left to say?...

Current-Out Circlotron
I was reminded of the current-out Circlotron by an e-mail from Zygmunt Jerzyński, who puts out a truly interesting and informative web site, hiend-audio.com. If you like this web site, you will also like his. Be sure to check it out. A few years ago, when I was obsessed with current-out amplifiers, I wondered what a current-out Circlotron power amplifier would look like. The circuit I came up with was based on the topology I created for blog 137...

Hybrid Circlotron Variations
Vacuum tubes boast many wonderful attributes, but high transconductance and current delivery are not among them. The 6AS7, for example, when run hard, say 100V cathode-to-plate voltage and 10mmA of idle current, exhibits a gm of only 7mA per volt. A typical power MOSFET, on the other hand, delivers gm of many amperes per volt and can release many amperes of current. Thus the appeal of combining the two technologies...

Circlotron Puzzler
Since I want to make sure that you get your money's worth, and since this post is fairly short (for me at least), here is a puzzle for you...
     06 Oct 2013

 

Move Over Texas, Colorado is Flooding
Although not up there with the universal deluge recorded in the Old Testament, we have gotten a lot of rain lately. Unlike snow that sits there and slowly melts away, the rain quickly flows and pools and overflows the creeks and rivers. Over 200 people are still unaccounted for and Boulder County must repair 100 to 150 miles of roadway and 20 to 30 bridges...

The Return of the SRPP+
Due to popular demand, the SRPP+ is back, minus the power supplies. The new PCB is 6in by 5in and, like all GlassWare PCBs, is crazy overbuilt, with extra thick double-sided PCB material and heavy 2-ounce copper traces. Each channel gets its own large power supply RC filter, whose voltage rating can be a low 250Vdc, perfect for high idle current applications, such as headphone amplifiers; or, 400Vdc, perfect for line-amplifier use...

The Upcoming Death of the CD
CD sales peaked in the year 2000; by 2010, they had fallen by 50%. The much of the shelf space once devoted to CDs is now covered with gaming software and DVDs. Many reasons are given for falling CD sales, such as pirate downloads and friends giving out home-burned copies, some might actually be right...
     14 Sept 2013

New Unbalancer PCB
After many a season returns the Unbalancer. This will be welcome news to many audiophiles, particularly for those who need to convert their DAC's balanced output to an unbalanced output signal, as the Unbalancer can readily receive its balanced signal. If a stand-alone DAC already holds unbalanced outputs, why should I bother using its balanced outputs? The answer is that the balanced outputs are usually taken earlier in the circuitry chain, which allows us to avoid having our delicate signal passing through anymore OpAmps....

The Unbalancer as
an Unbalanced Line Stage Amplifier

Last year, I described my efforts at interconnect design in Blog Number 241. I mentioned the problem of tying the interconnect's grounds together at the line stage (or stereo power amplifier). Because the two grounds in the line stage tie together in the power amplifier, signal current from one channel can take two paths back to the line stage....
    07 Sept 2013

New GlassWare Octal Aikido PCB & Kit
This new Aikido stereo PCB is a brother of the Noval Aikido. In fact the PCB is the same size, 6in by 6in. And like the Noval Aikido, the Octal Aikido PCB holds a large RC power-supply filter in each channel, but no power supply. The reason behind this is that you are free to whichever power supply you wish: tube rectification form some, regulated solid-state high-voltage for others...

Push-Pull Tube Operation
Most tube circuit topologies are single-ended. Of course, the output stage of a push-pull power amplifier, whether it be a transformer-coupled or OTL design, must use push-pull operation. These push-pull output stages can be run in strict class-A, wherein no output tube ever stops conducting and meaningfully engages the shared load impedance; or, in class-B, wherein the idle current is set as low as possible through the output tubes and one tube aggressively conducts while its partner is completely cut off; or, in class-AB, wherein a richer idle current is run, which allows a larger overlap in conduction between output tubes and thicker slice of class-A operation, before moving into class-B mode. The great advantage of class-A, push-pull operation is low distortion and consistent output impedance through the entire waveform at full output...

Aikido White Cathode Follower
My goal was to extract the Aikido PP circuit's output stage as a standalone buffer circuit. Such a buffer circuit assumes a low external-load resistance, which requires the advantages of class-A, push-pull operation. Such an external load is likely to be a high-impedance headphone element, such as 300-ohm Sennheiser headphones....

XPP Amplifier
The XPP amplifier circuit uses a push-pull output stage and an odd internal phase splitter. This circuit fuctions like push-pull grounded-cathode amplifiers, so it delivers gain and a relatively high output impedance...

Pentode-Input Aikido Push-Pull
In general, I much prefer triodes over pentodes. At the same time, I must acknowledge that pentodes do offer many advantages, such as higher gain and less input capacitance....

Cathode-Coupled Aikido Push-Pull
For line-stage use, high gain is seldom required, but low input capacitance is often needed. The cathode-coupled circuit offers a low input capacitance and no phase inversion...
     31 Aug 2013

PS-17
The PS-17 is a new power-supply RC filter. It is not a complete power supply, as it requires an external power supply to establish the B+ voltage. In other words, it is an add-on for an existing power supply. It could, however, be used with a GlassWare Rectifier-1, which would create complete high-voltage power supply. This PCB uses five cascading RC filters to provide suitably well-filtered B+ voltage for class-A tube circuits...

Excessive Math Fear
Imagine that you wake up late, discovering that your alarm clock wasn't set, and you have 15 minutes to get to work. You jump out of bed, grab something to eat and your car keys, and speedily drive off to work. Sadly, you forgot to get dressed, which explains the odd looks from your co-workers, as you walk around in your underwear. Well, what could possibly worse? Easy. You have just been told that you must give a speech before a large group. Just about everyone suffers from glossophobia, the fear of public speaking; or, at least, so say psychologists who study such things. Supposedly, public speaking is feared more than sickness, accidents, crime, war, or death...

Tube Math
Fortunately, for us tube-loving folk, tube math is relatively simple, which mirrors the conceptually simple operation of a triode. None of the essential formulas are too complex, although some can get quite long. A pencil, paper, hand calculator, and simple algebra are usually all that is needed...
    17 Aug 2013

GlassWare Tilt Control
In my previous post, I promised the unveiling of the new GlassWare Tilt Control. well, here it is. This is a simple affair: a small PCB (1.4in by 2.8in), a stereo, shorting, five-position rotary switch, four capacitors and eight resistors. It is a passive design, which entails a -3dB insertion losses and whose center frequency is 500Hz and its nominal input impedance is 100k...
14 Aug 2013

How To Use The GlassWare Tilt Control
Since the Tilt Control is passive, it does not require a power supply. Thank God. But also since it is passive, it assumes that the signal source presents a low impedance and that its output sees a very high input impedance. This does not seem to be that big an assumption, but it can prove to be....

Missing Sonic Controls
Regaining sonic control has been the underlining theme of the Tube CAD Journal since it was born, even when the topic was auto-bias circuits or I-to-V converters. Implicit and sometimes explicit, such as in my last post on harmonic restoration. Just over ten years ago, I wrote a…I am not sure what to call it, as it was certainly too long and structured to be a typical blog post and label "article" just doesn't feel right. The label "essay" is probably best suited to the task, but it has for many readers a fussy connotation, sadly...

Tilt Control
The tilt control imposes a shelving function, which attenuates half of frequency band and augments the other half. In other words, it is special type of a tone control that, unlike the typical tone control that boosts or cuts just the highs or mids or lows, shifts both highs and lows at once. As tone controls are rare and as the tilt control is a special type of tone control, it is very rare. The only piece of hi-fi that held one that I know of was the lovely Quad 34...
     23 jun 2013

Restoring Lost Harmonics
(What I am about to recommend here is, paradoxically enough, something I have usually opposed adamantly—namely that notion that tubes make nice distortion and they should be used as a form of tone control. Just about every day, I get an e-mail from someone who owns an all-solid-state system that sounds just too sterile, too brittle, so my advice is sought for what would be the best tube product for producing a fat, smooth tube sound as a sort of sonic overlay. This is not my cup of tea. Several decades ago, I was sickened to read an article in a popular electronic magazine that described a solid-state circuit that produced for those nostalgic that old-time tube sound a tube-sound generator; circuit introduced hum, motor-boating, and distortion! No thanks.)...

Designing A Harmonic Restorer
The next tube circuit that will come to mind is the cathode follower, as it neither inverts the input signal's phase nor produces and voltage gain. But the problem with a cathode follower is that due to the large amount of degenerative negative feedback is intrinsically employs in its operation, it doesn't add much harmonic distortion. We can certainly get more distortion by loading down its cathode with a too small-valued cathode resistor, but then we will engender a big gain insertion loss. The next non-phase-inverting tube circuit that comes to mind is the cathode-coupled amplifier. Yes, it doesn't invert the phase, but it presents the many of the same problems that the grounded-cathode amplifier distortion: too little distortion and a high output impedance...
   12 Jun 2013

PCBs Are Here
As anyone who has visited the GlassWare-Yahoo store knows, I have run out of many of the most popular PCBs, such as the PS-1. Well, they are here...

Simple Phase Inverter Circuit
In the previous installment, I showed howed how a simple impedance-multiplier circuit (IMC) could be made from descrete transistors. The design enjoyed the added benifit of sidestepping the usual bane of solid-state output stages, as it exhibited a constant gm, so no gm doubling can occur...
   31 May 2013

Flea-Power OTL Amplifier
Fleas are tiny, but potent. As the popular cliche puts it, it punches above its weight. Although a flea-power amplifier puts out only a tiny amount of power, it can prove more compelling than a big Great Dane of a power amplifier. Impossible? I have heard sonic shootouts between two tube power amplifiers, wherein the tiny David of a flea-power amplifier crushed a Goliath of a power amplifier; 10W versus 120W....

OTL Power Supply
As the OTL's schematic reveals, the OTL power amplifier requires two B+ voltages: 200Vdc and 400Vdc. The easiest way to establish the two voltages is to use a center-tapped secondary, as shown below....

Impedance Multiplier Circuit
Since every reader has read all my previous posts on the impedance-multiplier circuit (IMC) theory and design, there's no need to go over the same material again. Yeah, sure. Okay, a quick review wouldn't hurt. An IMC is made up from a unity-gain buffer and two resistors....
    18 May 2013

Power Booster Down Shift
As I wrote my last post, my ten-yearold son asked me what I was writing about and I told him it dealt with power boosters. Of course, he had never heard of a power booster, so I explained how it was a black-box that sits in between an existing small power amplifier and the loudspeakers and that it acted like an electrical magnifying glass, making the small appear much more powerful than it actually was. He looked worried and, after a thoughtful pause, he told me that he didn't like the name, "power booster," explaining that it lacked sparkle and oomph...

Boosting Transformer-Coupled
Power Amplifiers

Returning to the world of tube-based power amplifier and transformer-coupled outputs, let's quickly review the voltage and current relationships of a flea-powered 4W tube power amplifier at its various transformer taps. An output of 8Vpk into an 8-ohm load results in 4 watts of power, as does 11.3Vpk into 16 ohms, as does 5.66Vpk into 4 ohms. Any way you slice it, the result is 4W...
     27 Apr 2013

Power Boosters
Every audiophile knows what is a DAC or phono cartridge or CD player or power amplifier, but almost no audiophile knows what a power booster is. Why not? The answer is not hard to find: power boosters don't exist. (At least not for audio amplifiers, but they do exist for radio transmitters.) Look where you may, by night or day, in glossy audio magazines or catalogs, in high-end audio salons or the big-box electronics stores, you will not find a power booster for your amplifier...

An Interesting Design Problem, No?
Designing big power amplifiers is no bigger deal than designing a small amplifier, but designing a new category of audio gear is a bit more difficult. Think about it: designing a good headphone amplifier is not fundamentally different than designing a 500W power amplifier, as they only differ in magnitude and scale, not in function. Both amplifiers accept a voltage signal at their inputs and deliver a bigger voltage and higher current at their outputs. A power booster must accept—and relay—an input voltage and current flow to the output in an augmented form....

16X Power Boosters
Now that we know how to double the small amplifier’s output current into the loudspeaker and, thereby, increasing the power delivered into the loudspeaker by fourfold, we will look into how to design a more aggressive power booster that would increase the power deliver by sixteen-fold! Remember I² in the power formula; well, tripling the current will increase the power by nine-fold; and quadrupling the current will yield 16 times more power...

Actually Building a Power Booster
So far, my overview has been fairly abstract, as I wanted to get the circuit’s functioning across. But if we want to design an actual power booster, we have to dig deep into all the important details. For example, Which amplifier topology should we use? Can we use a GainClone chip power amplifier or must we use discrete transistors? How big do the rail voltage need to be? Does the IMC require a smaller or bigger heatsink than the inverting power amplifier?..

Other Power Booster Amplifiers
Chip amplifiers (GainClone or IC power amplifiers) are both cheap and convenient. They are also sturdy and safeguarded against shorts and overheating. Almost all of them are transistor-based, but not all of them. For example, the TDA7293 holds a DMOS output stage and its output can be supported by another TDA7293 in parallel with it....
     20 Apr 2013

CCDA Noval Stereo PCBs
Almost square, being 6 inches wide and 5.6 inches tall, the new CDDA noval PCB does not hold a raw DC power supply, much like the new Aikido Noval PCB. Note the pattern: no more All-in-One designs...

CCDA Applications
The CCDA is not limited to line-stage amplifier use, as it could be used the front-end of a single-ended power amplifier or as a high-output-microphone preamp. Some have even built CCDA headphone amplifiers for high-impedance headphones, which I do not recommend, but I know many are pleased with the results. For myself, I like the idea of using a CCDA as the frontend of a single-ended power amplifier. For example, a 12AT7-based CCDA will develop a gain of 48, which is enough to drive even a KT120 to full output....
    17 Apr 2013

Solid-State Supercharger Review
The key idea is that the tube does not work in parallel with the solid-state device, but in cascade with it, the tube driving and controlling the solid-state device. Any other arrangement would result in less tube flavor. In other words, we hope to impart the sonic signature of the tube, while obtaining the cost savings and ruggedness of solid-state devices....

Some Supercharged SE Amplifier Examples
Now that we have that review behind us, we can look at some single-ended amplifier designs. In all the following designs, I have set an unnaturally low power-supply voltage of 330Vdc. Why? Solid-state and high-voltage do not fit as well as vacuum tubes and high-voltage do. Even when the solid-state device is rated 1kV, I worry. And I particularly worry when solid-state devices are used with inductive devices such as chokes and output transformers....

Supercharged Inverted Single-Ended Amplifiers
The inverted single-ended-amplifier has made many appearances here before. It uses a floating high-voltage power supply per channel, which will make building a stereo power amplifier bit more complex. But it does terminate the output transformer’s primary into ground, which allows us to apply a negative feedback loop from the input tube’s cathode to the output transformer’s primary, which will lower the amount of power-supply noise making its way to your loudspeakers....
     09 Apr 2013

New Aikido 12Vac PCB
Why yet another Aikido PCB? A very good question that deserves a very good answer. I know that many customers are using four 6DJ8 tubes with the Aikido LV PCB and a 24Vdc power supply, as they write me to tell me about how great their Aikido sounds. Well, here is the thing: I have tried using 6DJ8 with 24Vdc and they sound so much worse than the 6GM8 tubes in the same circuit that I would never recommend the pairing. Yes, yes, I know that their setups my indeed sound much better than their previous solid-state or tube line stage. The solution is more B+ voltage. But how do we retain the advantages of low-voltage and still get a decent B+ voltage? Well, the Aikido 12Vac is my solution, as the key advantage of the Aikido 12Vac is that it can be operated from a lowly wallwart power supply, yet develop a fairly high-voltage B+ for the triodes...

Voltage Sextupler Circuit
Most tube fanciers know about voltage doublers, but few have heard of sextupler circuits. Where a voltage doubler circuit creates a DC voltage equal to twice the peak AC voltage, the sextupler circuit creates six times the peak AC voltage....

My Own Aikido 12Vac Line Stage
The Aikido 12Vac board holds stereo Aikido line amplifiers. If somehow you don't know what the Aikido circuit is, say Google dumped you here for the first time, the quick answer is that the Aikido tube circuit provides low distortion, low output impedance, and stellar PSRR (power supply rejection ratio)—all without the use of a negative feedback loop. It is both simple and effective....
     30 Mar 2013

Aikido Noval Stereo PCBs
The new noval Aikido stereo board is square not rectangular and does not hold a raw DC power supply, neither for the B+ nor for the heaters. No more All-in-One, in other words. Why no power supply portion? Simplest answer is that because no one size fits all. Some want to use an external power supply, some want fully-regulated high-voltage power supplies, some want many cascading RC filters and no regulation, some want only solid-state rectifiers, some want only vacuum-tube rectifiers, some want DC on the heaters, but no regulation, some want AC on the heaters... but all want the Aikido circuit...

Aikido Push-Pull HPA/LSA
I am building my own headphone/line-stage based on the new Aikido noval stereo PCB, configured as an Aikido PP. For those who hate headphones, stop here, as you might get the wrong impression that this new noval PCB can only be used as headphone amplifier, which is plainly wrong, as it is not limited to one application by any means...
    24 Mar 2013

Solid-State Superchargers
Perhaps you are not quite sure what a supercharger is, as only a few auto enthusiasts have had any experience with them. Much like its bigger brother, the turbocharger, the supercharger performs a mechanical feat of magic, effectively making a car's engine larger in volumetric displacement than it actually is...

Supercharged Totem-Pole Output Stages
We have seen how to make a simple supercharged grounded-cathode amplifier, but the same technique could be applied to cathode followers or cascode amplifiers or White cathode followers or Broskie cathode followers, even, SRPP amplifiers. The following is a a simple supercharged cathode follower...

Supercharged Circlotron
Here is the mandatory, but not gratuitous, Circlotron circuit, which was promised in my last post. Each output triode gets its own supercharging output transistor. (We can view this either as being an impedance-multiplying circuit or as a form of current mirror.) The batteries are only symbols for floating power supplies....
     18 February 2013

Aikido Push-Pull Revisited
I was going through some old SPICE circuit files on a back-up hard drive and I was overwhelmed. In just one directory, I found over 2,000 SPICE circuits that I had created. If I could corral all of my SPICE circuits, the number might exceed 2,500. Well, I discovered the following schematic that I had created about four years ago and I had forgotten. Well, not completely...

Solid-State Circlotron
The Circlotron is not limited to vacuum tubes, as solid-state devices, such as FETs, transistors, IGBTs, and MOSFETs can be used. Indeed, the vacuum tube is a very poor choice for driving an 8-ohm loudspeaker without a step-down output transformer. Why? The tube works well with high-voltages and low currents, just the opposite of what 8-ohm speakers require. On the other hand, if high-quality 600-ohm — or even 100-ohm — loudspeakers were available, then tubes would prove an ideal match....

Broskie Remake of
an Existing Circlotron Amplifier

Quite a few readers who own existing commercially made Circlotron amplifier have e-mailed me, asking what I would do if I were in their shoes. Just what do their shoes look like? Their existing Circlotron amplifiers hold two floating power supplies per channel and a fixed, high-voltage, bipolar power supply, with rails of +300V and -300V or more...

IMC and Hybrid Circlotron
IMC stands for Impedance Multiplier Circuit. I have covered this circuit concept many times before. A good starting point might be Blog Number 171. The idea is simple enough: an external amplifier drive a load, such as a headphone or speaker driver, but the load impedance seems higher to the amplifier than it really is, as it has been multiplied by the IMC...
     28 Jan 2013

Even More Circlotron
Not done? Really? Even more Circlotron? Yes, alas. I doubt that even ten more posts on the Circlotron would satisfy the demand for more explication of this puzzling circuit. In fact, I am surprised that no www.circlotron.com web site exists...

Circlotron History
The first instance of a Circlotron-esque circuit that I have found is for a lie detector. In the June 1934 issue of Electronics magazine, we find the following schematic in an article titled, "A Regenerative Null Detector," by Daniel E. Noble.Then in 1951, Cecil T. Hall applies for a patent (US 2,705,265) for his Parallel Opposed Power Amplifier, which is a beautiful circuit that uses pentodes and no output transformer. Mr. Hall received his patent in 1955...

Split-Load Phase Splitter and the Circlotron
Now it's time to let you in on a secret about the Circlotron: it is more of a phase splitter than a power buffer. We return to the topic of the ground being placed mid load and we will start with our old friend, the split-load phase splitter...

Push-Pull Cathode Followers
In contrast to the Circlotron, which places its ground mid-load, we can build an output stage that uses two push-pull, parallel opposed cathode followers. Such an output stage, if correctly laid out, would yield an output impedance equal to rp/2(mu + 1)....

Balanced Zo Split-Load Phase Splitter
I am often asked how we can make a split-load phase splitter that offers a balanced output impedance from each output relative to ground. We can do it, but it will not offer a balanced output voltage. First step is to the look at the math required to achieve equal output impedances....
    12 Jan 2013

 

Single-Ended Circlotron
Blasphemy! How dare I even consider displaying anything so irreverent as a single-triode Circlotron? The two-tube Circlotron is considered so inviolable, so sacrosanct that to posit an alternative is to blaspheme most egregiously. But then, that is what I am good at: blaspheming in a polite sort of way...

Circlotron with Differential Input Stage
Now that we have mentally stretched, we can move on to an interesting Circlotron design I created. It places the ground at mid load and uses two output tubes and two floating power supplies, so few will not accept it in family of Circlotron....

Horizontal Push-Pull Amplifier
The following schematic certainly seems Circlotron-esque, with the floating power supply on the left and two output tubes, but it is a closer relative to the Futterman OTL. Both output tubes function as GPK circuits (cathode followers, for those readers who skipped ahead), providing a low output impedance, but no voltage gain....
     02 Jan 2013

Circlotron Tune Up Time
Much like the comic irony of a career criminal, having escaped capture and punishment for his many crimes, becoming indignant over his being arrested for some petty crime he didn't commit, most of the trouble that I have gotten into in my life has been for transgressions that I didn't perform. Slights that I didn’t intend, slanders that I never uttered, indiscretions —legal, moral, cultural, and political— I didn’t perpetrate: all have been imputed to me... An example from the world of tube audio is the false quote often attributed to me that the Circlotron circuit is identical to the Futterman totem-pole design. It is not; nor have I ever said that it was....
     29 Dec 2012

A3-Mini Stepped Attenuator
The standard A3 36-position stepped attenuator is 9 inches long, whereas this new A3 Mini is only 5.8 inches wide, but is otherwise identical. Where the resistors lay flat against the standard A3 PCB, the resistors stand perpendicular to the PCB on the new A3 Mini attenuator....

Circlotron Redux
Only a few circuits garner instant interest and one of which is the circlotron. I have covered the circuit and offered many variations on simple version. Each time, I receive a huge response from readers. My last post described a class-G circlotron and it provoked e-mail from old friends and old-time readers whom I haven't heard from for a long time. But the circlotron circuit that received the most interest was the following...
     25 Dec 1012

Class-G Circlotron
It's been a while since I posted anything on the Circlotron circuit. First, I must mention that the Circlotron circuit that we will look at is not the original Circlotron circuit from the 1950s, which held an output transformer and used power pentodes, but the far simpler version, as shown below...

Circlotron in Class-G
Now that we have had a refresher course in class-G operation, let's move onto how to make a class-G Circlotron output stage. Our goal is to retain the Circlotron core topology, while adding the class-G variable rail voltages...

Circlotron & Iron
All tube-loving audiophiles know what an output transformer is and how it functions, if only dimly. Made of two independent coils of wire wrapped around the same iron core, the transformer seems simple enough...

Broskie OTL
In my search for my old drawings of class-G amplifiers, I found that I had long ago drawn a true power OTL amplifier based on my OTL topology. I meant to post it long ago, but I forgot to do so...
     04 Dec 2012

Broskie OTL
In Blog number 239, I displayed many tube-to-OpAmp translations, including the Broskie OTL. I had low expectations of creating a stir with this post, as I figured that no more than 100 people on Earth would be interested...

Broskie OTL & CCSs
Infinite-valued plate resistors cannot be bought and would require an infinite B+ voltage. A constant-current source, can be made easily and work well with only a few volts across them, yet yield the same results as an infinitely large pate resistor...

Current-Mirror-Based Push-Pull Buffer
I have gotten several thanks for posting the following images. Why? Apparently, these images helped several readers finally grok* how the current mirror based on two transistors worked...
    13 Nov 2012

Aikido Cascode
The Aikido Cascode amplifier is a compound circuit consisting of a cascode input stage amplifier and an Aikido cathode follower output stage buffer. ... In a nutshell, the cascode circuit offers two huge features: gain, lots of gain; and low, very low input capacitance; thus, it delivers high gain and wide bandwidth....

Aikido Cascode SE Amplifier
So, which sort of audio applications require lots of gain? Phono stages immediately come to mind, but there are many others. For example, the frontend of a single-ended power amplifier. The 12AU7 delivers a gain of about 50 in above cascode input stage, which is enough to the drive the output tube to full output. If even more gain were required, say to drive an 845 transmitting tube to full output, then a 6DJ8/6922/E88CC would do the job....

Aikido Cascode Mixer
As audio listeners, not professional audio producers, we seldom encounter a multi-channel audio mixer, AKA a sound board, mixing desk, audio production console. The mixer's job is combine various signal feeds into one output signal...

Aikido Cascode Microphone Preamp
Another audio application that requires high gain is a microphone preamp. ... A 6DJ8-based Aikido Cascode stage will yield a gain of about +44dB (x166), which might prove sufficient. If more gain is required, say +60dB, then a step-up transformer must make up the difference....

Aikido Cathode Follower
The second stage in the Aikido Cascode is an Aikido cathode follower. Its job is to both provide a low output impedance and to null the power supply noise from its output. Since the both of its triodes are in series, the bottom triode must counter power-supply-noise-induced current variations to null the noise at the follower’s output...
   31 Oct 2012

RMAF 2012
Another year, another trip down to Denver for the Rocky Mountain Audio Festival (RMAF). Last year's RMAF held fewer attendees than the year before and this year fewer still...Fewer attendees can have its silver lining, as it will result in less noise, less door openings, less of a wait at the elevators...

Best Sound at RMAF
In general, I was not that impressed. Of course, such a venue is profoundly ill-suited to serious listening and quality sound reproduction. The rooms are too small and too crowded with both people and equipment. The Brits were on to something when they demanded single-speaker listening...

A Good Omen, Perhaps?
After attending a funeral or an audio show, it is easy to be pessimistic. I noted, however, something that might count as an encouraging sign of better times to come. Sex. Sexy women in big ads. Before you turn me into the federal insensitivity police, hear me out...

High-Gain Tube Circuits
Some audio applications require gain, lots of it. Phono preamps and microphone preamplifiers immediately come to mind, but there other needs for high gain. For example, if the circuit employs a negative feedback loop, it is the unused gain that powers the feedback....

Aikido Pentode
Another approach is to use a pentode-based gain stage. One of the reasons that pentodes were invented was to get more gain...

Aikido Cascode
Another approach is to use the cascode topology, which was created to achieve many of the same goals as a pentode circuit, such as low-input capacitance and high gain, but without the pentode's higher noise.
21 Oct 2012

Inverted Current Mirror
Last time we looked at the current mirror, both solid-state and vacuum-state versions. Its function is simple a change in current flow in one of its legs creates an equal current draw in its other leg....

PP Current Mirror HP Amplifier
The previous designs rely on a balanced input signal, which might be seen as cheat of sorts. Well, we can create a push-pull amplifier that receives an unbalanced input signal by using two plain current mirrors. But we must take baby steps to get there. Let's start with the bootstrap amplifier topology....
      08 Oct 2012

Cathode-Coupled Circuits
The cathode-coupled amplifier is a compound circuit made up of a cathode follower that drives a grounded-grid amplifier. The result is an amplifier that does not invert the phase and that does not drag down the high-frequency response with Miller-effect capacitance. In addition, it offer lower distortion due to the two triode curvatures tending to cancel, producing a flatter transfer curve...

Cathode-Coupled OpAmp
The following circuit is one that I came up with over a decade ago. It offers several interesting features, such as no phase inversion, low distortion, low output impedance, and only two capacitors. But it holds one flaw, which was big enough for me to keep it within my sketch pad....

Current Mirror & Cathode-Coupled Amplifiers
I might have already covered this circuit here before. If so, sorry. The idea behind this circuit was to design a simple tube-based headphone amplifier that would allow 300-ohm headphones, such as the Sennheiser HD580 and HD600 and HD650 and HD800, to be driven to higher voltage swings than an MP3 player, such as the iPod, can put out. Adding a current mirror allows a class-A push-pull output, so twice the idle current (per triode) can be delivered into the 300-ohm load...
    03 Oct 2012

Mighty Diamonds
No, no, alas no. Not the truly fine reggae harmony trio, The Mighty Diamonds, from Jamaica; instead, the rather clumsy solid-state unity-gain buffer circuit is this post's topic. With only four transistors and four resistors, the diamond buffer does not require much....

Mighty Triadtron Diamond
Before leaving the topic of Mighty Diamonds, we must look at an alternative topology: the Mighty Triadtron Diamond....
    22 Sept 2012

Interconnect & Speaker Cable: Long or Short?
Your speakers stand 20 feet away from your line stage amplifier, so there are (roughly) three ways that you can arrange your system: use long 19-foot interconnects to deliver the line stage's signal to the two monobloc power amplifiers, each of which sits behind the speaker it drives through an embarrassingly short 1-foot length of speaker cable; or use 1-foot interconnects to the two power amplifiers and 19-foot lengths of speaker cable; or use 10-foot interconnects and 10-foot speaker cables, placing the power amplifier in the middle....

Auto-Bias Single-Ended Output Stages
Having just written the above section, I must quickly announce that the following design idea is not limited to power buffers; it just that power buffers were what I was examining when the idea came to me. What is this idea? It is a two-barrel affair that solves two tube-based amplifier problems, automatically biasing the output tube and improving the amplifier's PSRR...

Push-Pull Inverted Auto-Bias
Yes, it is possible. A push-pull version will require two isolation transformers and two RC filters, one for each grid. The constant-current source sets the idle current, with each output tube getting half, assuming matched tubes....
      18 Sept 2012

Loudspeaker Cables
Like audio interconnects, loudspeaker cable is a hotly debated topic. At one extreme, we read in glossy ads and the breathless reviews in audio magazines that the length of wire that spans between power amplifier and speaker is the single most important audio component....

Mystical Lengths of Speaker Cable
As I remember it, Fulton sold speaker cables (brown & gold) that adhered to an optimal length, or rather, optimal lengths, as the cable's length had to be a multiple of some magic length, which I have altogether forgotten...

Remote Negative Feedback & Speaker Cables
I have mentioned my experiments with remote feedback here several times before, see Blog 183 and 184. Back in the late 1980s, I had been studying regulated power supply design. I noticed that many industrial voltage regulators held four output terminals: negative, positive, -feedback, and +feedback. This intrigued me greatly but it made a lot of sense....

Computer Ribbon Speaker Cables
A friend of mine asked me for help. He had bought some good speakers and his wife didn't want ugly speaker cables snaking through the room. I recommended computer ribbon cable. You know, the wire that connected old hard drives to the mother board....

Thick Solid Copper
An audiophile friend called me one day, explaining that I had to rush over to his place to hear his new speaker cables. I reluctantly showed up. Well, he had something different...

The Broskie Interconnect Technique
In Blog 240 and 241, I described my 50%-signal-driven-shields interconnects. Well, could this setup be applied to speaker cables?...

Power Cords
I will be frank; although I am willing to believe that interconnect and speaker cables can be somewhat important, power cables do not send a tingle down my leg. Why not?...
09 Sept 2012

The Interconnect Problem
The RCA jack and plug were created as an easy solution for adding electronic equipment to an existing electronic chassis and power supply...

The Interconnect Problem
The RCA jack and plug were created as an easy solution for adding electronic equipment to an existing electronic chassis and power supply....

Old and Warm Wire
I once had an interesting talk with an audiophile. He claimed that wire requires a few decades to recover from the trauma of being made. He explained that this was why tube gear sounded so good: it was old, filled with old wire. Interesting, but why doesn't old transistor gear sound good then?...
     29 Aug 2012

Living in a Wireless World
Now that wires will soon be added to the list of superceded electronic parts, along with the vacuum tube and turntables, it is time for me to write about audio cables...

Interconnects
Having said all that, I do believe that cables make a difference, that they do sound different, some better, some worse. But before digging into the details, we should mention the huge practical difficulties in testing cables sonically...

My Interconnect History
Back in the late 1970s, while in college, a few of us audiophile types got together to do an interconnect shootout. We had gathered as many RCA-plug-terminated cords that we could find. Most were super cheap, the sort of cables that came with budget turntables and cassette decks. A few were more expensive cables that could be bought at electronic-supply stores. Only one cable was a high-end creation that cost a stately sum of $30, an outrageous expense back when we paid less $200 in monthly rent...

Broskie Cable
I was impressed with balanced XLR cables at the time and I found much to prefer over unbalanced RCA-terminated interconnects. Think about the voltage relationships within an XLR cable: two anti-phase signal wires, one ground wire, and shield, which attaches to one chassis to another...
     27 Aug 2012

Broskie CF in Solid-State
The Broskie cathode follower accepts a balanced set of input signals and delivers an unbalanced output signal. It offers low distortion and output impedance and an excellent CMRR. The easiest translation would be to replace triodes with FETs, as both are depletion-mode devices...

SPICE and Easier Solutions
The following does not hark back to last year, but this year. Back in May (2012), a reader wrote an interesting e-mail. He had read Blog number 231 and he thought that I had needlessly complicated matters in my Aikido PP headphone amplifier design, as shown below. As he saw it, too many triodes and just too complex. His alternative circuit was extremely interesting, but not for the usual audio reasons....
     18 Aug 2012

The Howland Current Pump Circuit
Almost a year has passed and ten posts since I last wrote about current-output amplifiers (blog number 218). No doubt, many readers are glad that I left the subject, as it was just too extreme a notion, even for tube-loving folk. Many readers, and I mean many, wrote to me, all stating the almost exact same sentence...

The Howland Circuit and Precision Resistors
Each amplifier type has its own its own dangers. A voltage amplifier must never see its output shorted to ground; a current-output amplifier must never see an open load. The Howland voltage-to-current converter circuit also must never see an open load connection...
      31 July 2012

Janus Variations
Janus was the Roman God of beginnings and endings, whose purview included doorways and gates. Unlike most Roman gods, Janus finds no Greek equivalent. Usually, Janus shown with two faces, one looking forward, the other backward. The month January is named after Janus, as this month looks on the previous year and on the new year to come.The Janus high-voltage regulator also looks two ways: towards the ripple from the rectifiers and to the signal-induced noise riding on the B+ rail. How this feat is accomplished is a bit difficult to understand, as the following schematic shows....
      08 Jul 2012

Octal CCDA Rev. A
The Noval CCDA has a brother. When I laid out the new version of the noval CCDA, I also brought the Octal CCDA PCB into Rev. A. The PCB is now 7 by 6 inches (1 inch longer than the old PCB), which allowed me to add an improved B+ and heater power supplies and to add a second set of output coupling capacitors....

CCDA B+ Power Supply
The old CCDA PCB held only one power supply RC filter per channel. The new Rev A boards hold a pi filter after the rectifiers (C7 & R17 & C8) and then each channel gets its own RC filter (R9 & C5||C4)....

CCDA Heater Power Supply
The old CCDA PCB's heater power supply circuit was a modest affair. The Rev A PCB holds a much more flexible heater power supply, which can turn 6.3Vac into regulated 12Vdc when its reservoir capacitors and rectifiers are configured as a voltage doubler....

Octal CCDA Tube Types
I know that 99% of Octal CCDA users will opt for the 6SN7 as the tube of choice. And it is a good choice. There are other types worth checking out, such as the 8SN7, 12SN7, 12SX7, 18SN7, and 5692. I have been extolling the use of oddball heater voltage for a long time. A decade ago, you could buy a first-rate 8SN7 or 12SN7 for a few dollars, whereas the 6.3-volt version of the exact same tube sold for $50 to $100. No more....
       03 July 2012

CCDA Noval Rev A.
Last year, I laid out a new version of the noval CCDA PCB, bringing it into Rev. A. The PCB is now 7 by 6 inches (1 inch longer than the old PCB), which allowed me to add an improved B+ power supply and to add a second set of output coupling capacitors....

Positive Feedback
We can all use some positive feedback. Not a lot, just some. Jumper J4 introduces some positive feedback into the circuit. With jumper J4 in place and capacitors C3 & C6 left off the board, the CCDA benefits from a small helping of positive feedback, which slightly increases the gain and slightly lowers the output impedance...

My New Project
Okay, back to tube electronics proper. I like the new CCDA a great deal and I want to combine it with Tetra Sans PS phono stage in a single box. I bought a black-painted, steel chassis box from Hammond, 17 by 3 by 10 inches big. In my mind the box was huge, but in reality it is much smaller than I expected....
       30 Jun 2012

Current to Voltage (part two)
Last time, we looked at some tube circuits that could convert a current-output DAC's audio output into a useable voltage signal. The big problem we faced was giving the DAC the low input impedance it required. (Voltage-output DACs desire a high output impedance, in contrast.) Low input impedance and tubes are not a natural pairing, so I devised the Broskie I-to-V circuit to greatly reduce a triode's cathode impedance....

Triadtron I-to-V Converter
The three-transistor Triadtron unity-gain buffer circuit was covered in blogs number 187 and number 188 and number 189....

Push-Pull I-to-V
The Triadtron is a single-ended circuit. Creating an I-to-V circuit that uses both emitter input and push-pull operation can be done, as the following schematic shows. This is an inverting amplifier whose input is taken at the input transistor's emitters. It is somewhat of a tweaky circuit, as it requires matched transistors and some fine tinning of values. Nonetheless, it is amazingly simple....

John Atwood's I-to-V Circuit
About 15 years ago, maybe more, John Atwood created a simple three-transistor I-to-V circuit for the new Dynaco company (Panor Corporation, I believe) for their CDV-Pro CD player....

5687 Aikido PP
I haven't mentioned it, but the in the last production run of the 5687 Aikido All-in-One PCBs I brought the PCB into Revision A. The board is slightly longer now and it now holds dual output capacitors. (The ability to choose between coupling capacitor types is just to essential to forgo.) In addition, the PCB can now be configured as an Aikido push-pull amplifier, for those in need of a excellent tube headphone amplifier....
       26 Jun 2012

DACs & Tubes
I have been hoping again to write on this topic since last year, since last year's RMAF... The topic isn't how to make a DAC out of tubes, but how can tubes receive the audio baton from the DAC. Once a DAC has converted its digital input signal into an audio signal, either in the form of a varying current or a varying voltage, the tube must convert the DAC's audio output into a useable voltage signal that can drive the power amplifier to full output...

Broskie I-to-V
Sorry, I cannot think of a whimsical name, such as Burning-Path I-to-V converter, so "Broskie I-to-V" will have to do...

DACs & Low-Pass Filtering
Most DACs require that their audio output signal, either in the form of current or voltage, be stripped of ultra-high-frequency contamination. A second-order, low-pass filter is often employed, although higher-order filters are sometimes used. The cut-off frequency can fall between 80kHz to 200kHz. Adding such a filter is easy with the Broskie I-to-V converter...

Solid-State Broskie I-to-V
Tubes are wonderful, but they are not the only devices that could be used in the Broskie I-to-V converter topology. For example, bipolar transistors or FETs could be used exclusively or in some hybrid arrangement...
       14 Jun 2012

Hybrid Headphone Amplifier
In blog 231, I described the Aikido SLPS headphone amplifier, which used a variation on split-load phase splitter to produce a low-noise tube headphone amplifier that could work over a large range of load impedances, unlike the SRPP or White cathode follower or SRCFPP circuits, which must be optimized to work into a single fixed load impedance...

Diamonds Versus Triangles
I have covered the diamond transistor buffer circuit here before, such as in blog 187. It is a simple unity-gain buffer that uses two PNP and two NPN transistors. Because it is so simple, it is beloved by many audio DIYers. The circuit is not perfect, however, which I discovered decades ago, when I played around with the National Semi LH0002....

Hybrid Triangle Circuit
The large-valued capacitor used to relay the output signal to the split-load phase splitter can be replaced by an NPN transistor or N-channel FET or a triode. The following schematic shows a hybrid triangle buffer....

Aikido Line & Headphone Amplifier
Returning to tubes, the transitor-based triangle buffer could be used with a PS-6 power supply and an Aikido line stage....
        26 May 2012

Aikido Split-Load Phase Splitter
A grounded-cathode amplifier stage (not an SRPP stage), made up from two triodes and two cathode resistors, directly couples to a split-load phase splitter, which delivers two anti-phase signals to the push-pull output stage. The Aikido aspect of this configuration lies in the phase splitter's two outputs holding identical amounts of B+ noise, which the output stage, being a difference amplifier, largely ignores, resulting in a noise-free output....

Aikido SLPS Headphone Amplifier
Now we can combine the Aikido split-load phase splitter with a totem-pole output stage to create a fine tube-based headphone amplifier.The following circuit would also make a fine linestage amplifier that could work well extremely low-impedance loads, such as Zen power amplifiers or 600-ohm input impedance amplifiers. Unlike an SRPP or White cathode follower or SRCFPP headphone amplifier, all of which require a known, fixed load impedance, the Aikido SLPS headphone amplifier can work with a wide range of load impedances, as the Aikido split-load phase splitter within it automatically adjusts the balanced output signals for each load impedance attached to the output....
        19 May 2012

Cathode-Follower Power
In my last post, I mentioned the possibility of building a cathode-follower output stage (OPS). This wasn't a first, as I have covered the topic many times before; see blog 149 and 148, for example... This got me thinking: well, why not use two power supplies, one 300Vdc for the output stage and a 600Vdc power supply for the input and driver stages?...
      12 May 2012

BCF-2 All-in-One PCB
The BCF-2 is a new PCB design that will make it easier for many to assemble a Broskie cathode follwer circuit for converting a balanced input signal to an unbalanced output signal...

BCF-2 and DACs
The BCF-2 can be used with voltage-out DACs that offer balanced outputs. If the DAC presents current outputs, then the Unbalancer circuit is a better choice, as quite a bit of gain will be needed. On other hand, most voltage-out DACs put out plenty of signal....

Aikido Single-Ended Output Stage
No, this is not the old Aikido technique that I came up with back in the 90s, wherein a cathode-biased output stage receives a small injection of power-supply noise at the cathode, so the output tube undergoes no alteration in current conduction in the face of B+ ripple, as shown below. See TCJ's first issue....

Warning!
One problem with using a single cathode resistor with several output tubes is that the assumption is that all tubes will be in place. What happens if only one output tube is plugged in and the amp is running?...
      09 May 2012

SRCFPP
Great. Just what we don't need: another damn acronym. On the other hand, "Series Reflexive Cathode Follower Push Pull" is a mouthful. But before I jump into its explication, I will begin with a quick overview of its two cousins, the White cathode follower and the SRPP....

SRCFPP History
Back in the late 1970s, when I was in college, I owned a pair of Sennheiser HD-414 headphones. I just loved them and I built many solid-state headphone amplifiers for them. I found that none of the OpAmp-based amplifiers sounded as good as my small, discrete-components, solid-state power amplifier that I had modified for headphone use. At the same time, I was reading about the SRPP topology, which was taking off in Japan...

Aikido SRCFPP
Audio Aikido is a family of many electronic techniques that all share the same characteristic: the power-supply noise is used against itself, rather than relying on brute force techniques. Could I perform an Aikido transformation upon the SRCFPP? My first thought was no, absolutely not...

Remember the Tringlotron?
Back in blog number 185, we examined the Tringlotron, which was three-transistor circuit that also held a floating load, i.e. not ground referenced load, which also accepted an unbalanced input signal....

MOSFET SRCFPP
Could an SRCFPP be made with solid-state devices? Absolutely. The obvious choice is a FET (or depletion-mode MOSFET), but a transistor or MOSFET could also be used....
28 Apr 2012

Essential Book:
Designing Power Supplies for Tube Amplifiers

On a whim, I bought Merlin Blencowe's book. I have to admit that I had low expectations, not because I was prejudiced against its author, but for the simple reason that most books intended for DIY audio electronics, and perhaps particularly for those intended for tube-audio, tend to be too simple-minded. It is almost as if the publisher and writer assume that making the big jump from the remote control to the soldering iron is in itself a feat of such towering skill, endurance, strength, and imagination that it is asking too much to expect the poor audiophile to learn and understand complex concepts. Thus, they give us too little—for our own good, as they see it. Mr. Blencowe's book, on the other hand, is not a piece of fluff, as he does not insult our intelligence nor withhold the goods....

New HV Regualtor Topology
While on the topic of high-voltage power supplies, let me show you a new design I devised. Starting with the compulsory throat clearing, let's look at two old designs. The first uses the famous LM317 positive, adjustable voltage regulator and a high-voltage triode. I remember seeing this schematic in an old National Semiconductor datasheet over 30 years ago....

New Series High-Voltage Regulator
The question was: Can a positive, high-voltage, series, voltage regulator be made out of an LM337? It can. The following schematic is stripped down, so as to leave its operation on display without the clutter of the safety and performance-enhancing components. We see a 34V zener providing the safe DC voltage window in which the LM337 hides from the silicon melting high voltages....

The Safe Approach
When building a new high-voltage regulator design, I always first try it with low voltages, for if it fails to work putting out 20V, it will never work with a 200V output voltage. The following low-voltage version would be a fine place to start. If no problems are encountered with the 35.25V output voltage, then we can move on to the high-voltage version....
        07 Apr 2012

Water Under the Bridge
Since my last post, I have had at least four interesting and prolonged exchanges with sevral TCJers and friends on different tube-audio subjects....Well, all of this got me interested in playing with stereo summing and difference circuits again; you know, the old L+R and L-R circuits for the glory days of quadraphonics. So, I set upon seeing how to implement these electronic functions with tube circuitry and my first efforts were producing far too complex topologies that required far too many tubes...

Current-Output Amplifiers
Esa Meriläinen, a Finnish electrical engineer, sent me a review copy of his amazing book, Current-Driving of Loudspeakers: Eliminating Major Distortion and interference Effects by the Physically Correct Operation Method....

PS-12 New Bipolar Low-Voltage Regulator
I have been playing with OpAmps lately and I needed a small bipolar low-voltage regulator that would fit in small boxes that were too small for my big B-PS-1....

Trim-1: 11-Position Stepped Attenuator
The Trim-1 single-channel, stepped attenuator offers 11 positions and can viewed as high-quality replacement for a potentiometer....
    26 Mar 2012

New RTI Capacitors
I sold out of the 1µF\600V film-and-foil RTI Capacitors, but a I just received a new shipment of the film 1µF polypropylene RTI Capacitors, but with a difference: they are 600V capacitors, not 400V, and a good bit larger than the original 400V capacitors....

Loudspeaker Diffraction Loss
Or, better still, the last system would be a near-field setup, wherein you would sit in the middle of the room and the two speakers would be with touching distance. In the near field, room reflections come too late to blur the sonic image or muddy the tonal colors. At first, near-field listening sounds wrong. But after only ten minutes or so, the brain and ear figure out what is going on and a beautifully precise image results. In many ways, near-field listening is the best compromise between headphone and loudspeaker listening....

Making the Ultrapath Work with Push-Pull
I have made over ten years worth of posts on the Ultrapath configuration, which Google will readily reveal. Thus, there is no need to carefully go over old analysis. Instead, my aim is to help the Ultrapath along....

Virtual Ground
The following schematic shows radical departure from the standard power supply. Here a monopolar power supply is transformed into a bipolar power supply. Note where ground is found; it is at the shared cathode connection, not the bottom of the power-supply's main reservoir capacitor. Ideally, the B+ and the ground connection are perfectly noise free, as the constant-current source perfectly isolates the ripple from both....

Single-Ended Zero PSRR Frontend
As I stated at the beginning, most Ultrapath power amplifiers are single-ended affairs. How do we use a zero-PSRR input stage with a single-ended output stage. Well, many options are available to us. We can use a pentode input stage, with the same cathode-to-B+ resistor in place (or even a high-mu triode). Or we can use two triodes in a cascode topology...

Virtual-Ground Power Supply and SE
If we used the same virtual-ground power supply shown before in the push-pull example, we could use a modified cathode-coupled amplifier, as shown below....

Standard Power Supply
and Cathode-Coupled Fronted

If a standard tube amplifier power supply, with a real ground center tap and negative-bias voltage rail, were used, then the modified cathode-coupled input stage could be used, as we could attach its constant-current source to the negative bias voltage. In general, it is never a good idea to make to great current demands on this negative rail, as it is created by half-wave rectification, which induces an unbalanced current draw in the transformer's secondary....
      08 Feb 2012

Aikido Push-Pull
It is time for me to fix a small problem. In blog 221, I described a push-pull, low-noise circuit variation on the Aikido—but I failed to give it a name. Why not?...

6AS7 Special Case
If we examine the formula for Rk, we see that the triode's mu (amplification factor) must exceed 2, or we will end up either zero or a negative resistor value. Does this mean that we cannot use the 6AS7 in the Aikido Push-Pull topology? No, as we can forgo the use of a cathode resistor to bias the triode and use a fixed bias voltage instead. In actual, practice, I always use a 10-ohm cathode resistor with the 6AS7 triodes, as the triode requires some degenerative negative feedback to keep from running away with itself. Thus, the following circuit achieves all the above goals and still fits in the Aikido Push-Pull topology....

Too Hot Aikido PP
Okay, what happens when the above formula specifies a cathode resistor value that is far too low, as it would result in excessive idle current? We have at least three options available to us...
      18 Jan 2012

Happy New Year
Wow, this will be my 13th year of posting the TCJ. That is a long time and a lot of words, schematics, and photos. Well, like any other reasonable fellow, I hope for the best, but I plan for the worst. Since 1980, I have been reading about audio's impending demise. Yet, I am certain that tubes will still rule and are not about to be vanquished. On the other hand, I can imagine wire going the way of the dodo bird...

5687 Aikido All-in-One PCB
The 5687 Aikido PCB is back, but different. Now, all the tubes are 5687s and the PCB is an All-in-One effort that holds both the high-voltage B+ and heater power supplies, just like the octal and 9-pin Aikido All-in-One PCBs. I know that I am not alone in loving the 5687 and after I assembled the PCB shown above and listened to the first few notes, I felt as if I had reunited with dear friend, affection and devotion swelling within me...

Single PS Circlotron
My renewed love affair with the 5687, got me thinking about building a balanced-output headphone amplifier for my Sennheiser HD650s using the 5687 in a modified circlotron circuit. I agree; it does not look like a circlotron circuit. Nonetheless, it functions like a circlotron, exhibiting the same Zo, gain, and distortion...
      09 Jan 2012

A-5 Stereo Stepped Attenuator
Amazing, just amazing. This stereo attenuator offers 36 volume settings, but only holds two 6-position rotary switches and 24 resistors. This cleverly designed stepped attenuator exploits both the series-attenuator and the shunt-attenuator configurations to yield the best compromise between flexibility, performance, and cost....

Select-4
One of the best sellers at the GlassWare-Yahoo store is the Select-2 three-input signal selector switch and PCB. I love this signal selector, as it switches the grounds along with the signal hots...On the other hand, most audio gear is wired with all the input and output RCA jack grounds tied together at the rear panel by a long strip of bus wire. In other words, you don't get to choose how to handle the ground connections, as the descission has been made for you. In such a setup, the following selector swith can come in handy, as it only switches right and left chanel hots. Moreover, the Select-4 allows up to six input signal sources to be used....

Select-Phase Switch
The following item has been available, but I haven't made a big deal about it, so many do not know that it exists. With a balanced system, we can easily flip the phase of the signal with the GlassWare Select-Phase switch and PCB. I get a lot of e-mail asking if I believe in phase. In other words, do I believe that we can hear the difference that phase reversal makes? I do....
      26 Dec 2011

The PCBs Have Arrived!
The shipment showed up this Wednesday, but they had to sit for a day or so, as I recovered from a bad cold. In fact, my entire family has it. ... The boards are beautiful and many of the out-of-stock boards and kits are back in stock, such as the Janus Regulator, 9-pin Aikido mono, Aikido stereo 9-pin, Tetra phono stage, PS-4 power supply, and the H-PS-1....

New LV-Regulator
The LV-Regulator uses a simple RC filter (1 ohm & 10kµF) as a pre-filter before the LDO regulator and holds bypass capacitors for all the electrolytic capacitors and a 4.7µF/400V polypropylene shunting capacitor at the output. The 1-ohm resistor is a 4W device, so the maximum current output is 2A....

Dead Quiet
The following story has a surprising ending…well at least for me. A customer, Richard, bought a pair of Aikido mono 9-pin PCBs, with the aim to building a headphone amplifier for his 300-ohm Sennheiser cans. Richard wrote me asking for help in building the best headphone amplifier for his headphones—the best being, from his perspective, the quietest headphone amplifier....

Chris Paul to the Rescue (update Dec 16 2011)
If you do not know who Chris Paul is, you should. He has been writing good, solid articles on audio circuits for the last 30 years. He is also a circuit-analysis wizard. I asked for his help, as I had run into many dead ends in my mathematical inspection of this new topological variation on the Aikido circuit...
       10 Dec 2011

What the Heck is Aikido Anyway?
Confusion grows. In my last posting, I described an Aikido cathode-coupled amplifier (ACCA), but it does not look anything like the famous Aikido amplifier circuit that I revealed back in blog number 11 in 2004....

Aikido History
Aikido is a relatively new Japanese martial art created back in the 1920s by Morihei Ueshiba (1883 – 1969). Originally, this martial art was named "Aikijutsu," but in 1942 its name was changed to "Aikido."...

Aikido Single-Ended Output Stage
Now here is another topological technique to add to the family of Audio Aikido. I have long wanted to assemble a single-ended amplifier that held a grounded-grid output stage, wherein the grid was grounded and the cathode received the input signal. Why? If nothing else, no Miller-effect capacitance. Recent correspondence with my good friend Vance Lohoff has revived my interest in the grounded-grid amplifier output stage...

Danger, Danger!
One big danger presents itself with the Aikido grounded-grid amplifier shown above: What happens if the input tube is not in its socket but the output tubes are in theirs? Not good...
     06 Dec 2011

Once again, the Cathode-Coupled Amplifier
Over the last 12 years, I have written much about the cathode-coupled amplifier. The reason is easy to find: I like the circuit a great deal and I believe that it is under appreciated. Two triodes and a few resistors is all that is required....

Aikido Cathode-Coupled Amplifier
I have presented this circuit, which injects a portion of the power supply noise into the second triode's grid to cancel the power supply noise at its plate, before. The only new addition is the resistor in parallel with the capacitor, which introduces a positive grid-bias voltage for the second triode's grid, allowing both triodes to conduct an equal amount of current, in spite of the dissimalar cathode-to-plate voltages....

Aikido Cathode-Coupled Amplifier Variation
The following variation uses four triodes, so two dual triodes can be used. In addition, a higher B+ voltage can be used, as no triode sees more than half the B+ voltage....
    25 Nov 2011

V-to-I Power Amplifiers
Same topic, but a more precise name. A "V-to-I" amplifier receives an input signal in voltage and delivers a proportional output current, whereas the "current-output amplifier" label can describe either the I-to-V or the I-to-I amplifier. I know many readers are wondering why I am spending so much time on this topic, particularly as V-to-I amplifiers are so rare....

Zenlike I-to-V Power Amplifier
First, I believe that I rather jumped the gun last time by revealing the I-to-V Moskido amplifier, as that design was a push-pull type and I should have begun with only single-ended MOSFET-based designs, so let me backtrack a bit. But before we get to the actual circuits, bear in mind that single-ended means strict class-A, wherein the maximum peak symmetrical output current swing equals the idle current. This is a sober reality, hot and brutal, not the the make-believe world of high-end audio and glossy ads, where "class-A" operation means whatever we think we can get away with....

Simple Push-Pull I-to-V Power Amplifiers
Now that we have examined some simple single-ended I-to-V power amplifiers, we can move on to their push-pull brothers. The simplest push-pull V-to-I amplifier would be one that used only two active devices. Remember the Sagaris amplifier from blog number 153? This design was a push-pull V-to-V power amplifier that used a floating power supply per channel and grounded the output MOSFET sources, taking the output from their drains....

All-Tube PP OTL V-to-I Amplifier
I know all these solid-state devices have many readers in cold sweats, but not to worry, as the tubes are back. The following V-to-I amplifier (super-buffed line-stage or headphone amplifier) uses low-voltage bipolar power-supply rails (+/-40Vdc) and a high-voltage B+ voltage (+200Vdc). Such a power supply is easy to build up with the PS-6 kit....
      15 Nov 2011

Knowledge & Understanding
I know that I am about to swim against the stream here. I know that most audiophiles (and far too many audio DIYers) just want stuff, new shiny, plastic-film wrapped stuff—stuff not knowledge nor understanding. But at the same time, I know that you are different; you wouldn't read my posts unless you wanted to know more about tube electronics and electronics in general, so that you can understand what is going on inside the circuit...

Simple Hybrid Current-Output Amplifiers
Baby steps are not a bad idea, so I will start with a truly simple current-output amplifier. The following current-output amplifier uses a voltage-output DAC as its signal source. The DAC's voltage output is referenced to half its B+ voltage, which in this case is 2.5Vdc...

Moskido I-Out Amplifier
The venerable Moskido power amplifier can be transformed into a current-output amplifier. The conversion require that the output be taken from the center-tap of a floating power supply and that the MOSFETs' sources be grounded. In other words, each channel must receive its own non-grounded low-voltage, bipolar power supply....

LT1166 I-out Amplifier
Remember the Linear Technology LT1166 IC? We saw it back in blog number 211. From the LT1166 datasheet:

 

The LT1166 is a bias generating system for controlling class AB output current in high powered amplifiers. When connected with external transistors, the circuit becomes a unity-gain voltage follower. The LT1166 is ideally suited for driving power MOSFET devices because it eliminates all quiescent current adjustments and critical transistor matching.

Well, with a little reconfiguring, we can create a current-output amplifier using this device at its core.
      27 Oct 2011

2011 RMAF
Another year, another Rocky Mountain Audio Festival. This was my second attendance and I noted a few changes. My first impression was that although there were fewer attendees, just as many exhibitors occupied the rooms, which is certainly better than having both fewer attendees and fewer exhibitors. Still, this says something about our moribund economy. My second take on this year's RMAF is that this was the year of the small speaker....

First Watt and Current-Output Amplifiers
Back to the RMAF, what I like best about the show is meeting my old friends and making new friends. Additionally, I become inspired by what I saw at the show (and by what I didn't see there). One piece of equipment that fascinated me was Nelson Pass's First Watt power amplifier, which was in use in the Lowther America room. Very interesting....
      21 Oct 2011

RIP Steve Jobs
Steve Jobs died this month and I am iSadden. Over 300 million iPods have been sold since 2000; and for many, the iPod is as close as they ever come to experiencing Hi-Fi. And for a few, the iPod was their first step to becoming an audiophile. Indeed, it's far too easy to dismiss the iPod...and Steve Jobs for that matter. Both have been vitally important. Now, I have to admit that I have never owned either an Apple computer or an iPod, but I am glad they both exist; likewise, I am glad that Steve Jobs was born and I am sadden at his demise. Can you imagine how hard it must have been to run Apple? (I remember talking to Apple employees who referred to Steve Jobs as an Apple virus.) Or how desolate, sterile, and vacuous the PC world would be without Steve Jobs or Apple? A world with only IBM and Microsoft equals iBored....

PCB Update
Well, all the Tetra Sans PS PCBs are sold. It was quite a ride, although far too short. Do not dispair; more will be made. In the mean time, the following new PCBs might help you out in your next project. These are small, 2 by 2 inch, PCBs that are designed to hold a single tube socket and a few resistors. Many, myself included, like to see tube protrude from the chassis top panel, like missles leaving thier silos. These boards help make that possible. Each PCB comes with a ceramic tube socket and four sets of aluminum hex standoffs (0.75in) and rubber O-rings and screws. The PCB materail is thick (0.93in) and tough, so there is no chance that it will bend, as the tube is pulled from or pusshed into its socket....

Over/Under OTL
I love shotguns. Although I am a much better shot with a pistol, shotguns thrill me. I love the blast, the shoulder-bruising kick, the savage raw power of magnum-load 12 gauge shells. And I long to own a fine gas-operated shotgun, such as the famous Remington Model 1100 or the Browning Maxus. But I never understood the appeal of the over/under shotgun, both from a design and function standpoint. Nonetheless, over/under shotguns is what came to mind, as I inspected this phase-splitter/OTL topology, seeing the two transistors as two barrels...

Over/Under True-Phase-Splitter Variation
The over/under OTL output stage presents the input signal to both thetop tube's gridand the bottom PNP transistor's base. But what if the circuit only took its input signal at the bottom PNP transistor's base, placing both the top and bottom tubes under the controll of the two transistors? Here is the naked topology, sans grid-stopper resistors and part values....
      10 Oct 2011

Tetra Sans PS Phono Preamp
The new Tera Sans PS PCBs are now available at the GlassWare-Yahoo store. ... Well, in testing the new Tetra Sans PS, I have been spinning a bunch of vinyl—and I don't want to stop. As I write this, I am spinning Peter, Paul and Mary's (Ten) Years Together album, which has never been a favorite, but dang if I am not being pulled into the music. Even the dreaded Puff the Magic Dragon is altogether compelling. I have to say again; it's amazing just how amazing LPs sound. Yes, I know they tick and pop sometimes rumble, but dang it, they can sound so...so dang real....

Positive Feedback
Once again, I configured this new Tetra Sans PS phono stage to utilize a bit of positive feedback. Not a lot of positive feedback, just hint. Originally, about 25 years ago, when I first played with the above circuit, I simply sought to eliminate the need for a cathode- resistor-bypass capacitor, but now I am convinced that something extra results from the technique and I like it. The wee bit of positive feedback seems to lend a restored vigor and liveliness to the sound, almost as if a slight electronic expansion were being introduced....

Ripping LPs
I have a huge project ahead of me. First of all, the test that cries out to be performed is to run the Tetra's signal out to a high-quality ADC, then save the digital file to the hard drive. How much of this sonic joie de vivre will survive? And at what cost in terms storage space? I have ripped a few of my LPs before and I was neither thrilled nor distraught by the results. One problem I encountered was overloading the ADC; not pretty....

Sans PS?
Why did I make power-supply-less Tetra PCB? What's wrong with including a power supply? As I mentioned in the blog post, many already own or have access to a power supply. For example, some own old tube equipment and can jettison the old guts. Others own power supplies, such as the Janus and PS-1 regulators. And others still own the All-in-One PCBs that hold both a high voltage and low-voltage power supply, with extra solder pads for just such an external-load use. Only four wires are needed to energize the Tetra PCB from an All-in-One PCB....

Quatrode-2 with Gain [OTL]
When I come up with a new circuit, my notepad fills with an array of variations. So it was with the quatrode-2 topology. Over twenty circuits blossomed from the three-device scheme. Not all seed circuits are this bountiful and not all of the deviations from the initial circuit worth keeping. One variation, however, I must look into further is the B & B CF that has been forced to provide voltage gain....

Quatrode-2 Circlotron
This Circlotron OTL amplifier will need twice the number of tubes that the hybrid would need, as negative voltage swings will have to be provided by an opposing bank of tubes—but then this design is pure tube. Note how it is the output tubes that get the coupling capacitors and AC input signal (and bias potentiometers) form the MJE350, while the input tubes (the 6922) see the PNP collector's DC voltage, but not its AC voltage swings. Also note, how huge the drive signal to the output tubes can be, roughly +/-115Vpk....
       29 Sep 2011

More B & B Cathode Follower Circuits
Blog No. 212 introduced the B & B cathode follower; this time we will examine more variations on this theme. "Theme" is the right word, as a unifying or dominant idea, motif, configuration, structure is what I want to highlight....

Quatrode-2
Like the first quatrode and A. J. van Doorn's octode, this scheme defines a basic building block, which finds application in many different, wildly different, applications....

Quatrode-2 Series Voltage Regulator
Here is a straightforward example of a high-voltage regulator built up with a quatrode-2 scheme at its heart. ...

Quatrode-2 Shunt Voltage Regulator
Here is an example of a high-voltage shunt regulator designed around a quatrode-2 cluster. In so many ways, this regulator is the inversion of the series regulator just described....

Quatrode-2 Single-Ended Amplifier
Now for something different, a single-ended power amplifier based on the quatrode-2 scheme....
       01 Sep 2011

RIP Bob Pease & Jim Williams
Two analog-electronic giants died last June. Pease was killed in an auto accident (ironically and sadly enough) while leaving a memorial service for Williams, who died Sunday, June 12. Pease was only 70 years old and I fully expected him to go on another thirty years. Both will be missed greatly.

Cathode Followers
The poor cathode follower; it gets little respect...or too much. Many audiophiles discount it instantly, believing that it is a sonic blight. Yet, many counter examples exist, great sounding tube-filled pieces of equipment that hold cathode followers. And we can quickly perform our own sonic test by reconfiguring a CCDA line stage to forgo its cathode follower, which just as quickly sonically reveals why the cathode follower was there in the first place. (Even the distortion meter shows a decrease in distortion after the cathode follower, albeit a small one; but when we realize that no global feedback loop is employed, we should be astounded by this result.) At the other extreme, many tube lovers falsely ascribe superpowers to the cathode follower, imagining it able to melt away yards and yards of capacitance-heavy cables, drive 16-ohm headphones to ear-bleeding levels, and leap tall buildings in a single bound....

Hybrid CF+
Okay, you knew all of this already; but did you know that the following circuit was possible?...

Brains & Brawn Cathode Follower
No doubt it is a bit confusing. What is that transistor doing there, for example; and are there no internal coupling capacitors? At first glance, it appears as if both triodes are driving the cathode resistor and the external load, but actually the input triode pretty much draws a constant current, which means it cannot directly dump a varying current into the load. That job is held by the right triode, which gets its instructions from the input triode, the brains of the circuit...
      27 Aug 2011

Aikido Mono Solo
The standard Aikido mono 9-pin PCB and the power supply from the Aikido All-in-One stereo PCB have been wedded, creating an All-in-One mono Aikido board. The new Mono Solo PCB is 6 inches by 6 inches, extra thick, 2-oz copper traces, silk screened on both sides. It holds one channel of Aikido amplification and the needed B+ and heater power supplies, with the heater power supply getting a voltage regulator....

Aikido Mic Preamp
The above schematic displays a simple microphone preamp based on the Aikido Mono Solo. Much more could be easily added, such as anti-RFI ferrite beads, a resistor-capacitor network shunt for the transformer's secondary, and, possibly, a phantom power supply...

Moskido Power Amplifier
At the other extreme, we see below a hybrid power amplifier. In other words, a Moskido power amplifier based on the Aikido Mono Solo PCB. As shown, the MOSFET output stage uses a mono-polar power supply, but a bipolar power supply can be just as easily be used, by just lifting resistor R13's connection to the ground and attaching it to the negative power supply rail...

LT1166
The LT1166 is an interesting device from Linear Technology. I first read about it in Robert Cordell's must-buy book, Designing Audio Power Amplifiers. Thanks Robert. The LT1166 is so interesting that I am amazed that it was ever created. Okay, just what does it do? It is an auto-bias/driver circuit for solid state output stages, mostly MOSFET-based output stages....

Back to the Aikido Mono Solo PCB
Of course, these two audio applications for the new Mono Solo PCB are not the only ones possible, as a dual-mono stereo line amplifier or a tube-based mixer or a tube headphone amplifier could also be made from this new PCB. In fact, I plan on using two of the boards myself, as I plan on building two monobloc single-ended power amplifiers using the new PCB....

VIAS
I found the following website by accident and I am glad that I did. These seem to be my kind of people, as my motto is conserve and improve; conserve everything worth keeping and improve all that you can....

Passive Line Stage Vs Unbalancer
I finally got around to wiring up my passive line stage that is based on two M1 mono stepped attenuators and one Select-1 switches and ten RCA jacks....
    20 Aug 2011

Something New: MP3 of Blog Post
Everyday I use a software program that converts text into speech. Yes, I know that all computers come with such a program, but these universally sound dreadful, with odd mechanical, robot-like sounding voices that quickly irk and weary the listener. The program I use makes use of new high-quality voices that sound natural. So, here is an example. Blog210.mp3

Cathode Bias with a Constant Current Source
A few years ago, I received many e-mails asking for help on how tube-based, push-pull amplifiers that used a constant-current source at the output tube's connected cathodes worked. I believe that it was Allen Wright that had caused the stir, with his Vacuum State dpa300B power amplifier...

TCJ Workaround
I am planning on building a push-pull power amplifier and I have had to face many design choices. Although I like the ease of the class-A, constant-current source approach, I would like to cheat the system a tad, by running a lower idle current and still getting an occasional burst of wattage beyond the strict limits of class-A....
    13 Aug 2011

PS-7 High-Voltage Power Supply
This new power supply board is only 2 by 3 inches big and holds a very simple circuit. Yet the PS-7 can pack a wallop, as it can be used to make a power supply of up to 400Vdc and it holds two 47µF/450V capacitors; a lower voltage version holds two 220µF/200V capacitors, for a power supply of up to 170Vdc; and still another version holds two 1kµF/63V capacitors, for a power supply of up to 50Vdc....

Bipolar Shunting Regulators
My postings on the hybrid tube Op-Amp and the hybrid spilt-load phase splitters have all relied on using a bipolar power supply, much like most solid-state Op-Amps run on, but at much higher voltages: +/-100V instead of +/-15V. So, how do we design a high voltage bipolar shunt regulator? The simplest shunt regulator uses a string of zeners to set the fixed positive and negative power supply rails...

Tube-Based Bipolar Shunt Regulators
In the schematic below, the triode compares the AC signals on its grid and cathode, which move in anti-phase to each other. Thus when the positive rail swells positively, the negative rail will swell equally, but more negatively....

At Last, the Beginning
We arrive at where I wanted to begin, but couldn't.... The following all-tube, bipolar, shunt regulator uses a 12DW7, a dissimilar tube that holds 12AX7 and 12AU7 triodes. The 12AU7 section does all the heavy current swinging, while the 12AX7 provides all the voltage amplification to drive the 12AU7 triode....
    07 Aug 2011

Hybrid Op-Amp Part Three
Part three is here and we arrive at where I wanted begin, but knew that I couldn't. In part two, we examined inverting hybrid Op-Amps. Now, we will look into their non-inverting brothers. The following circuit uses a single (PNP) transistor and two triodes....

Fancy Hybrid Op-Amp Variations
Okay, long time readers will know that once I come up with a new circuit, I immediately come up with a dozen variations. I can't help myself. This superabundance both dazzles and dismays my friends. As I look at the circuit, I note that although the output tube runs in a strict single-ended class-A mode, there is nonetheless a definite push-pull quality to the circuit, because of the two triodes drawing the same idle current and operating in anti-phase to each other...

Back to Single-Ended
The big advantage that a push-pull output stage bestows is more power into the load, typically four times more than the comparable single-ended output stage would deliver. But if we are driving a power amplifier's 47k input impedance, almost no power delivery is needed. Thus, let's look into how we can return to the single-ended version—but hot-rodded...
    25 Jul 2011

Mono 66-Position Balanced Stepped Attenuator
Good news for the those audiophiles who run balanced systems. The BM-1 is a new stepped attenuator that I have designed. It uses 24 resistors and two rotary switches: one 11-position, 1-pole, shorting switch; one 6-position, 2-pole, shorting switch. The configuration is interesting, as it consists of a shunt stepped attenuator cascading into two series stepped attenuators, one for each phase....

My Passive Line Selector
and Stepped Attenuator Box

I was about to title this section "My Passive Line Stage," but this is a really a stage-less design. My latest project is made from two GlassWare M1 66-position stepped attenuators and one GlassWare Select-1 input selector and a black-painted Bud box (2.5 x 17 x 7 inches) and ten RCA jacks; this simple project uses no active devices, relying on me to provide the active knob turning. It can choose between three input signal sources, say a CD player, a tuner, and a phono preamp. Once a signal source is selected, each channel's volume can be set independently by the two mono stepped attenuators, each of which holds two knobs, one for fine (-1dB) decrements and one for coarse (-6dB) decrements....
    10 Jul 2011

Faux Glass
Did you know that that one of the hottest fashion accessories today is clear eyeglasses, glasses that hold flat lenses? Just do a web search on "Retro rocker clear glasses" or "Fashion nerd eyeglasses." Apparently, millions are wearing non-prescription eyeglasses in order to make a fashion statement....

New ACF Octal PCB
I have a new version of the ACF (Aikido Cathode Follower) to offer those who do not need any voltage gain, but do need an active tube buffer stage. Like its 9-pin predecessor, the octal ACF PCB holds a high-voltage bipolar power supply and a low-voltage heater power supply (regulated) and two channels of Aikido cathode followers....

New Toroid Transformers at Mouser
Good news. Mouser Electronics is now selling Triad brand toroidal transformers, which include some nice high-voltage models that are perfect for tube-audio use....

Tube Hybrid Op-Amp part-2
Last time we looked into designing a tube-hybrid, inverting Op-Amp that used but a single PNP transistor and ran off high-voltage bipolar power supply rails....
    02 Jull 2011

12B4 CCDA
The 12B4 is a great little triode that has a small but supremely devoted following—and with good cause, as it is both inexpensive and great sounding. An inspection of the 12B4's specifications reveals that it is not your typical audio triode....

Hybrid Op-Amp
Operational amplifiers fill just about all audio products and with good cause. Small, efficient, inexpensive, sturdy, and supremely capable, today's solid-state Op-Amps are a truly marvelous. And it's their near perfect functioning that allows us to treat them as simply being an encapsulation of a electronic generic function—namely, an operational amplifier. Thus, we seldom have to worry about what goes on inside the Op-Amp, only about what Op-Amp's function is in the circuit...
    24 Jun 2011

New Mono Step Attenuator
I have a new stepped attenuator design to offer the DIY audio world, the M1 mono stepped attenuator; and I just love it. The M1 stepped attenuator combines both series and ladder stepped attenuators into a single functional attenuator. The ladder attenuator's job is to provide six fine steps of attenuation, each step being -1dB; the series attenuator, eleven coarse steps of attenuation, each step being -6dB. The result is that 66 attenuation settings are possible. In other words, we can set the attenuation to any value from 0dB to -65dB in -1dB decrements....

M1 Mono Stepped-Attenuator Applications
I can envisage DIYers building mixer consoles using several of the M1 mono stepped attenuators or dual-chassis dual-mono line-stage amplifiers or slick headphone amplifiers (imagine four knobs with the headphone jack in the center). I myself plan on building a passive linestage out of two M1 mono stepped attenuators and one of my Select-1 3-position signal selector switch....

RTI Film& Foil Coupling Capacitors
As most GlassWare customers know, I ran out of RTI Audio capacitors a few months ago. When I placed an order for more capacitors from RTI months ago, however, I decided to try something new: polypropylene-and-metal-foil capacitors. I had listened to this type of capacitors from RTI before and they impressed me a great deal, so I ordered a shipment of 1uF at 600Vdc film-&-foil capacitors...

PS-9 New Heater Power Supply
Over the years, I have hard-wired together the following circuit many times, as I often have used solid-state rectification in my own tube power amplifiers and I didn't want the 5Vac rectifier winding to go to waste. The circuit is a simple voltage doubler that converts the 5Vac into 12Vdc for the power amplifier's frontend circuitry...
    26 Apr 2011

Unbalancer
Good news. I can finally reveal my mystery project that I mentioned at the bottom of blog 199, as I have thoroughly tested the PCB and I finished the 20-page user guide. Behold the the Unbalancer: a tube-based, balanced-to-unbalanced converter that accepts a balanced input signal and puts out a single-ended, unbalanced signal, much like a Broskie cathode follower, but with gain....

Unbalancer as I-to-V Converter
The Unbalancer could be used to build a I-to-V converter following a DAC. The idea here is that a differential current-output DAC can work into two 60-ohm resistors, which will develop enough just voltage gain to allow a 12AX7-based differential amplifier to bring up to a useable voltage swing....
    22 Mar 2011

Hybrid Split-Load Phase Splitter
The following hybrid split-load phase splitter circuits allow huge voltage swings to be created, as the solid-state devices do not suffer from the big voltage drops that that triodes incur. The first circuit is quite simple, apearances to the contrary. The triode's current conduction is varried by the input signal at its grid. This current variation is then relayed to the PNP adn NPN output transistors....
     18 Mar 2011

New Balanced Step Attenuator
Like its unbalanced predecessor, this stepped attenuator offers 36 steps of attenuation for two channels of signal. The center rotary switch controls both channels and presents coarse decrements, while the two flanking switches afford fine volume decrements for each channel....

Aikido Super Triode Correction
I screwed up last time. In describing how the Aikido-Super-Triode circuit that held a floating power supply worked, I said that the current path through the tubes traveled into the solid-state's negative power-supply connection. It doesn't. It flows into the positive power-supply connection. I am sure that 99% of readers didn't catch that mistake, as such circuit is difficult to wrap one's mind around...

Aikido Hybrid Amplifier
Here is another topic that I thought would make its appearance in last year's posts. The idea is to create a hybrid Aikido power amplifier that replaced the Aikido's cathode follower with a solid-state power amplifier. Sixty blog posts back, in May of 2008, I posted a good-sized blog, number 141, on using gainclone chip amplifiers, such as the LM3886 and LM4780, in an Aikido hybrid amplifier configuration. What I am going to offer this time is tweak on that topology, which grants us greater flexability when using a gainclone power OpAmp....

The Aikido Hybrid Workaround
Finally, I arrive. The workaround allows us to use two identical triodes with identical cathode resistors as the input stage, just as in the all-tube Aikido amplifier, and run the solid-state power amplifier with an AC gain of 10....
     09 Mar 2011

Blog Number 200
Two hundred blog posts! Unbelievable, just unbelievable! Most blogs, like most scandals and love affairs, are short lived. Like a match bursting into life, briefly burning hot, then quickly smoldering out of existence, many promising blogs begin sincerely and earnestly, with great fanfare, but soon falter, then stall altogether and finally die, littering the web with many grand, but abandoned, endeavors. In contrast, this blog has been more like a candle, burning long, occasionally seemingly extinguished by the wind, but always igniting again....

Tetra Phono Stage Update
Yes, it's been a long, long time. I remade the Tetra PCB, bringing it into revision A. The B+ power supply now sports an extra RC filter and the heater power supply now allows more rectifier configurations (a 6.3Vac or 12.6Vac or 24Vac CT winding can be used to create a 12Vdc regulated heater power supply). In addition, all the phono stage parts have been shifted into better positions on the board....

Tetra & Positive Feedback
I decided to do something different this time. While building up a Tetra phono stage to test the new revision of the Tetra PCB, I choose to try a little bit of positive feedback; not a lot, just a tad....

More Super Triode Circuits
I have a big backlog of circuits that I had planned on posting last year. More split-load phase splitters, more hybrid power amplifiers, more buffer circuits, more voltage regulators... One cluster of circuits follows the theme of Super Triodes, wherein a triode superimposes its sonic signature over a solid-state power amplifier's transfer function....
28 Feb 2011

Quatrode
I planned to reveal this electronic trope—scheme, grouping, figure, composition, pattern, configuration—last year, but the year flew past me. The inception for this circuit cluster began when I tried to come up with new uses for the 12DW7/ECC832/7247 tube. (This dissimilar triode tube effectively holds 12AX7 and 12AU7 triodes in one envelope and the current production version from JJ is quite good.)...

Quatrode Single-Ended Power Amplifier
Here is one possibility, an SE power amplifier. The triode on the right controls the triode on the left, much like the super-triode circuits I have shown before. A change in either or both the right triode's grid or plate voltage will cause a change in its current conduction. This variation in current flow is then relayed to through the PNP transistor to the collector resistor, which in turn is relayed to the left triode's grid...

Quatrode Baby Steps: the Cathode Follower
The more I think about it, the more I am sure that the single-ended amplifier was too big a jump for many. Let's backtrack to a simpler circuit, the cathode follower. Plugging a Quatrode into the cathode follower topology is easy enough....

Quatrode Grounded-Cathode Amplifier
A Quatrode-based grounded-cathode amplifier displays super-triode qualities. For example, the gain is limited to something less than the input triode's mu. The following example uses a -50V power supply rail and would deliver a gain of 70 into a 100k load....

Quatrode High-Voltage Series Regulator
Let's try something altogether different, a high-voltage series regulator, as that would truly show off the Quatrode's portability....

Quatrode High-Voltage Shunt Regulator
If you stop and think about it, the Quatrode in itself would make a fine shunting device in a high-voltage shunt regulator. If very little regulated current were needed, we could even get away with using the 12DW7. But where higher current delivery is required, the 6AS7 would be a better choice...
    30 Jan 2011

Aikido Cathode Follower Version 2 [ACF-2]
Here is one of the new PCB designs that I mentioned here before. The ACF-2 is an All-in-One design that holds both the B+ and heater power supplies and two Aikido-ized cathode followers. In other words, the ACF-2 is a unity-gain buffer for those who do not need any extra voltage gain, but do need more current delivery than a passive line stage would allow...

Logitech Squeezebox™ Duet & Touch
The ACF-2 is a perfect partner for the Duet or Touch (or the old Squeezebox unit), as these products offer an easy way to set up a WiFi networked audio system. In addition, they use high-quality DACs and remote-controlled volume attenuation. What they lack, however, is a tube-based audio section....

Impedance Multiplier Circuits and the PS-6
With my new PS-6 power supply PCB that holds a low-voltage bipolar power supply and high voltage quintupler circuit, I have been thinking about what designs to build around it. This has forced me to reexamine some of the many hybrid power amplifier circuits that I have covered here....

IMC Baby Steps
Let's begin with a simple impedance-multiplier circuit with a 50-ohm load...

IMC Headphone Amplifier Example
By the way, don't assume that only big, powerful, hybrid impedance-multiplier circuits are possible. Topology stands apart from application. We could use the same topology to design either a 1kW power amplifier or a 1W headphone amplifier or even a 10mW line-stage amplifier. It's only a matter of scale. As design example, here is a rather powerful headphone amplifier that can deliver 4W into a 32-ohm load...
    23 Jan 2011

PS-6
The PS-6 power supply offers a low-voltage bipolar output (up to +/-50Vdc), suitable for solid-state power amplifier or OpAmps; in addition, the PS-6 power supply holds a voltage quintupler that yields a single high-voltage B+ output voltage roughly equal to five times the rail voltage....

Hybrid Power Amplifiers & Power Buffers
In the last year, quite a few novel circuits have appeared here that could make excellent use of the PS-6 power supply. For example, do you remember this super-triode version of a GainClone power amplifier from blog 179?...

Low-Voltage OTL
Or how about an OTL line/head-phone amplifier that ran on +/-40Vdc rails? The 6082 dual triode (the industrial verion of the 6AS7 with a 25V heater) would be a perfect choice, as we could run its heater on the low-voltage PS rails...

Hybrid Phono Stages
Or how about a hybrid phono stage that made use of a super-quiet OpAmp, such as the LT1115 or AD8599, as the input stage, followed by a passive equalization network, then followed by an Aikido gain stage....

PS-6 and Shunt Regulators
No doubt some have grown nervous over the OpAmp getting a regulated power supply, while the tube stage just gets an RC filter. The high-voltage B+ can also be regulated, shunt regulated no less, with the addition of just a few parts....

New Phase Splitter:
the Aikido Split-Load Phase Splitter

I promised a new phase splitter design and here it is, the Aikido phase splitter. I know that this phase splitter looks like it cannot possibly work, but it does. It functions just like a conventional spilt-load phase splitter, except that the split-load's cathode resistor has been replaced by an active resistive load, created by the bottom triode and its cathode resistor. If we chose its cathode resistor carefully, the impedance presented by the bottom triode's plate will equal that of the plate resistor at the top triode's plate....
     29 Dec 2010

PCBs At Last!
I came to doubt that the boards would ever arrive, I must admit. But they are here and they look good. Many of the most popular boards are back in stock...

Split-Load Phase Splitter
After reading Stuart Yaniger's article in Linear Audio, "Taming the split-load inverter" and after designing my 6AS7/6080/6082 based OTL, it seemed to me that many more topological tricks remained latent within the humble but capable split-load phase splitter. But first, let us do a quick recap on the circuit....

Broskie Spilt-Load Phase Splitter
This phase splitter not only provides power-supply-noise-free outputs, it offers a solution to the two-triode problem. What's the two-triode problem?...
    21 Dec 2010

OTL for 50-Ohm Loads
Well, two months later, I finally get to start on this OTL design; but I want to deviate a bit first. Note how I have made no mention of headphones yet--purposely. I know many audiophiles just don't do headphones. Disliking them intensely, these audiophiles immediately tune out any topic that's headphone-related. One's preferences are just that—one's preferences, which exclude debate. (Although I enjoy headphone listening, I can readily appreciate why headphones are despised by many...

72-Ohm Loudspeaker
Nine 8-ohm loudspeaker drivers, like four 8-ohm loudspeaker drivers and sixteen 8-ohm loudspeaker drivers, is a good number of drivers to use, as when placed in series-parallel with each other yield an impedance of 8 ohms. In other words, we can always backtrack to a safe, conventional 8-ohm load if we wish. Moreover, 72 ohms is high enough to make OTL power amplifier both much easier to design and build. Yes, 128 ohms (sixteen 8-ohm drivers in series) would be better still for the OTL, but not for our wallet....

Back to OTL Headphone Amplifiers
I mentioned that, while attending the RMAF, I was astonished to learn that relatively big watts from headphone amplifiers are now favored, with 4W (into 32 ohms I assume) considered ideal. This increase in required power comes from the recent development of a wonderful 50-ohm, planar magnetic headphone, the Audez'e LCD-2...

Biasing OTL Output Stages
The above design examples show two grid-bias voltages used: -36V and -136V. When actually building an OTL amplifier, more care will be needed in setting the optimal bias voltages. The following passive circuit was designed for an OTL power amplifier that used many parallel output tubes and was meant to drive 8-ohm loudspeakers, but the same topology can be applied to our little 4W OTL headphone amplifier....

OTL Power Supply
A suitable power supply for this OTL amplifier is shown below. A center-tapped secondary can create the four required power supply rail voltages...
  16 Dec 2010

Cathode-Coupled Amplifier (CCA)
I have covered this circuit many, many times before....The last entry is actually a good place to start, as it shows several ways to overcome the problem of dissimilar cathode-to-plate voltages in a CCA. This problem is the CCA's one big hassle to overcome. In fact, if the using CCA circuit didn't entail dealing with disimilar cathode-to-plate voltages, I am sure that this topology would be much more popular, as it has much to offer, such as low input capacitance and no phase inversion and a high-impedance feedback port....

The CCA Power Supplies
The CCA PCB is an All-in-One affair, wherein the audio and power-supply circuitry reside on the one board. The B+ power supply is a simple design that uses cascading pi filters to reduce ripple and rectification nasties....

More RMAF Details
Last time, I mentioned several of the loudspeakers that I liked at the Rocky Mountain Audio Festival, which prompted a friend to ask why I didn't mention any new tube gear. Well, I did see and hear some interesting tube amplifiers....

With a Name Like Schiit,
You have to Sound Good

I had great time talking to Jason and Rina at the Schiit table. (Jason is a TCJer and he even used my software in designing his tube headphone amplifier.) Check out the specifications on his cool tube-based headphone amplifier (actually, it is hot to the touch and draws 40W from the wall socket)....

At the Other $$$ Extreme
I saw a lot of interesting audio products at the RMAF and when I asked the selling price, the answer was usually $20k, $30k, $40k, $50K, $60k, $70k...

New Books
The RMAF was not just about audio gear; other items were on display and for sale, such as records, CDs, and books. Finding any new audio-related books is amazing enough, but finding two exceptional new audio books a treasure find. The first is by Robert Cordell and it is titled, Designing Audio Power Amplifiers...
     30 Nov 2010

Rocky Mountain Audio Fest
I came, I listened, I chatted. The Rocky Mountain Audio Fest (RMAF) 2010 was held in Denver earlier this month. It was my first attendance and I'll be back next year. While the RMAF is certainly closer to a typical stereo show than a true festival, never approaching the truly festive or jubilant, it was nonetheless fun....

Best Loud Speakers
Expecting to hear great sound at a stereo show is as futile as expecting to find true love at a singles bar or haute cuisine at McDonalds. I know this well, yet I am always amazed at how disappointing the sound can usher from $200k worth of stereo gear. Reasons are not hard to find....

Turntables, Computers, and a Few CD Players
The last stereo show I attended was the CES show in 2005. What struck me immediately was the absence of CD Players. I know that I will never buy another CD Player, but I was surprised to find so many others making the same decision....

CanJam
Headphone aficionados had big room to themselves, wherein headphones and headphone amplifiers were the staring attractions. Unlike loudspeakers, headphones should not be venue dependent, although the problems of contaminated wall voltage and loud background noise persist. I was quite surprised to learn that, with headphone amplifiers, big watts are now desired...
     30 Oct 2010

New Output Stage Topology Variations
Of late, it seems that I have been thinking about little else other than output stage (OPS) topologies—at least that is what my sketch pad shows. OPS design can be difficult....

Coupling-Capacitor-Free OPS
I know several tube-loving audiophiles who despise coupling capacitors. I feel their anguish, as coupling capacitors can be a real pain in the amplifier. Large, expensive, and sonically befogged, coupling capacitors are certainly a liability. Many of us tube-loving audiophiles secretly envy solid-state signal-capacitor-free designs, wishing that we too could listen to our music naked, free of the sonic overlay imposed by coupling capacitors. So, is it possible to design a power amplifier that is coupling capacitor free? Yes, indeed, but it will require interstage signal transformers...

Differential Gomes Amplifier
I know that many are thinking that we have replaced one reactive component (the coupling capacitor) for an equally reactive component (the interstage transformer), so have we made nothing more than one giant step sideways? Can we not forgo coupling capacitors and interstage transformers? One easy answer would be something along the following lines, a differential Gomes amplifier...

Broskie CF Amplifier
The Broskie cathode follower receives a balanced input signal and delivers an unbalanced output. Two Broskie cathode followers placed in opposition will deliver a differential output that looks quite similar to the four output tubes at the heart of the previous design examples...

BCF Headphone Amplifier
Using the same functioning, but simplified, the following circuit accepts an unbalanced input signal and drives a relatively low-impedance (300-ohms) headphone transducer. Per channel, two 6SN7s are used, resulting in a voltage gain of +20dB into the 300-ohm load. (A pairing of 6N1P and 5687 or ECC99 tubes might prove much more effective, but four octals are more eye pleasing.) Two coupling caps are used, as centering the OPS would be difficult otherwise...
      10 Oct 2010

Plate-Driven Phase Splitter
Although a triode's grid is the usual signal input port, all three of a triode's elements, its plate, grid, and cathode can function as a signal input...

Second Last Sunday Circuit
The more I pondered this plate-driven Aikido-esque circuit, the more I wanted to simplify. If only I could apply the same V1 cathode-to-plate voltage swings to V4 without having to use an active load for V1 and the cathode follower (V3). Then it hit me: V4 was effectively in parallel with V1, so why not make actually in parallel with V1. The following circuit was my last circuit drawn in church and it places the two triodes in parallel....

Tubes at Last
I know that I have already lost many readers, as they do not feel comfortable with solid-state devices; period. But the joke is on them, as I am now going to present an all-tube version of the plate-driven buffer (Boomerang Buffer). The following power buffer uses a 6AS7-based OTL output stage....
     16 Sep 2010

Aikido All-in-One LSA/HPA Rev A
I ran out of this PCB a while ago. I then made a few improvements to the PCB, so it is now in Rev. A and in stock at the the GlassWare/Yahoo store. I tested the new board by building a tube-based headphone amplifier, this time with 6N1P and 6H30 tubes. I am running the output tubes hard and I am using a higher B+ voltage (+240Vdc) than last time (200Vdc). It sounds impressive. The bass is rock solid and it slams. I swear I can feel the bass notes in my chest. The music comes out alive and forceful. On the other hand, my octal equivalent sounds more mellow, in a radiant, glowing way, which is altogether beguiling. It isn't sloppy by any means; it's just more yang than yin. These two HPAs prove that no one HPA can ever be perfect, as different music requires different amplification....

Aikido LV PCB
The old 24V Aikido PCB was a hit. I sold many PCBs and kits and I have gotten great comments from users. (I still listen to headphones with it and it sounds amazingly good as a line-stage amplifier.) The 24V Aikido, however, faced a problem: no 6GM8/6N27P/ECC86 tubes; well, at least no moderately-priced tubes. Not too long ago, the tube was selling for $5, or less; today, they go for closer to $25 to $30; next year, $40 to $50?. Moreover, when a supply of a trendy tube grows small, two things happen: obviously, the price goes up and, all too often, the quality goes down. Diminished quality? ...

LV Aikido Power Supply
The key advantage of the LV Aikido is that it can operate under relatively low B+ voltage. Where the typical tube-based line-stage amplifier requires a B+ of 200V to 400V, the LV Aikido can get by with only 24Vdc with 6GM8/6N27P/ECC86 tubes; or 48Vdc with 12BH7 or ECC99. The heaters are all placed in series and this heater string is placed in parallel with the B+ and ground. Thus, a single power supply voltage is all that is required...

LV Aikido Power Switch
The LV Aikido can switched on in two ways. The first is to place a switch in series with the heater string (two eyelets are provided below C1 on the PCB), leaving the LV Aikido's power supply constantly on (which works well with external power supplies, such as wallwart and switch-mode power supplies)....

Why No Headphone Buffer?
I decided to leave the solid-state headphone buffer off the LV Aikido PCB for several reasons. I have mentioned one already: the extra required real-estate. Another is that many do not own high-quality headphones. Why should they pay extra for something that they are not going to use? Thus, it seem to make more sense leave the solid-state buffer off the board. Which is not to say that I have forsaken the headphone option, as the relatively low-voltage operation cries out for a headphone amplifier service. Think about how crazy it is to build a tube-based headphone amplifier that runs a 240Vdc B+ voltage and an insanely high idle current just to swing 1V into a headphone driver. Well, a 24V B+ is ten times less insane...

Balanced Power Buffers
While on the topic of buffers, I have playing with the idea of building a tube-based power buffer that could drive loudspeakers. Because it offers no gain, in fact suffer from an insertion loss, the buffer would need to be presented with a large differential input signal, say 50 to 80Vpk-pk. Fortunately, such an input signal are not difficult to create with tubes. I greatly prefer the simple over the complex, so I would like to get away with just two output tubes, probably EL34s or KT88s. In addition, it would be fun to experiment with pentode operation of the output tubes....
    16 Aug 2010

BCF History
The Broskie cathode follower (BCF) is old news, having made its first appearance back in the June 1999 issue of the TCJ, and reappearing in October of 1999 . (I actually came up with the circuit about a decade earlier, but it mostly languished in one of my notebooks.) After I published the circuit, I received laudatory e-mail mostly from electrical engineers who worked with balanced audio signals. (DIYers and audiophiles either didn't need the circuit or they didn't understand how it worked; probably, mostly the former, as I have met very few audiophiles who ran balanced systems.)...

Balanced to Unbalanced
The BCF receives a balanced input signal and converts it to an unbalanced output. It is a unity-gain buffer that offers a high input impedance, a low output impedance, low distortion, and great CMRR. In addition, because the BCF uses a push-pull topology, its use is not limited to line-stages, as the BCF can be used as a headphone buffer-amplifier if the headphone's impedance is high enough (say, 300-ohms). In addition, much like a signal transformer, the BCF offers common-mode signal rejection (CMRR); this means that BCF passes differential input signal, but largely ignores what is common to both input signals. Why is this a feature?...

To bypass or not to bypass?
To get the lowest output impedance, the BCF's cathode resistors must be bypassed, as shown in many of the above schematics. What happens to the BCF's output impedance if the resistors are unbypassed, as shown in my mixed input configuration? Normally, an unbypassed cathode resistor will greatly increase the output impedance. For example in a grounded-cathode amplifier, the effective increase in the value of rp is equal to the (mu + 1) times the value of the cathode resistor. Here the value one cathode resistor is simply added to what the output impedance would be with bypassed cathode resistors....

Re-Balancing the Input Impedances
The BCF presents mismatched input impedances, with the positive (the top) input being much higher than the negative (the bottom) input. Because the top two-resistor voltage divider terminates into the output, the resistor string's effective impedance is magnified, as the output is in phase with the positive input signal. The top resistor string's effective impedance is increased by 1/(1 gain); for example, if the BCF output gain is 0.9, then the resistor string's effective impedance will be 1/(1 0.9) or 10 times greater than its nominal resistor values would indicate...

More Triadtron
The many Triadtron circuit variations I have already posted are not exhaustive, as I have many more to post. But first, let's look into the "Triadtron" as a name. Mallory, a long-time TCJer, wrote the following:

 

Dear John,
having read your articles on the tringlotron and now the triadtron, it seems there may be a gap in the bulk of your knowledge, regarding the meaning of "tron"!

Triadtron Rail Splitter
Just about every electronic device I buy now comes with a switcher wallwart power supply. And with good cause: these devices are small and cheap and universal, working with any wall voltage between 90V to 260V. The one problem with these technological wonders is that they usually are only available with mono-polar DC outputs. Which is great if the intended load is a lamp, but if you want to power an OpAmp, a bipolar power supply would be easier to work with. What is needed is a rail-splitter circuit that will effectively create a bipolar power supply out of a mono-polar DC voltage. At its simplest, all that is needed is two resistors and two large-valued capacitors. But if better performance is needed, which it almost always is with audio equipment, the following triadtron-based circuit offers hope....

Inverted Triadtron Headphone Amplifier
The inverted triadtron configuration results in a voltage amplifier, rather than a buffer. The example I gave last time was of a hybrid amplifier. A completely solid-state version can be made using three NPN or three PNP transistors. Below is a simple headphone amplifier based on the inverted triadtron....
27 Jul 2010

Aikido Cathode Follower [ACF]
Born from the rib of the two-stage Aikido amplifier topology, the ACF is basically the last half of the Aikido amplifier, which uses a modified cathode follower circuit to provide unity gain and a low output impedance. Much as in the Aikido, this modified cathode follower scrubs away the power-supply noise from its output and provides a complementarily non-linear load for the top triode’s cathode. In addition, the Aikido CF produces lower distortion by using the triode’s own nonlinearity against itself....

Aikido Cathode Follower [ACF]
Born from the rib of the two-stage Aikido amplifier topology, the ACF is basically the last half of the Aikido amplifier, which uses a modified cathode follower circuit to provide unity gain and a low output impedance. Much as in the Aikido, this modified cathode follower scrubs away the power-supply noise from its output and provides a complementarily non-linear load for the top triode’s cathode. In addition, the Aikido CF produces lower distortion by using the triode’s own nonlinearity against itself....

No Gain, No Pain; No Hummer, No Bummer
Tube-based buffer line stages that provide no voltage gain are, surprisingly enough, rare. As far as I know, no commercially-offered, unity-gain, tube-based line-stage buffer exists. This is an odd situation, as passive line-stages are popular, which proves that extra signal gain isn’t always required. Additionally, the passive line- stage does not require plugging into the wall and it adds no extra active devices into signal path. It is the purest of the pure. Yet passive line stages often prove inadequate, incapable of adequately driving high- capacitance cables or low-input impedances. Moreover, most active line-stage amplifiers can often impart the missing heft and solidity that are missing in many passive setups, even when the load is wimpy, but at the cost of greatly increased complexity and cost—and with some added noise and distortion....

More Triadtron
The Triadtron circuit exploits a feature of transistors: a well-defined and abrupt base-to-emitter voltage. Unlike triodes and pentodes, transistors exhibit a sharp turn-on voltage, as shown in the graph below....

I-to-V Triadtron
Moving in another direction, we can create a current-to-voltage converter out of a Triadtron by placing a resistor in series with the two output transistors....

Inverted Triadtron
Speaking of hybrid Triadtron circuits, we can use the same Triadtron Vbe reflecting trick to create a voltage amplifier, rather than a unity-gain buffer....
    30 Jun 2010

Triadtron
Since writing about the Tringlotron circuit, I have had an itch to create my own "_t ron" topology. The first step was to pick a name, a name that implied three, as I wanted to limit myself to three active devices, just as the Tringlotron does. "Tritron" was a tube brand from the 1930s, so it would not do. After much head scratching, I chose "Triadtron." Surely "Triadtron" is such a cool name that it demands an equally cool topology. Now all I had to do was come up with a new topology. Mind you, this is not how it is supposed to work, you don't come up with a name and then create a new topology to match it. But, of course, that is precisely how much of the stuff that fills our modern life is created, from movies to kitchen appliances; just ask any marketing department....

Diamond-Buffer Topology
This is an old topology, at least 40 years old. And because it is a good topology, it persists. It is DC coupled throughout and part parsimonious....

The Triadtron at Last
Having examined the Tringlotron and holding asymmetry so dear, I knew that I wanted a a simple buffer that would not conform to the diamond template. My first try was the following circuit. This circuit provided low distortion and an SE-flavored harmonic structure....
    21 May 2010

PS-3: High-Voltage
Power Supply & Heater Regulator

High voltage power supplies are a pain. One thing that I love about OpAmp circuits is being able to hook up two 9-volt batteries and run, with just a single bypass capacitor across the positive and negative power supply pins—no lethal voltages and no heater power supply to worry about. Low-voltage power supplies are a breeze....

Tringlotron Once Again
A friend asked me how to pronounce "Tringlotron," but I have no exact answer. Maybe its something like, trĭng-low-tron or trĭn-glow-tron or trīne-glow-tron. Although it is the least likely, I do like the last possibility best, as the word "trine" means a group of three (in astrology, it means a favorable positioning of two celestial bodies 120° apart) and I like things that glow, as they say, if it doesn't glow, it doesn't go. In addition, I forgot to add the Syclotron to my list of "tron" suffixed electronic words; sorry Stewart...

Tringlotron Deconstructed
A fancy word is "deconstructed," probably too fancy. I have made fun of deconstructionism in these pages before, so I will not belabor my disdain for the destructionist's efforts. (Perhaps I should have titled this section "Tringlotron Disassembled," as I wish to dismantle, break into sub-sections the Tringlotron topology. Maybe my misuse of the word “deconstruction” will help obcure its literary-theory meaning, thereby promoting the technical and methodological “metaphor” that should necessarily attach to the word “deconstruction.”) Why do I feel the need to dismantle, break into sub-sections the Tringlotron topology? As I wrote my explanation of the Tringlotron in my last blog entry, I knew that it would provoke much head scratching....

Rk-Based Tringlotron
In the blog number 185, I listed the need for a constant-current source as a liability for the tube-based Tringlotron circuit, as it adds extra complexity and cost. But does the Tringlotron actually need a constant-current source load at its bottommost triode's cathode? I ran some SPICE simulations with the constant-current source replaced with a simple resistor.

Variations on the Tringlotron
The Tringlotron admits modification beyond replacing the constant-current source with a cathode resistor. For example, the topmost triode can be replaced with a pentode or transistor or MOSFET or FET or IGBT or…. In the schematic below, we see a MOSFET in place. Note the B+ voltage, which is substantially lower than in the pure-tube version....

More on the Broskie Buffer
I ran out of time when writing my last blog entry, as I had much more to cover on the circuit I called the Broskie Buffer. One disadvantage to this circuit that I mentioned was the relatively low input impedance (10k with a 6DJ8). The easy workaround is to drive the buffer's input with a cathode follower....

Letters to the Editor
Back in days when I tried to produce a true monthly Tube CAD Journal webzine, I regualarly posted e-mails that I received from TCJ readers. Unfortunately, this created a quite a bit of stress, as many did not want their trade secrets revealed and other readers wanted me to give them the letter writer's e-mail address. Now, my life has been made a bit easier by not posting reader e-mail, but many wonderful e-mails that other readers would love to read languish in my e-mail inbox. The following e-mails from Christian and Louis have been cleared by then and they both raise several important points....
    23 Apr 2010

Tringlotron: An Interesting New Topology
After a bit of web searching, I found that the Tringlotron topology is reported to have been created by a Ludwig Von Bürnmoll and that this topology is related to the Biglotron, of which I know nothing and no amount of Googling yielded any results. The word "Tringlotron" is a contraction of TRIplet of NPNs Grouped in Line. I first read about this topology in April 8th issue of Electronic Design magazine in an article titled, "Novel Buffer Topology Cancels Nonlinearities," by Louis Vlemincq. The circuit shown in the article used three transistors to create a super-fine unity-gain buffer, but as Mr. Vlemincq rightly points out that the topology can be applied to many other device types, even tubes....
    19 Apr 2010

The Ultimate Dynaco ST-70 Mod:
Remote-Sensing Feedback

To be frank, I didn't expect much response to my mentioning of remote-sensing negative feedback, but I did expect the few e-mails it would garner to be extremely interesting and well-informed. I was right on both counts. In general, negative feedback, because the topic is either too complicated or too gauche, seldom provokes much attention. Remote-sensing negative feedback techniques only add to the complexity, but they also subtract from the uncouthness of negative feedback. A paradox of sorts. Two questions were paramount: Is this technique limited to balanced systems, holding both balanced signal sources and power amplifiers? In other words, do we have to have a balanced signal source? The second question: Can this technique be applied to tube-based and solid-state OTL power amplifiers? In other words, is the output transformer necessary to making this technique work?...

Ultra-Path Amplifier with
Remote-Sensing Feedback

This amplifier's name is a bit of a joke, but the underlining design is sound. This circuit was covered in blog number 147, but here is a quick recap. The cascode input stage both provides a lot of gain and offers almost no PSRR at its output, both of which are highly desirable features in this amplifier. The high gain will power our negative feedback loop and the poor PSRR will decouple the audio signal from the ground, superimposing it on the B+ rail's noise...

Added Zo Trick
A few years after my experiments with remote-sensing negative feedback, a brilliant EE friend of mine told me that he had discovered why tube amplifiers sounded so much better than solid-state amplifiers: high output impedance; most speakers are over-damped, needing to be loosened up by a high Zo...

Remote-Sensing Feedback on the Cheap:
The Single-Remote-Wire Approach

I know readers will be troubled by the four connections required by remote-sensing feedback, as they will not see in value in adding the second remote-sensing connection to the loudspeaker's negative input terminal. In other words, why can't we just use three wires, two for delivering the power into the loudspeaker and one for remote negative feedback sampling?...

Balanced Remote-Sensing Negative Feedback
Can this technique be applied to balanced power amplifiers? Indeed, yes. First, let's look at a simple balanced power amplifier....
  28 Mar 2010

PS-4 Tube Power Supply
On this small four by four inch, extra thick (0.094), 2oz-copper traces, USA-made PCB resides both a simple high-voltage power supply and a low-voltage power supply and low-voltage regulator, with each finding its own raw power supply, including the rectifiers and power-supply reservoir capacitors. The low-voltage regulator is meant to power the tube heaters; the high-voltage power supply, the rest of the tube circuit...

EL84/6BQ5 Push-Pull Power Amplifier
Few tube amplifiers sound as sweet as two triode-connected EL84s in push-pull. These amazing little power tubes are quite efficient, requiring only 15Vpp to bring them to full output. Thus, with just a single input/phase-splitter tube, such as a 6N1P or 12AT7, we build a fine push-pull amplifier...

EL34/6CA7 Push-Pull Power Amplifier
In fact a bigger tube power amplifier could use a PS-4 power supply, if the output tubes ran their heaters on AC, while the input and driver tubes ran their heaters on regulated DC. I can hear the Huh? That many are thinking after reading that last sentence. Let's say that you build a push-pull tube power amplifier that uses a total of four EL34s and four 6SN7s....

Remote Sensing (was Remote Output Trans.)
Back in 1994, John Atwood wrote about this design in the rec.tubes user group...
Mar 22 2010

Balance
Balanced audio confuses many—and would confuse many more, if they knew it existed. For those who are acquainted with balanced audio what immediately pops into mind are XLR connectors and professional audio equipment. Then soon follows the varied rumors, such as "Although balanced audio is used extensively in making LPs and CDs, balanced audio never sounds good in home systems" and "Balanced systems are the only possible way to get low-noise, high-quality sound and the cheapest, most-humble balanced cable sounds vastly better than the best, most-expensive unbalanced cable." I have heard both of these rumors and many others in between and a few crazy ones, such as "Balanced systems will strip away even-order distortion from a music signal, leaving only odd-order harmonics in place." Indeed, balanced audio confuses many...

Unbalanced
Sad to say, but home audio systems seldom embrace the logic and beauty of balanced signal transfer. Instead, most home audio equipment is solely configured for unbalanced signal transfer. Why? Tradition and stinginess and electronic myopia—all play a part. The evil RCA jack has burdened us for the last 70 years, forcing us to make needless compromises. The RCA plug could just as easily have held two prongs and an encompassing ground cover, something like a two-prong DIN connector; such a design would have allowed for two-wire balanced or two-wire unbalanced signal transfer. Instead, we have been straddled with a clunky connector that indecorously exposes its hot prong and makes a feeble connection with its mating jack, whereas you can tow your car with a few XLR-connected cables....

Balanced-Input Aikido
I have added a 1:1 input transformer to my own Aikido line-stage amplifier. This transformer is a fairly high impedance affair that likes to be terminated by a 15k resistance. It's made by CineMag, model CMLI-15/15B..
    11 Mar 2010

Octal All-in-One LSA/HPA
I have received many requests for an octal All-in-One LSA/HPA PCB like the 9-pin version that weds a PS-1 solid-state, high- and low-voltage regulator to a 9-pin Aikido stage on one PCB. But I decided that since octal tubes seem more retro than 9-pin tubes and the Janus regulator is a pure-tube design that even uses a tube rectifier, why not wed a Janus regulator to the Aikido stage instead?...

Construction Details
I decided to try something different this time. Rather than place all the parts on the top of the PCB, I mounted only the tube sockets on the top of the PCB and all the other parts on the bottom. The tube sockets and standoffs are all that attaches to the top of the PCB. All the other parts attach to the bottom....

11-Position Series Stepped Attenuator
Did you notice the 11-pos series attenuator I built? It is made up from 22 carbon-film resistors (another nod to retro-coolness) and an 11-posistion, 2-pole Elma hard-gold-contact rotary switch. This volume control holds two 11-position series attenuators. It works very well indeed, even with its coarse -6dB steps, as I use the headphone amplifier with either my Zune HD or my Squeezebox Duet, both of which allow for fine volume adjustments, which, surprisingly, are rarely needed....

How Does This
12SX7-Based Headphone Amplifier Sound?

At first it didn’t sound at all. I wanted to use all NOS RCA 12SX7s, but I had a bad RCA 12SX7 cause huge problems, so it runs with two GE and two RCA 12SX7s. The output stage is configured as a White cathode follower that runs hot, with a B+ of about 280Vdc and a high idle current of 11mA. The coupling capacitors are 30µF Dayton 250V polypropylene capacitors. I am using an RCA 5R4GB rectifier (remember, I was trying to make an all-RCA headphone amplifier). So once again, how does the headphone amplifier sound? The headphone amplifier sounds quite good: extremely civilized and smooth....

Octal All-in-One LSA/HPA
as Line-Stage Amplifier

Having just gone through all the work of building the 12SX7-based headphone amplifier and having just written all I've written, I must confess, that the Octal All-in-One LSA/HPA would work much, much better as a line-stage amplifier. The 12SX7 is a wonderful tube, but like any decades-old NOS tube, you can encounter functional problems; nor are they cheap, and this tube is much happier with loads greater than 300 ohms. Besides, the new Russian Tungsol 6SN7s and NOS 12SN7s sound quite good and the PCB does offer enough room for line-stage amplifier coupling capacitors (the big Daytons had to be squeezed onto the PCB). Moreover, while the 12SX7 may not work as well as the output tube in a headphone amplifier, it certainly sings in a line-stage amplifier....
    14 Feb 2010

Tube Clock
Tube clock? Is it a piece of artwork, worthy of window display in exclusive art galleries? Or is it the long-awaited analog replacement to the famed Tice digital clock? Or is it a chronometer of exceptional precision? Or is a timepiece of uncanny beauty, suitable for adorning walls of Beverly Hills mansions and Manhattan penthouses? Or is it something like a badge of affiliation to an almost cult-like adoration of vacuum tubes? Well, it’s all of the above. It is also my latest kit offering....

Ultra-Linear Line-Stage Amplifiers
Back in late December, an avid TCJer wrote to me asking about ultra-linear line-stage amplifiers, as he owns a huge collection of interesting 9-pin and octal pentodes (and pentode-triode dissimilar tubes, such as the 6AN8, 6AU8, 6BH8, and 6BM8) that are just sitting idle in his garage. He had discovered some of my past posts on this topic, such as the ultra-linear Aikido, which I covered in blog 69. He didn’t need a lot of gain and he wanted to experiment with different ultra-linear ratios, so I recommended the following circuit as a useful test bench and starting point....

Ultra-Linear Cascode
The cascode circuit was created to squeeze pentode-like gain from triodes, while still retaining the triode’s low noise. Adding an ultra-linear flavor the cascode is easy, as the top triode’s grid functions much like a pentode’s screen. Back in the early 80s, I came up with the following circuit on my own...

Food for Thought
We know that if the screen in a pentode sees a steady fixed DC voltage, the pentode will functions as a pentode, with high gain and poor PSRR; we also know that if the screen sees 100% of the plate’s output signal, the pentode will function like a triode, with lower gain, distortion, and improved PSRR; moreover, we know that we can feed the screen some ratio of plate signal in an ultra-linear fashion. So at one extreme, we have pure pentode; at other, pure triode; and in between, ultra-linear. Now, what would happen if we fed the screen more than 100% of the plate output signal, say 130%? In other words, what would happen if we amplified the plate output signal, preserving its phase, but increasing its amplitude; and then we fed this bigger signal to the pentode’s screen? Super-ultra-mega-triode operation?...
     10 Feb 2010

The Swinish H1N1 Flu
In the last half of December, I fell ill with what I can only surmise was the H1N1 flu. It was brutal—most certainly the worst flu I have ever suffered. I ached in places that I didn't know I could ache; I burned with fever; I coughed and hacked constantly; I grew weak, surprisingly, unbelievably weak; I could not stomach the taste of most foods, with the blandest crackers seemingly made with corrosive salts; and, saddest of all, I lost two weeks of productivity in 2009. Adding to the torment, my wife and daughter also contracted the flu. (My son, who has an immune system made of titanium, fell ill and recovered in less than eight hours.) Thus, I only made one post in December and I fell far behind in shipping orders and in answering e-mail. A thousand apologies.

SuperTriodes
Blog 178's topic of supertriodes provoked much interest, which surprised me, but really shouldn't have, as the potential supercharging of a small triode is an obviously compelling goal. Power is satisfying; it’s like money in the bank, good looks, strength, charm, and wit—more is always welcome. Triodes often frustrate us by providing a taste of sonic glory, but only a small taste. A 12B4-based single-ended amplifier beguiles our ears with its superb resolution and amazing dynamics, but then disenchants us, as we raise the volume and the little amplifier thwarts our desire to reproduce a huge orchestra in our living room. Ultimately, we want to keep the 12B4’s delicious delicacy and add the heroic heft and unperturbed might of a 400W solid-state power amplifier....

SuperTriode Meets the Aikido
Building a SuperTriode circuit with power MOSFETs seems such an obvious and natural choice, but power MOSFETs have their own issues to overcome. For example, many power MOSFETs require a relatively huge turn-on gate voltage, say 5Vdc, which will make direct cathode-to-gate coupling difficult. Second, the turn-on voltage can vary quite a bit from one sample to another. And third, at low currents, most power MOSFETs only realize a small portion of the transconductance that manifest at high currents. Transistors, on the other hand, require less than 0.8V applied to their bases in order to begin conducting in earnest. The turn-on voltage is marvelously consistent between individual transistor types. And transistors develop relatively huge transconductance figures, even at low currents. Finally, transistors are much cheaper than MOSFETs....

Electrostatic Speakers
I have been thinking about ES speakers lately, but not the conventional ES speakers that you can buy in audio salons. But before jumping ahead, let’s do a quick review of how a conventional ES speaker is assembled and works....

Inverted Electrostatic Loudspeakers
No doubt, if you have read my blog entries even casually, you know that I am much tickled by inverted topologies and radically altered circuits and electronic approaches. Well, an alternative approach to building electrostatic loudspeakers exists. The justly famous David T. N. Williamson and Peter J. Walker were awarded US patent No. 3,008,014 in 1957. Their patent basically covered the idea of running an electrostatic loudspeaker in constant-charge mode, by means of the large-valued series resistor between the high voltage polarizing power supply and the diaphragm. In figure 1 of their patent, we see the inverted electrostatic loudspeaker configuration...
   06 Jan 2010

Super Triodes
Triodes rule. Triodes stand out, much like a Roll Royce in a parking lot otherwise filled with GM clunkers. What makes the triode stand out in audio terms is its inherent linearity, its intrinsic good manners and self restraint. Self restraint? “What are you adding to your eggnog?” many are thinking. The self restraint I am speaking about is the triode’s relatively low rp, plate resistance. MOSFETs, transistors, JFETs, and pentodes all offer transconductance, the ability to efficiently control current conduction through an electronic device, but only the triode presents a low parallel resistance. Yes, MOSFETs, transistors, JFETs, and pentodes also present drain, collector, and anode resistance, but far too much of it...

P-channel MOSFETs & SuperTriodes
By the way, we could eliminate the cathode resistor by using a P-channel MOSFET instead. In the schematic below, we see a BUZ906 attaching at the triode’s plate resistor. This configuration works in much the same way that the N-channel-MOSFET-based version does; the triode governs the MOSFET’s current conduction. As the triode’s current conductance increases and decreases, so too the MOSFET’s. If the triode ceases to conduct, so does the MOSFET. The only real problem with using a P-channel MOSFET is their low 500V upper limit on maximum drain-to-source voltage...

OTL Output Stage & SuperTriodes
Building a Futterman-style OTL power amplifier with SuperTriodes is certainly possible. And truly staggering power output could be possible with a 300Vdc B+ voltage, but I would prefer not to resort to parallel MOSFET devices, as I fear losing the clarity provided by one or two output devices. Thus, in order to remain within the maximum dissipation limits for a 125W power MOSFET, a much lower B+ voltage will be require, say +60Vdc, which should allow about 40W of power output into 8-ohm loads....

The SuperTriode & GainClone
Speaking of implicit feedback loops, since I have started down this path, I should point out that a SuperTriode can be made out many other solid-state devices, such JFETs and transistors. In fact, one can be assembled from a triode and a complete solid-state OpAmp or power amplifier....
     06 DEC 2009

Class-AC
A. J. van Doorn’s new topology and the impedance-multiplier circuits helped to remind me of my own efforts in taming wild output stages. One such effort gave birth to what I label class-AC mode. First of all, I should point out that Douglass Self has labeled “class-AC” Class-G amplifiers that run under a high, class-A-rich idle current. This is not what I mean by class-AC. Mr. Self's aim is a low-dissipation class-A amplifier, whereas my goal is a non-gm-doubling output stage that runs primarily in a rich class-AB. My approach places two output stages in parallel, a richly biased class-AB and a true class-C stage. (So, maybe a better name might be class-ABC or class-A-to-C.)...

Class-AC with Only Two Output Devices
Another thought I had was: What if we use just two transistors in the output stage, but still maintain the class-AC functioning? In the above schematic, we see two emitter resistors per output device. At idle and at low signal levels, the super-fast rectifiers never conduct, so the two resistances add together, effectively creating a 0.76-ohm emitter resistor. But once the output stage leaves class-A operation, the rectifiers kick in, thereby bypassing the 0.47-ohm emitter resistors, which effectively doubles the sole active transistor’s transconductance....

Class-AC & Tubes
Returning to Douglas Self's class-AC, wherein he has configured a low-voltage class-A power amplifier in series with high-voltage class-C power amplifiers. In contrast, I have placed a high-voltage class-A power amplifier in parallel with a high-voltage class-C amplifier. My approach dissipates more heat, but is much less complex; his runs cooler, but requires many more parts. Furthermore, the principle behind my approach applies to all output devices, tube, transistor, or MOSFET....

Class-AC Impedance Multiplier Circuit
Finally, we arrive at where I wanted to end up: the class-AC IMC. Remember that all the previous impedance multiplier circuits I have shown have run in true, honest-to-God, no-marketing-hype, class-A mode. Class-A is wonderful in terms of sonics, but is is also massive, expensive, and hot. Creating a slightly leaner, cooler IMC requires something like my Class-AC configuration.

Class-B Auto-Bias
Pierre Corbeil's super clever design overcomes one of the huge problems with class-B amplifier design: getting the bias right—over time and over device heating... Too little bias and the amplifier falls into class-C and high distortion; too much bias, the amplifier overheats, possibly blowing a fuse. Many complex and finicky circuits have been devised to overcome the class-B transistor amplifier’s bias problems. Mr. Corbeil has come up with a clever auto-bias scheme that automatically adjusts the idle current through the output stage devices...
    15 Nov 2009

A. J. van Doorn's New Amplifier: the Octode
Buy the November 2009 issue of audioXpress today; well, tomorrow, if you must.... Mr. van Doorn’s goal is to wed the triode’s linearity and low output impedance to the pentode’s big power delivery. The triode cannot compete with the pentode in the power race, because the triode gives up half of the potential voltage and current swings that a comparable pentode can fully exploit.

Tetra Phono Preamp
I have complied a list of matching plate and cathode resistors for use with common tubes in the Tera's CCDA stages....
     06 Nov 2009

Introducing the Tetra Phono Preamp
Somewhat along the lines of the last blog entry’s walk down Nostalgia Lane, I must admit that I find the LP’s allure powerful.... Combining the Aikido-All-in-One power supplies with the CCDA topology allows an excellent tube phono stage to be made easily and inexpensively. The Tetra uses only four tubes, not the eight tubes that the Aikido phono stage requires. And it holds two power supplies, one for the high-voltage B+ and a low-voltage regulated power supply for the heaters. The high-voltage power supply is unregulated, but uses cascading RC filters to smooth away ripple...

CDA Topology
The CCDA’s low distortion, fairly high gain, and low output impedance—all are desirable attributes in a phono preamp. The Constant-Current-Draw Amplifier is a compound circuit that holds a grounded-cathode amplifier directly cascaded into a cathode follower. So what; what's so special about this obvious pairing? Its special status lies in the details. Each triode sees the same cathode to plate voltage and the same load resistance and same idle current draw. Both the grounded-cathode amplifier and the cathode follower are in voltage phase, but in anti-current phase....

Broskie Impedance Multiplier
I have come up with an interesting tube-based IMC, which I shamelessly have named the Broskie Impedance-Multiplier. The circuit offers push-pull impedance multiplying, with top and bottom triodes responding to the AC current signal presented at its input...
     23 Oct 2009

The Good Old Days
Born in 1956, I cannot be expected to remember much of the 1950s proper. As a child in the 60s, I disliked intensely the residue of the '50s I could see around me —the cars, music, fashions, and black-and-white movies. I was smitten with all things new, the bright colors, day-glo posters, bold drug-inspired patterns, color TV, and electronic rock ‘n roll music of the '60s. Back then, the '50s reminded me of Goodwill stores and the homes of old people....

Stereo Cabinets
What has prompted my most recent pangs of nostalgia was seeing a beautiful stereo cabinet at a garage sale last weekend. You know the look, a large, wide piece of furniture made from quality wood and decorative grill cloth and ornate trim work, standing on four or six spindly feet. These all-in-one music centers usually held a record player, radio, power amplifiers, and loudspeakers. No interconnect, no fat speaker cables, no power strips, no hassle...

Even More Impedance-Multiplier Circuitry
Because the topic of impedance-multiplier circuits is virtually unknown to most solder slingers, I feel compelled to chart out more unmapped territory. In blogs past, I showed how a cathode follower could work into an impedance-multiplier circuit and how several triodes could work into a single impedance-multiplier circuit, thereby greatly increasing the potential current swing ino the load...

Solid-State SRPP+ Output Stages
The SRPP twist has been applied to solid-state devices for almost half a century, so I expect the technique to be invented soon and used in some $10,000 high-end power amplifiers....

Zen² MOSFET Power Amplifier
I leave for last that which most wanted to see first, as that’s what my e-mail is telling me. In blog 172, we saw a Zen-looking amplifier that used an impedance-multiplier circuit to undo the Zen’s single-ended amplification. Sevral readers wanted to know how this amplifier differed from the Zen, as they look so similar. A true Zen amplifier is a single-ended affair that uses a single MOSFET working into a constant-current source to deliver up to the idle current into an external load...
    18 Oct 2009

SRPP+ Part Three
The key point I hope that I have made is that the SRPP holds two sub-circuits and that one of the two, the impedance-multiplier circuit, is quite interesting, if no other reason that so few knew it was there in the first place. The impedance-multiplier circuit, as topology, as a basic-circuit function, is little known. If anyone has mentioned it in the audio press, audioXpress et al, I haven’t seen it. A search of www.diyaudio.com for “impedance multiplier” yielded only five results, four of which dealt with a transistor’s current gain and one referred to my blog number 172. On the other hand, searching through patent applications does deliver many authentic hits, although few of the patented impedance-multiplier circuits concern audio...

The Mu Follower
The SRPP is about a simple as a compound tube circuit can get, holding only two triodes and only two key resistors, which goes a long, long, long way to explaining its immense popularity. But as we have seen, simple is not always easy to understand. Used as mini power amplifiers of a sort, amplifier that can deliver big current and voltage swings into a low-impedance load, the SRPP and SRPP+ excel, as both circuits can deliver up to twice the idle current into a load, if designed correctly and offer a great PSRR figure (better than -20dB with a 300-ohm in this design example and much better with a 32-ohm load)...
   06 Oct 2009

First SRPP, Then SRPP+
In Part one I promised that a deeper look into the SRPP would be forthcoming. In a topological nutshell, in the SRPP circuit, one triode stands atop another, with a resistor, Rak, spanning the bottom tube’s plate and the top tube’s cathode. Usually Rak equals the bottom triode’s cathode resistor, Rk. The circuit’s output is taken at the top tube’s cathode....

The SRPP’s Impedance-Multiplier Circuit
I said in the last post that the SRPP circuit consisted of two primary circuits, not one; that it was a compound circuit that held a grounded-cathode amplifier and a impedance-multiplier circuit. Well, let’s pull the impedance-multiplier circuit out of the SRPP and examine it in isolation. The first step is to get rid of the bottom tube, replacing it with a single resistor, Rk....

SRPP+ Versus Plain-Jane SRPP
The SRPP+ differed from the generic SRPP by using two resistors in place of the SRPP’s single Rak resistor. Thus, we should incorporate the improvement into the inductor-loaded impedance-multiplier circuit....

Solid-State
Impedance-Multiplier Circuit Variations

The following circuits make use of three-pin, adjustable voltage regulators. These little wonders contain a power transistor, a voltage reference, and an OpAmp. The OpAmp sees a 1.25V voltage reference in series with positive input, which allows the regulators to be used in many different configurations, from adjustable voltage regulators to constant-current sources to AC-clipper circuits....

Zen Meets the Impedance-Multiplier Circuit
The following circuits may look like one of Nelson Pass’s Zen amplifiers, but they all differ in one major fundamental way: they push and pull, whereas the Zen amplifier only pulls: What is the sound of one MOSFET output device? The following circuit is a current-output amplifier or a power voltage-to-current converter circuit, depending on your perspective. In any case, the output impedance is about 1k and only because of the 1k terminating resistor at the output. How does this amplifier work?...
     26 Sep 2009

SRPP
Most classical music fans have heard of the curse of the ninth. The curse, which started with Beethoven and ended with Shostakovich, would claim the life of any composer who wrote a ninth symphony, leaving them dead soon after its completion. Gustav Mahler, in a sneaky attempt to cheat death, titled his actual ninth symphony Das Lied von der Erde, but nonetheless died soon after writing his official ninth symphony. Well, I am greatly relieved that there is no curse on writing about the same circuit topology nine times. For it there were, I would be in trouble, as I have written on the famous SRPP circuit at least ten times so far...

The Secret Behind the SRPP
In spite of its immense popularity and few circuit elements, few understand how the SRPP circuit works. The first step is to discern just what the SRPP’s function is. For example, is it a unity-gain buffer? A voltage amplifier? A phase splitter? An argument could be made for all three answers, but none would prove completely satisfying. Yes, the SRPP appears to encompass a cathode follower of sorts, making the unity-gain buffer answer seem at least partially right. And it does provide gain, making the voltage amplifier answer partially right. And it is capable of swinging positive and negative current swings into a load in excess of its idle current, making push-pull operation and, thus, making a portion of the phase splitter answer seem reasonable. So what is its primary function?...

Higher-Ratio Impedance-Multiplier Circuits
So far, only impedance-multiplier circuits that doubled the load impedance have been covered. Now we move on to greater impedance-multiplication ratios. What happens if R2 is greater than R1 in the following circuit?...

Power-Booster Amplifiers
The idea behind the power-booster amplifiers is that a small, flea-power amplifier and a huge, heavy, power-booster amplifier are wed and the result is the ability to greatly magnify the wimpy amplifier’s output into hundreds of watts. The commercial example was the Musical Fidelity 550K power-booster, which presented a 50-ohm load to the small power amplifier. I have no idea what the 550K schematic looks like, but I am sure that it does not represent a true impedance-multiplier circuit. This is not a failing, just an observation. Now imagine a big impedance-multiplier circuit capable of delivering 500 watts into a loudspeaker and imagine a tube OTL power amplifier capable of delivering 1Apk with 100Vpk voltage swings. Such an OTL would require a 100-ohm load, which the impedance-multiplier circuit could create out of an 8-ohm load. In other words, the tube amplifier would deliver 1A and the solid-state impedance-multiplier circuit would provide 11A into the 8-ohm load. This could prove quite interesting, as 12A equals 576W into 8-ohm loads.
   22 September 2009

Logitech Duet Power Supply Upgrade
We could fairly easily build a small box that would hold a passive filter, made with air-core (or common-mode) inductors and film or ceramic capacitors; such a box would be placed in series with the Duet and its switcher wallwart. This might work quite well, depending on how much noise and what frequency the noise contained. Heck, if placed in an expensive enough, heavy enough, pretentious enough box, such a scrubber box could sell for more than the Duet itself...

Cascaded Regulators
The two big problems with passive solutions are cost and bulk. Improving a Plain-Jane linear regulator by active means is certainly cheaper, but possibly much more complex. The easy active approach is to cascade regulators, so the final regulator sees a fairly clean input voltage, greatly unburdening it...

Low-Voltage Feedforward Shunt Regulators
Finally, I arrive. The idea here is to use a feedforward-shunt regulator to scrub away the rectifier ripple and a series regulator to offer a steady, fixed, and clean output voltage. The feedforward-shunt regulator has been covered here many times before, albeit in a vacuum tube format. In a simple solid-state variation, the following circuit uses an adjustable voltage regulator in a feedforward-shunt regulator configuration....

Cash for Clunkers
I cannot expect every reader to have heard about the US government’s bold, new initiative: CARS, the Car Allowance Rebate System, aka "Cash for Clunkers." The plan was simple: trade in your old car for a new car and the government (the taxpayers) will write a check for $4500 to the new car dealer on your behalf. Obviously, only cars worth less than $4500 would be traded in; and government restrictions apply, for example the new car must get better milage than the old car. Amazingly, there was no Buy-American clause and, in fact, most cars (80%) bought were Japanese brands. Since old cars can be a pain, polluting, unsafe, and downright ugly, who could complain?....

New Government
Program to Buy Tube Amplifiers?

After I watched the famous Demi Moore and Ashton Kutcher “I Pledge" video on YouTube, I sat devastated. As soon as the nausea passed, no doubt due to deeply hidden guilt, I cursed myself for not being able to offer up a big enough sacrifice, I mean, service to Him. I am not in Oprah’s rolodex. Demi and Ashton do not know that I exist. I don't have a single tattoo. How could I possibly hope to compete with these worthies? I was crestfallen....
    06 Sep 2009

Asymmetrical Amplifiers
One reader asked if the solid-state asymmetrical amplifier discussed in blog 168 could be reconfigured to forgo the global negative-feedback loops, as the idea of the OpAmp controlling the power MOSFETs didn’t sit well with him. I can see his point, but probably for different reasons. My big worry is that the solid-state asymmetrical amplifier will clip harshly, like most solid-state power amplifiers. So if I were to give this design some more thinking time, I would strive to find a way to incorporate a soft-clipping circuit, such as the one I offered in blog number 66...

Rethinking the Hybrid Asymmetrical Amplifier
I ended the last blog entry with a hybrid asymmetrical amplifier that used a single twin-triode tube, such as a 6DJ8/6922 or 6N1P, to drive two MOSFET output devices. The idea was to lose the OpAmps and add some tube magic to the mix. Well, a quick run through SPICE revealed that the circuit would take a bit more tweaking than I expected. For example, the bottom output device never sees anything close to unity gain from its connection to the bottom triode's cathode. So, after a little part value shifting, the following worked out fairly well....
      15 Aug 2009

Look Ma, No Heater Power Supply
Although a regulated heater power supply would seem an unquestionably good idea, not everyone agrees. Some simply prefer AC on the heater elements, arguing that the AC heater voltage lends a mellow tone and retro sound that is much desired. I am not about to quarrel with fashion and taste preferences, but I am willing to argue that many have not heard DC on the heaters in the purest sense. For example, I have often employed motorcycle batteries to heat the heaters in test circuits that I have thrown together. Believe me—the battery truly improves the sound. Taking the heater’s transformer’s winding and rectifying it, however, is not the same. Why not? Using a big battery unloads the power transformer, whereas rectifying a winding only further burdens the power transformer; in fact, even more than just running AC into the heaters....

Solid-State Email
Unity-gain power buffers are the hot item today, judging by my email. This interest is understandable, as a power unity-gain buffer can easily be mated to a vacuum tube line-stage amplifier, thereby creating a simple hybrid power amplifier. Ideally, no negative feedback loop would be required, but one could be added without too much extra labor....

Solid-State Transmogrification
So, how do we go about turning the tube Gomes amplifier into a solid-state equivalent? We cannot just replace the triodes with transistors or MOSFETs, as they will not bias up correctly and are certain to provide far too much voltage gain in the input stage. What we do want is for the top output device to work as a unity-gain follower and the bottom device to share equally the current swings, but offer little output impedance on its own. Ideally, the bottom output device should work as voltage-to-current converter, delivering a specified current swing into the load, much like Nelson Pass’s 1st Watt amplifier, while the top device delivers a specified voltage swing into the load, much like all standard voltage amplifiers....
     10 Aug 2009

Split-Load Phase Splitter & Zo
In the last two issues (June and July of 2009) of audioXpress magazine letter section, in response to an article by Stewart Yaniger, we find a raging debate on the split-load phase splitter’s output impedance. Is the output impedance low, high or dissimilar? Before I give my answer, I must mention that I like the split-load phase splitter a great deal. I believe that it provides the best balance of any phase splitter and that its dissimilar PSRR figures can prove useful in many applications. Not everyone is so impressed....

Birds of a Feather Must Flock Together
The split-load phase splitter works best when presented with an equal, balanced load, as equal loading results in equal output levels and frequency responses. Starting with the simple case of pure resistance, if each output sees the same load resistance, the split-load phase splitter will deliver identical voltage swings and frequency response from both its outputs. This makes sense, as each load resistance is effectively in parallel with partner plate or cathode resistors; for example, plate and cathode resistors equal to 20k and a balanced load of two 20k resistance effectively creates a plate and cathode resistor equal to 10k. Now, what is more impressive is equal reactive loading...

Zo and Split-Load Phase Splitters
It’s now time to answer the following question: Is the output impedance low, high or dissimilar? We have to proceed carefully here, as it’s easy to lose our way. There are two ways of approaching output impedance: the first is by way of voltage sources; the second, current sources. A power amplifier approximates a voltage source in that it should present an infinitely-low output impedance and an infinitely-high output current delivery. Solid-state, high-feedback power amplifiers come close. If we drive an circuit’s output with a perfect voltage source, the voltage swings forced onto the output will induce a varying current through the connection between voltage source and output driven. By placing an ammeter in series with the power amplifier and the output driven, we can read the current swings induced directly. For example, forcing an output to swing 1V peaks, will induce peak current swings equal V/Zo. Assuming a 10-ohm output impedance, we would see 100mA of current swing between voltage source and output...

Split-Load Phase Splitter’s Achilles Heel
I must point out a potential problem when using a split-load phase splitter in a tube power amplifier. As we have seen, dissimilar loads are a no-no, when using the split-load phase splitter....
     20 Jul 2009

Differential Cross-Coupled Circuits
We have touched on a few cross-coupled circuits in previous posts; the Blumlein "garter" circuit is one example, albeit a primarily DC cross-coupled circuit. The XPP amplifier also holds an “X” at its core. And the OTL computer amplifier from blog 19 back in 2004 held a cross-coupled arrangement. But what good is cross-coupling? Cross-coupling can come in handy....

Differential Cross-Coupled Amplifier
The following is an interesting circuit. As far as I know, it has no name; so, let's call it the “differential cross-coupled amplifier.” I built up such a circuit back in the late 1980s as a frontend in a Dynaco ST-70 I was rebuilding. How did it work? To be honest, the amplifier oscillated, but I had so many trick circuit enhancements in place, including regulated power supplies and an elaborate remote feedback arrangement. I did however test the frontend I isolation and it worked gloriously well...

Still More Cross-Coupled Circuits
The following hybrid circuits are interesting because they invert the cross-coupling, building down, rather than up....

CCDA Once Again
I have built up 9-pin and octal CCDA PCBs into line stage amplifiers and I have been quite pleased by the results. On the 9-pin board, I used a pair of 12AU7s and metal-film resistors and polypropylene coupling capacitors; on the octal, new Russian Tungsol 6SN7s and carbon-film resistors and PIO coupling capacitors. Both sound good, but different. The octal version offers a sweet easiness that belies its simplicity and low cost. I know that many long for this type of sound from their systems, as it produces an archetypal representation of what most consider being “tube sound,” but without the usually concomitant hum and power-supply noise....
     14 Jul 2009

All-in-One LSA/HPA Update
Well, today I got enough free time to do a few listening experiments. I tried the 6H30 as the output tube in my Aikido All-in-One headphone amplifier. I have played with the 6H30 before and I was never overwhelmed, in spite of its stellar specifications. This time, however, this little powerhouse seemed to deliver the sonic goods...

Existing All-in-One User Guide Typo
Once again, cutting and pasting caught me...

6GU7 Tubes
I just received an e-mail from a TCJer who says that 6GU7s sound fantastically good and yet they only cost $5 each. He might be right, as the curves look good...
     31 May 2009

New: The All-in-One
Line-Stage/Headphone Amplifier

The original Aikido All-in-One design combined a 9-pin Aikido line stage amplifier and high-voltage B+ power supply and a regulated low-voltage heater power supply on one PCB, thereby greatly reducing the stress level of many Aikido fanciers. The new PCB is bigger and it holds a PS-1, high- and low-voltage regulated power supplies, and an Aikido line-stage/headphone amplifier. (The original All-in-One design could only be used as a line amplifier, as the RC B+ filter layout does not allow a White-cathode-follower configuration of the output tubes.)...

Loudness Distortion
Here is an odd aspect of audiophile life that is seldom mentioned, loudness distortion. For most audiophiles, the louder the better, just as a film buff might clamor for the biggest movie screen possible. And having watched movies on my Zune’s 3 inch screen, I can go along with a desire for a bigger picture screen, but hearing is not seeing. For example, too big a movie screen will never cause your eyes to hurt, whereas too loud sounds will make your ears bleed. In addition, the ear can only tollerate a small amount of distortion, whereas the eye feels no pain at seeing near infinte visual distortion. Moreover, a larger movie screen will not cause the image discoloration. Small green looks the same as big green and, for example, the following chromatic shift is not a natural product of image magnification....

Now for something completely different...
I have created small, 4 by 6 inch PCB that holds a low-voltage bipolar regulated power supply for solid-state use. Why? I have been experimenting with OpAmp circuits lately and I needed a killer low-voltage, bipolar power supply....

Chinese Cathode Cuffs Circuit
I assume that everyone played with some variation on the Chinese thumb/finger cuff toy as a child. They were small woven tubes that allowed both thumbs to be inserted readily, but clamped down, when the fingers tried to escape the cuff. Well, the following circuit works in a similar way: it allows the output tube’s cathode to slowly shift its DC voltage, but it bucks any fast changes in cathode voltage....
       08 May 2009

Blumlein Garter Circuit Revisited
Alan Dower Blumlein’s garter circuit appeared here before, in blog 46. This is a simple, cross-coupled, four-cathode-resistor-current-balancing circuit...
      30 Apr 2009

FFSR
The feed-forward shunt regulator has been covered here before, such as in blog numbers 95, 96 and 97; in addition, it is the basis of the Janus regulator. Well, here is a simple solid-state version....

CCDA Circuit, Part Two
The constant-current-draw is a compound circuit that consists of a grounded-cathode amplifier directly coupled to a cathode follower. Its goal is to draw a constant current from the B+ connection, thereby greatly relieving the power supply from having to work to squelch signal-induced perturbations at the B+ connection, which would otherwise re-circulate through the signal path. Achieving this goal requires that both circuits use the same valued load resistor, the grounded-cathode amplifier’s plate resistor and the cathode follower’s cathode resistor. By sharing the same valued load resistors, both circuits will experience the same peak current swings, but in anti-phase; thus, attaining a constant current conduction....
     15 Apr 2009

Introducing an Old Friend: The CCDA
As a response to the darkening economic sky and the big success that the All in One PCB and kit have been, I have resurrected an old trusted circuit that is tube-stingy, the CCDA line-stage amplifier. Back in September of 1998, I had posted at the GlassWare web site a Circuit of the Month article on the CCDA circuit. CCDA stands for constant-current-draw amplifier. The CCDA consists of a grounded-cathode amplifier directly cascading into a cathode follower. So what; what's so special about this obvious pairing? Its special status lies in the details....

New AC Selector Switch
I have created a new small, 1.4 by 1.7 inch, PCBs that hold a single rotary switch and allows two transformers to be switched on in a staggered fashion. The three positions are: all transformers off, transformer 1 on and transformer 2 off, and both transformers on. With this setup, you can turn on the heater transformer first, so the tubes are given a chance to heat up, which will create an electron cloud over the cathodes, protecting them from the B+ voltage....

Graigslist: Eight 7591 Tubes Go to Heaven
Thanks to Eric Barbour and John Atwood for the following:
   7591 vaccum tubes in original boxes - $50
   (westchester)...

    19 Mar 2009

TCJ 10 Year Anniversary
In March of 1999 the Tube CAD Journal was born. I had been writing the Tube Circuit of the Month articles for my GlassWare website and the response was just tremendous. This surprised me, as schematics were not rare on the Web, but enthusiasm-dripping e-mails I received told me that something extra was going on. I asked and the answer I got was that I just didn’t show how a circuit was laid out; I explained why it was designed that way. I realized that something more than one short article a month was needed. Thus the Tube CAD Journal was created. It, however, did not become what I had intended....

Aikido All in One with Tubes
I know that many want one-stop shopping, so I bought a bunch of tubes, JJ ECC802s (fancy 12AU7), EH 12AX7 and 6CG7 and 6922-Gold (fancy 6DJ8), to go with the All in One line stage amplifier kit. Now the kit can be bought with or without tubes at the GlassWare Yahoo Store....

Tube Tester: Ig & Vg
Back to testing tubes. Last time we saw how an auto-bias circuit could be implemented that forced the tube under test to conduct a fixed current by adjusting its ground voltage, while its cathode-to-plate voltage remained fixed. (Had we tried a constant-current source in series with the cathode, the cathode-to-plate voltage would decrease by the cathode voltage, which would throw our results off; maybe not a lot with a 12AX7, but by a huge amount with a 6AS7.) Now, we look at how we can derive some useful information with the auto-bias circuit, such as the grid voltage and the grid current....
     07 Mar 2009

New Aikido All in One
It's new and it's sweet. It's the All in One PCB, which holds two Aikido line-stage amplifiers and it includes the heater and high voltage B+ power supplies—all the same PCB. Thus, the All in One board makes building a standard-setting line stage amplifier much easier. The All in One assembled board with a chassis, volume control, selector switch, power transformer, and a fistful of RCA jacks is all that is needed. Of course, the All in One can be used for other audio purposes; for example, it could used with a DAC or even converted into a mic preamp or audio mixer or frontend to an SE power amplifier...
      28 Feb 2009

Slightly Newer and Slightly Improved
A while back, I rearranged all the regulator PCBs layouts so that they could accept fatter heatsinks, which was move in the right direction, as a fatter, short heatsink is better than a tall, skinny heatsink. But better still is a fat, tall heatsink. Starting now, the PS-1, H-PS-1, and Janus regulators will contain Aavid Thermalloy 530002B02500G heatsinks. This fat and tall heatsink shares the same footprint as the old 529802B02500G they used to ship with, but is 1 inches taller. The extra inch decreases the heatsink thermal resistance down to 2.7 from the old 3.7, based on a 75°C rise in natural convection. In addition, the power dissipation @ temperature rise is a healthy 8W @ 30°C, over the 5W of the previous heatsink. Why is all this important?...

JRB's Dream Tube Tester
Almost two decades ago, somewhere 1989 to 1991, I came up with a novel idea for creating a modern tube tester, one that would test gm, linearity, bias voltage, grid current, heater-to-cathode leakage noise, rectifier conduction, and balance between triode sections or between two output tubes...
       25 Feb 2009

Stimulus: It's Not What It Used To Be
I imagine that somewhere atop a seldom-traveled mountain or deep inside some unruly jungle there people who do not know that the World’s economic fortunes have turned bleak. I envy them...

Getting Electronic Parts
I have been going through hell trying to source all the parts I need for the Aikido kits. Last year, this was no big deal, but right now it is a small nightmare. After asking around, the answer I keep hearing is that no one wants to tie a bunch of cash in inventory that may not sell. I understand and I am the same situation. What is weird is that I will visit a website and see that they have 160 of the capacitors that I need, so I order 100. Normally, the capacitors would arrive in a few days. But they don't arrive until 10 days later. What has changed?...

Back to Power Boosters
This will be the last post on the topic of power-booster amplifiers (well, at least for a few months). Let’s pull back a bit and think about power and power amplifiers. It’s a dang pity that amplifier hold a power rating, as power is confusing, because it doesn't linearly correlate well with our perception of acoustic power. For example, a 20W power amplifier does not play nearly twice as loudly as a 10W power amplifier, something closer to 200W would be needed. In addition, power isn’t linearly related to output voltage, as a doubling of the output voltage will quadruple the wattage...
       10 Feb 2009

New and Slightly Improved
The PS-1 regulator PCBs were part of the shipment; but now the boards are in revision B. I made the PCB half an inch taller, which allowed a fatter heatsink to be used. In general, a fat, short heatsink is better than a tall, skinny heatsink. The intrinsic thermal resistance of the metal is effectively placed in parallel in a fat heatsink; in series, a tall heatsink. The upshot is that the PS-1 regulator now holds the same Aavid Thermalloy 529802B02500G heatsink that the Janus and H-PS-1 regulators use. This chubby heatsink boasts a thermal resistance of only 3.7, based on a 75°C rise in natural convection....
     17 Jan 2009

Back to Power Boosters
The ideal that we long for would be a setup wherein a small, high-quality power amplifier, say a 2A3-based, parafeed-single-ended amplifier worked into what effectively would be a an electronic magnifying glass, which would hugely expand the wee amplifier’s paltry 2.5W into 1kW at the loudspeaker’s terminals. This power-booster amplifier would stand like a heavenly megaphone perfectly magnifying an angel’s voice. Preserving every detail, neither adding nor subtracting from the original signal, this ideal power-booster amplifier would only augment power by flawlessly magnifying voltage and current output...

Diamond Buffer Topology
This famous four-transistor buffer topology has many adherents and a long history; see the datasheet for the LH0002, which came out in the late 70s. Precisely because it is so simple, because it is so easy to understand, it is venerated by many solder slingers. (It is in many ways the solid-state equivalent of the SRPP tube circuit.) Well, now it’s time to put on our mind-stretching caps. In my last two blog entries, I showed several voltage amplifier topologies that looked like unity-gain buffers, such as my Sagaris Amplifier...
    09 Jan 2008

New Signal and Capacitor Selector Switches
I have created small, 1.4 by 2 inch, PCBs that hold a single rotary switch and nothing else, other than termination pads for hookup lead. The selector switch assembly accepts three stereo inputs, with both the hot and grounds of each signal source to be selected. So if a signal source, say a CD player, is not selected, neither its outputs or grounds make any connection to the line-stage amplifier...

Resurrection of TCJ 36-Pos Stepped Attenuator
This simple three-rotary switch attenuator was the GlassWare Yahoo Store’s selling product. I still use two of them myself and still marvel at its elegance and effectiveness. The attenuator was offered with either metal-film or carbon-film resistors, in either -2dB or -1dB decrements, and its sturdy open-frame switch allowed easy contact cleaning. So if this stepped attenuator was so great, why did I stop selling them?

A Few Comments on Power Booster Amplifiers
I received some interesting e-mail because of my last entry on power-booster amplifiers. I must admit that I quickly wrote that blog entry, so it was a bit too brief, too formless to avoid some misunderstandings. So, I will try to sharpen the focus at tad. I am intrigued by the idea of a small power amplifier driving a large power amplifier, not just with fairly large voltage swings, but with concomitant current swings, driving in the most robust sense. Otherwise, why bother? If all that is needed is voltage swing, then use a tube line-stage amplifier with a high-power amplifier and be done with it. An Aikido line-stage amplifier with a 12AX7 input tube will provide a gain of close to 50 and big voltage swings, maybe not enough to drive a large unity-gain power buffer amplifier directly ala the Moskido amplifier, but surely enough to drive 500W power amplifier with a gain of 3.16 to full output. A peak voltage swing of 90V into an 8-ohm load equals a 506W and 90V divided by 3.16 equals 28.5V, which is well within the high-gain Aikido’s range...
      21 Dec 2008

Power-Booster Amplifers
If you do a web search on "power booster amplifier," most of the search results will refer to radio transmitters, not audio amplifiers. This makes sense, as few of us would consider cascading power amplifiers. If we need a more powerful amplifier, we buy a more powerful amplifier. Still, the idea is an intriguing one: take a small, flea-power, single-ended, tube amplifier and a huge, heavy, solid-state, power-booster amplifier that greatly magnifies the wimpy 2.5W into hundreds of watts. The best of both technologies.

Or would it be?...

Sagaris Amplifier
How do we get the voltage gain down to a more useable amount? The following circuit shows how third feedback loop can be applied....

Tube-Based Power-Booster Amplifier
Super high-wattage tube power amplifiers can be built. We just need many output tubes and a high B+ voltage. But a conventional tube power amplifier is hardly novel nor would such an amplifier take advantage of the smaller power amplifier’s ability to drive a low impedance....
       09 Dec 2008

Cathode-Coupled Amplifier
A few years ago, I received many requests for more information on the cathode-coupled amplifier, but not much interest the last two years. Until, just recently, as two readers have wrote asking for help with their cathode-coupled amplifier circuits and a friend of mine mentioned interest in this circuit in the last two weeks. Is this a new trend or just a coincidence? Has an article appeared in audioXpress that featured cathode-coupled amplifiers? Well, every time I think that I cannot say anymore on a particular circuit, I discover something new to write about; so it is with the cathode-coupled amplifier....
      28 Nov 2008

PS-1 Solid-State Regulator Kit
Finally, after many a tease and far too many false starts, arrives the new PS-1 regulator. The PCB is only four by six inches, yet it holds an all-solid-state two regulated power supplies, a high-voltage regulator for the tube B+ and a low-voltage regulator for the tube heaters. Each voltage regulator also finds its own raw power supply, holding the all the rectifiers and power-supply reservoir capacitors required for feeding each regulator its raw DC voltage. In other words, except for the power transformer(s), the PS-1 PCB holds all that is needed to make a superb regulated power supply for tube-circuits....

Janus Regulator User Guide
Although I quickly sold out of the Janus regulator kits, I have more boards on order and I have completed its user guide. This fat beast is 24 pages long and it covers much more than just the Janus regulator. In other words, I recommend that everyone interested in high voltage power supply design to download the PDF version of the user guide...

H-PS-1 Rev. A PCB Error
If you have recieved a H-PS-1 PCB marked "Rev. A," then please email me and I will send you a replacement, as I messed up the PCB. I was moving the old H-PS-1 layout about to make room for the new larger heatsinks I had bought and I assumed that the circuit was intact, as only a few parts positions had changed, so I didn't build up an H-PS-1 PCB to test. Huge mistake. The repositioning of D6 has both pads on the PCB bottom connected to the same trace, which shorts the input and output pins together on the regulator.
     19 Nov 2008

Janus Regulator PCB, Rev A
The revised Janus regulator PCB features a one inch increase in height and now all four high-voltage electrolytic capacitors hold bypass capacitors...

UltraPath Push-Pull
Blog number 147 gave an overview and critique of the Ultrapath circuit. Here is a quick recap: the Ultrapath circuit is a small variation on the grounded-cathode amplifier, differing in the B+ termination of the cathode bypass capacitor, rather than the usual ground termination. Depending on the rest of the circuit, this may or may not prove to be a good idea.

Ultra White Cathode Follower
Not all tube-based push-pull amplifiers hold an output transformer. The SRPP and White cathode follower need no center-tapped output transformer to work. The White cathode follower, which I have covered many times before, holds some promising results when its cathode resistor’s bypass capacitor is terminated into ground, but not in the way you might envisage. Put differently: if the White cathode follower is used with positive power supply rail, then having the cathode resistor’s bypass capacitor terminate into the B+ connection is a bad idea, as the bypass capacitor will relay the power-supply noise into the output signal....

Aikido White Cathode Follower
About a decade ago, I set about trying to apply an Aikido-like elimination of power supply from a standard White cathode follower. The result was the following circuit.

Tubes and Religion
Well, it looks like I just got promoted. I have been called a tube guru many times, which is not is not something I want to be called . (On the other hand, I have friends who would give anything—anything—to be called a maharishi or guru or sage or czar or….) But now I am being hailed as the American tube pope, "Der amerikanische Röhrenpapst John Broskie;" well, at least I am at the German HB HIFI website.

Janus Regulator Kits
I have finally gathered all the parts needed to offer 10 Janus regulator kits. The Janus shunt regulator made its first appearance in 2007, at the end of Blog 112 and in the middle of blog 117. In a nutshell, the Janus shunt regulator uses both feedforward- and feedback-based shunt regulation to reject both the rectifier-induced power-supply noise and the signal-induced power-supply noise from its output...
     28 Sep 2008

Hybrid SE CF Power Amplifier
Cathode follower output stages were covered back in 2005. The nutshell-fitting conclusion was that the cathode follower output stage, because of its low gain, which is always less than unity, must make enormous demands on its input and driver stage supply a voltage swing even greater than what the cathode must swing. In other words, although the cathode follower output stage offers low distortion, the input and driver stages are so greatly burdened that their combined distortion contribution can easily undo the gains made by the cathode follower output stage.
   The following hybrid amplifier uses a cascoded input stage develop a high amount of wide-bandwidth gain to drive the second gain stage to full output....
      07 Sept 2008

UltraPath = Ultra-Simple (Except that it's Not)
While talking to John Atwood the other day, the fellow behind the Clarisonus website, the topic of Ultrapath circuit came up. I was surprised at my instant expression of exasperation with the Ultrapath circuit and its ardent supporters. What surprised me most about my emotion-laden response was that Ultrapath storm has long passed. For a period of about four years, starting around eight years ago, I was asked often was Is the Ultrapath the ne plus ultra? (Never, however, in those exact words, sadly.) I never liked the question. Why? To start with, the question was always asked in the same manner, excitedly, wide-eyed, an energized agitation worthy of a diehard Star-Wars fan recounting his favorite scenes in the latest installment from George Lucas. Second, the question, when examined closely, reveals itself to be incomplete, empty of a referent or even a context that would make the question answerable....

How the Ultrapath Works
Let’s look into the Ultrapath circuit. Picture a perfect power supply, differing from ground by only a large DC offset, but otherwise offering just as a perfect “ground” connection. In other words, such an ideal power supply would be free from the real-world liabilities that actual power supplies suffer from, such as frequency-dependent output resistance, rectification and signal-induced noise. In fact, so perfectly would the B+ connection mimic ground that the Ultrapath circuit would perform identically to an equivalent grounded-cathode amplifier, yielding no sonic difference whatsoever. In this fantasy world, the Ultrapath would not be so ultra, being jinxed by its requiring a high-voltage cathode-bypass capacitor, rather than the low-voltage capacitor the conventional grounded-cathode amplifier can get away with...

Ultrapath Single-Ended Amplifiers
What about using the Ultrapath technique with a single-ended amplifier, as many audiophiles do? Once again, the same egregious amplification of the power-supply noise pollutes the output signal, as the output transformer’s primary presents an impedance that is fairly flat across the audio band and against which the triode develops a signal gain. Unlike the plate resistor, the output transformer’s primary only transfers the differential across its leads, so a small portion of the power-supply noise gets rejected. Still, the single-ended amplifier below will be ultra noisy....

Parafeed and Ultrapath
I know that many solder slingers will try anything; God bless them. However, applying the Ultrapath technique to a parafeed line stage amplifier or power amplifier can be a stunningly bad idea....
      31 Aug 2008

One More Phono Preamp Design
More phono preamp circuits. It all started with the use of a battery as a coupling-capacitor/voltage-reference between the phono cartridge and the preamp's input. The battery qua coupling-capacitor/voltage-reference opens up a thick deposit of possible new circuits. The following is a hybrid effort that provides enough gain to accept a low-output MC phono cartridge....
      26 Aug 2008

Inverted Grounded-Cathode Amplifier
My last blog entry provoked some head scratching, as I deduced from the e-mail asking for clarification on the following circuit. It wasn't just the two batteries that confused, but the cathode-follower-looking appearance of the topology. The e-mails asked, "How can there be any gain at all, when this circuit is clearly a unity-gain cathode follower?"

Transformer-Coupled
Aikido Line-Stage Amplifier

Now that we are mentally limber and the inverted-grounded-cathode-amplifier topology no longer seems strange, let's move on to some interesting circuits. At the top of this blog entry, we see the Aikido phono pre-preamp circuit, but there is no reason that we cannot use this topology elsewhere, in a line stage amplifier for example. The following circuit uses an input transformer to DC isolate the signal source completely from the line stage amplifier; note the absence of a ground connection on the transformer’s primary. Once the input signal has been reflected across the input transformer, the input triode amplifies, while the second tube buffers the output signal. Power-supply noise is dropped from the output and the output coupling capacitor provides a safety net...

Aikido-Unicorn Phono Preamp
Just to help further establish my non-guru status, I present the above original circuit. Non-guru? If I were a tube guru (or a better businessman) I would never distract you with more phono preamp circuits; instead, I would claim that the Aikido PH-1 phono stage was perfect and that all other phono stages are laughingly inferior. As for the name, Aikido-Unicorn, back in 2002, I wrote an essay titled, RIAA Preamps Part 2, wherein on page 10 the Unicorn topology made its first showing...
      06 Aug 2008

MC Pre-Preamplifiers
Moving-coil cartridges often put out so little voltage that +20dB to +30dB of voltage gain must be added before they can be used with a standard +40dB phono preamp. The extra gain can be had either actively or passively. Both approaches are difficult to realize without compromising the tiny and delicate signal’s integrity, as quality transformers are supremely difficult to design and all active devices and resistors add noise and distortion to the output signal. Well, what about all those new super-linear, super-quiet OpAmps that are being touted as the easy solution to all audio problems? Aside from the perceived sellout implicit in all hybrid efforts, the OpAmps all require a negative power supply, which many tube-loving folk fear as much as many solid-state-loving folk fear high-voltage power supplies....

Aikido Phono Pre-Preamp
Since the Aikido is known for its low noise and low distortion, it would seem a natural for a pre-preamp—and it is, as long as one is willing to accept the higher noise that all tubes must intrinsically add, because of their low transconductance and high plate resistances. Even the unbypassed cathode resistors will add noise to the circuit. The following circuit uses two batteries to create an Aikido phono pre-preamp that forgoes cathode resistors in the first stage....
      25 Jul 2008

At long last: the Aikido Phono Preamp
This Aikido phono preamp uses passive equalization, rather than active, feedback-based equalization. By cutting the highs and boosting the bass, the phono stage’s inverse RIAA equalization of the LP’s RIAA equalization curve returns the signal to flat. The passive equalization network sits in between two Aikido gain stages...
      13 Jul 2007

Sharing the Workload
Consider this, 99.9% of hybrid amplifiers just pass the signal from one technology to another—usually from vacuum tube to solid-state, but not always. AJ van Doorn's hybrid amplifier strove to be different, to have the tube and solid-state portions of the amplifier work together at driving the loudspeaker. As I have stated before, I do not believe that Mr. van Doorn has succeeded. Now let’s look into an established method for truly running two amplifier in parallel...
        09 Jun 2008

24V Aikido PCBs & Other PCBs
If you have visited the GlassWare Yahoo Store recently, you probably have noticed that almost everything is sold out. Well, I have finally relented and placed a large order with my PCB fabrication house and the PCBs should arrive soon. A mix of old and new PCBs is coming. The old: stereo, mono, octal, 9-pin, and stepped attenuator boards. The new: a new rev of the 24V Aikido board and an Aikido phono preamplifier....

More GainClone Thoughts
I asked if any reader knew of a unity-gain-stable power OpAmp last time. Thanks from me to those who made suggestions, such as the LM12 and the Apex line of high-voltage/high-power OpAmps. Why was I interested in finding such a power amplifier? I have several topologies in mind that could only be used with a unity-gain-stable power amplifier, for example: an Aikido-gain-clone power amplifier that uses a mono-polar power supply....

Differential GainClones
A common design choice is to run two solid-state power amplifiers differentially, so that the loudspeaker never makes a connection to ground, as each power amplifier swings output voltage in the opposite direction. Such an arrangement allows twice the voltage and current swings into the load, so four times the potential power output should be developed. (Well, that’s the theory; reality is a bit more stingy.) For example, two 50W power amplifiers should be able to deliver 200W into the same 8-ohm load....

What JRB Would Really Do
Moving away from theory and moving closer to practice, I would never actually build the above circuit as drawn, as I do not think that the LM3886s would be up to the task. A quick scan of DIY postings on the web reveals that the National Semiconductor Overture™ series of OpAmp power amplifier all hold the same Achilles’ tendon: heat dissipation. An LM3886 might be big for an OpAmp, but it is miniscule for a 60W power amplifier. It will get hot and the heat must be wicked away or the LM3886 own thermal management circuit will shut the amplifier down. Placing the amplifier in differential configuration doubles the problem, as twice the current must be delivered with the same rail voltages. Given only two LM3886s, the only practical solution might be to use a fan to force cool air through a large heatsink...
     19 May 2008

New Hybrid SE OTL Design in audioXpress
In the May 2008 issue of audioXpress magazine, we witness a new hybrid OTL topology: a class-A, single-ended one at that. AJ van Doorn's article, "Build a Hybrid SE OTL Amp," describes a novel circuit that directly couples a vacuum tube to the loudspeaker, while a solid-state power amplifier also directly couples to the output....

High-Level Audio-Autopsy
Mr. van Doorn has created a low-distortion, high-wattage, affordable, hybrid, single-ended, OTL power amplifier—or has he?...

John Broskie 's Rant
Today we live in a caste system of consumers and technocrats. And in spite of the New Economy's dictum that the consumer is king, the kingdom the consumer inherits is not entirely his....

Putting the AI-kido into the gAInclone
Gainclone amplifiers are wildly popular. I know this because the few times that I have mentioned a power OpAmp in these pages, I have received a ton of e-mail from gainclone fanciers. Although I own many of the power OpAmps used in the gainclone amplifier, such as the LM12, LM3875, and LM3386, I have never actually assembled my own gainclone power amplifier. Slacker....

Circlotron Lie Detector
With the likely election of a new Clinton presidency, what the world needs is a good tube-based lie detector. Well, it turns out that such a beast existed back in 1934....
      28 APR 2008

More Circlotron Circuits
I had a good time revisiting the Circlotron topology last time, but there was much more that I hoped to cover. So let’s start at the beginning again and work our way up to something more complex. Below is a simple Circlotron circuit. The two 1k resistors define a two-resistor voltage divider that sets the signal reference at the midpoint between both cathodes....

Floating Power Supplies
One of the many big problems with using two floating power supplies is the temptation to treat them as something other than a power supply. In other words, many simply assume that the floating power supply is a different kind of beast from the typical clunky, noisy power supply that normal push-pull amplifiers use. It’s not. All the problems that come with a power supply are still there and they still make a big difference in the sonic fingerprint left by the amplifier....

Dynaco ST-70
I think that the Dynaco ST-70 would make an excellent platform, as it comes with two output transformers and four octal tube sockets. In other words, we could convert this stereo amplifier into a mono-bloc amplifier. Instead of using four EL34s, we could use two KT88s; instead of the silly stock driver PCB, we could use one 6SL7 and one 6SN7 and use the PCB space for a board that would hold the supporting parts....

Creating Ultra-Linear Without UL-Taps
Before I explain how we can get ultra-linear operation from non-UL-tapped output transformers, let's look at a simple tetrode/pentode mode output stage...
       07 Apr 2008

It's Time to Get Serious
None can deny that the Earth has grown warmer in the past few decades. This increase in global temperatures must be in response to external forces, including variations in its orbit around the Sun (orbital forcing), volcanic eruptions, and atmospheric greenhouse gas concentrations. The exact and complete causes of the recent warming remain an active object of research, but the growing scientific consensus is that some of the increasing warmth is due to the concomitant increase in tube amplifier popularity (who would be willing to deny the coetaneous relation between the growing popularity of vacuum tubes and increasingly balmy Scandinavia?). The logic is hard to refute: tubes glow red hot and the planet is warming; thus, tube amplifiers are contributing to the that warmth.,,,
      01 Apr 2008

Circlotron Amplifiers Once Again
We near the Tube CAD Journal’s ten-year anniversary and during that near decade I have written much on just about every tube-related topic: a veritable omnium-gatherum of tube technology. Most of the topics have proven, although no doubt enthralling and informative, uncontroversial. But not so with the SRPP circuit, the Rosenblit-patented Transcendent OTL, and the Circlotron output stage topology—these three topics are controversial. In fact, I have received amazingly angry e-mail (and even downright insulting e-mail) because of what I have written about these three most hallowed circuits....

The Magic Behind the Circlotron
Just as when you see a magician saw a woman in half or pull a rabbit from his hat, the magic lives only in your eyes. The topology neither redefines the laws of physics nor falls outside of established electronic theory; in fact, the Circlotron topology was not discovered at the UFO crash site in Roswell, New Mexico, as so many seem to believe. It can be—and almost always is—run under class-AB mode and in no way is it exclusively limited to class-A operation; nor does it consist of two independent single-ended amplifiers, no more than any other push-pull amplifier does; and nor is this topology confined to tubes, as solid-state Circlotrons can and have been made....

Solid-State Circlotrons
The solid-state Circlotron was patented in 1959 by A. W. Donald III and, in 1964, A. S. Goldsmith patented a simple diode-biasing method for a transistor-based Circlotron. More recently, in 1980, the great James W. Bongiorno patented the basis of the clever Sumo Nine power amplifier he designed...

Thorens TEM 3200
The venerable German turntable company makes an interesting hybrid Circlotron, the TEM 3200. The amplifier uses an input transformer to isolate and split the phase; six tubes to provide voltage gain and two N-channel power MOSFETs to deliver high output current; and no coupling capacitors. Stereoplay magazine says that this is the world's best power amplifier. Perhaps it is, but our concern is "How does it work?"...

New Tube Circuit
Now, let's create something new. (I have been told many times that it is impossible to create a new tube circuit—and I always agree that it would be impossible for the person telling me that to create a new tube circuit.) Where to start? One path of exploration might be to examine a functionally identical circuit to the Circlotron....

Amplifier Qua Speaker Stands
As you can readily see, had we the time spare, we could go on forever creating the impossible. Now let's say that we built such a hybrid amplifier; we would still face the problem of having to house the amplifier. Remember, half of the cost of producing high-end equipment must be in cosmetics. So here is my suggestion: a monobloc amplifier that doubles as a speaker stand....
       26 Mar 2008

Balanced Aikido
It looks like an octal, stereo Aikido line-stage amplifier, but it’s not; instead, it is a single channel, balanced Aikido line-stage amplifier. Yes: four tubes, two interstage coupling capacitors, and two output coupling capacitors—all for one channel. No one ever said that balanced was cheap. I designed the PCBs late last year and I just got around to testing them...

Differential Amplifiers
The differential amplifier is a simple circuit that holds two triodes (or pentodes, transistors, FETs, MOSFETs, heptodes…) and two plate resistors, but only one common cathode resistor. When a balanced signal is applied to the two grids, one triode conducts more, while the other conducts less, thereby creating balanced output signals at the triodes’ plates...

Broskie Cathode Followers
If a high CMRR is so desirable, why not use a differential amplifier? Or, put a little differently, how are we going to get an equally high CMRR without a differential amplifier frontend? The solution is found in the output stage, which consists of two Broskie cathode followers. This follower accepts a balanced pair of input signals and it delivers a single phase output, which explains why two of them are requireded. In addition, the Broskie cathode follower offers an excellent CMRR...
      16 Mar 2007

TCJ Attenuator User Guide
I have finnished the user guide booklet that comes with the TCJ Attenuator kit. It is six pages long and it offers a good overview of stepped attenuators in general and the TCJ Attenuator in specific...

Back to the Future
In the past year, I have sensed a significant resurgence in interest in tube push-pull amplifiers. E-mails pour in asking about new push-pull circuits; and I am surprised by how many of my friends have retreated from single-ended amplifiers, moving back to the more powerful push-pull tube amplifiers. What could explain such a re-examination and re-adoption of the push-pull amplifier?..

Aikido Single-Ended-to-Balanced Topology
With the increased interest in push-pull power amplifier, there has been a concomitant increased interest in balanced circuits in general. Besides, many new high-end audio products are sporting RCA and balanced XLR output connectors, so why not give them a try? And if you are going to run a push-pull power amplifier, wouldn’t it be a good idea to feed the amplifier an equally push-pull input signal? Even if your power amplifier is an entirely single-ended affair, with nothing pulling while something else is pushing, having an easy phase selection option is fun. In other words, with a balanced output, we can choose which output phase to send to our otherwise single-ended amplifier....
      22 Feb 2008

After Many a Month,
Returns the TCJ Stepped Attenuator

Reloaded and ready for action, it's back; but it's not the same—it's much better. First of all, the PCBs are meant to hold resistors on both sides; the switch spacing is now 3” instead of 2.5 inches; and, as a result, the PCBs are now shorter, 1.4 inches tall, and a tad longer, 9 inches long. Why? Now the attenuator will fit within a 1U rack-mount enclosure. Second, the TCJ stepped attenuator now offers many more positions, a total of 66 steps with 1dB resolution, as the center switch now presents 11 positions, rather than the old 6 positions. Third, and most importantly, the old open-frame rotary switches have been replaced by Elma switches. Swiss-made, gold-heavy, precisely-designed and exquisitely-made, Elma rotary switches are justly famous as the gold-standard in switches. And like all things golden, they are obscenely expensive. But when only the best will do…
       14 Feb 2008

24V Aikido PCB User Guide
I have made a few improvements and corrections to the user guide booklet that comes with the PCBs. To download a PDF, click here....

24V Aikido PCB Heater Capacitors—
IMPORTANT!

On the new 24V PCB, the heaters are all placed in series, so each heater sees one fourth of the B+ voltage. So we might assume that each heater bypass capacitor will only see the same one fourth of the B+ voltage; and they do, when all the heaters are conducting. But what happens when one tube is removed from its socket or when one heater element becomes open?...

More About the 24V Aikido
Line- Stage/Headphone Amplifier PCBs

I was wrong: I didn't expect the enthusiastic welcome that the 24V Aikido PCB would receive. My original plan was to sell ten blank PCBs and eight kits, but before I could turn off the availability of the new 24V Aikido boards, 15 had been sold, leaving with just three PCBs for kits. Thus, I will have to order a new production run, as no doubt the new PCB and kit will continue to be popular, much more than I imagined at first...

High-Voltage Aikido
Line-Stage/Headphone Amplifier

Because I held such low expectations for the 6GM8 tube, I built in to the PCBs an escape hatch, whereby I could still use the PCBs with a high-voltage power supply and a separate heater power supply, as the heater string is not hardwired to the B+ connections....
      04 February 2008

24V Aikido Line Stage & Headphone Amplifier
It’s been almost two years since I first wrote about building an Aikido line stage amplifier based on the low-plate-voltage, dual-triode tube, the 6GM8 (AKA ECC86 and 6N27P). And I have finally built one, as shown above. While I had the tubes, the Aikido PCBs, the supporting parts, the design, and the desire, somehow I just never had the time. (Something to do with having two small children and an infinite amount of e-mail to answer.)...
        27 Jan 2008

New Zune
My Christmas present was deferred. Like many others, I had to wait before I could get my new 80G Zune, as no one had them in stock. In fact, after many phone calls and when at last I had at last found one in stock, I had to promise to pick it up within 30 minutes, my Zune being the last one in the store...

Sliding-Fixed-Bias Circuits
Yes, I know that “sliding” and “fixed” are irreconcilable, so a better label might be “sliding grid-bias circuits.” The logic behind the following circuits is that a push-pull, tube-based, power amplifier should be run under a relatively high idle current at low-signal levels, thereby ensuring the benefits offered by class-A operation, such as low output impedance and low distortion. But once the envelope of class-A operation is pierced by high input signal levels, the output stage should see a quickly dropping bias voltage, thereby shifting the amplifier into a lean class-AB mode of operation, which will offer the advantage of much greater power output, but at the cost of higher distortion and output impedance. Once the input signal relaxes back within the class-A envelope of permissible current variation, the grid-bias voltage should climb slowly up to its idle current value...
      15 Jan 2008

More Classic Articles?
I plan on offering more OCRed articles in the future. The only stipulations are that the article be well-written (almost all of the articles before 1965 are), that the article deal with tube electronics and be interesting, and that the article be out of copyright. I happen to own all the original audiocraft magazine issues, except for the last issue. This was a grand magazine that ran from November 1955 to June 1958. In its pages, you will find great articles on horn loudspeakers and tube power amplifiers. I am tempted to perform an OCR recreation of these magazines, but I am worried....
       06 Jan 2008

Two More OCRed Classic Articles
I asked a friend what he thought of my re-created article on the Brook amplifier, "High-Quality Audio Amplifier With Automatic Bias Control," that I had OCRed from scanned images of the original article. He told me that I had surely messed up and posted the original images, not the text-filled PDF I had planned on posting. He was wrong; and it took some persuading to get him to test the PDF’s contents for text that could be copied and pasted. This is what all art forgers (and plastic surgeons) hope for: a forgery so good that no one can be convinced that it is a fake. No doubt what fooled my friend was the use of the Bookman Old-Style typeface....

Push-Pull Output Stages
and Sliding Idle Current

Last time, we looked into how a single-ended amplifier could enjoy a relatively low idle current that would swell with load passages, allowing more headroom than the low idle current would otherwise imply. This time, we will look into the opposite scenario, wherein a low-wattage class-A, push-pull amplifier starts with a relatively high idle current. Then, when it is provoked by large input signals, a greater negative bias voltage is applied to the output tubes’ grids, bringing down the idle current, and shifting the class of operation to a lean class-AB. Yes, this was the operating principle behind the Brook amplifier, but it used extra tubes and a complex power supply to achieve this goal. In contrast, I am interested in exploring how the same task might be performed more simply and inexpensively....
      27 Dec 2007

The Brook Amplifier:
An Amplifier with Automatic Bias Control

To help continue the topic of variable-bias power amplifiers, John Atwood sent me a scan of a great little article on the Brook amplifier, "High-Quality Audio Amplifier With Automatic Bias Control." I have OCR-ed the scan and it is available by clicking on its title....

Sixty years ago, this interesting and fun-to-read article appeared in Audio Engineering magazine (the precursor of Audio magazine). Written by J. R. Edinger, of Brook Electronics, the article lucidly explains how the push-pull Brook amplifier uses a dynamically shifting bias voltage to create an output-mode-shifting amplifier. Simply put, the Brook amplifier offers two faces: a push-pull, class-A, low-distortion, low-power amplifier, when at idle or under low signal levels; and a lean, mean class-AB, higher-distortion, high-power amplifier when provoked by large input signals....

Broskie Sliding-Bias SE Amplifier
To transition push-pull class-A operation into lean class-AB operation requires a drop idle current. To transition a low-wattage, light-current single-ended amplifier into a high-wattage (for a single-ended amplifier), high-current single-ended amplifier requires a boost the idle current. The obvious route would be to rectify the secondary voltage swing and reduce a negative bias by an addition -15V or so. But what about cathode-biased single-ended amplifiers? How can these amplifiers achieve the same goal?....
      17 Dec 2007

Essential Gadget
Anyone who has read this blog/webzine will know that I like headphones—more than I should perhaps, as I already own six pairs of headphones (AKG, Grado, Sennheiser, Stax, V-Moda...) and I would love to buy more! Why? Each headphone holds a different perspective on the music, just as each loudspeaker does. But unlike speakers, headphones are relatively cheap, they take up much less space, and they are portable. And here is where the troubles begin. To doubly mangle Robert Burns, The best laid headphone cables of iPods and laptops often go astray. A cat jumping into your lap or a unexpected knock at your door, or a branch extending into the sidewalk—suddenly, the headphone cable catches, your head goes one way, the headphones go another, the laptop or iPod fly in some other direction, all at once....

Philips DVD Micro Theater MCD908 Mods
More details trickle in. The tube type used is the 12AX7. The MCD908 does in fact hold the very-listenable TDA8920 stereo class-D amplifier module from NXP, not Philips, as I had mistakenly mentioned before (NXP was founded by Philips). The power amplifier derives is power from a fairly large toroid transformer. In addition, the MCD908 will play just about anything that you throw at it...
        08 Dec 2007

Sledding-Bias Output Stage
My idea is a simple one: slowly vary the idle current on the single-ended output stage to meet the demands of the music being played back. When the music pauses or falls into a near-silent pianissimo, let the idle current fall to a diminutive trickle; but just before it swells to an ear-bleeding crescendo, let the idle current climb to a near-dangerously high torrent. In other words, use only the amount of idle current that is needed to trace the music signal at the desired volume level...

Before-It-Happens Clipping Indicator
Clipping sucks. When an amplifier clips, harsh harmonics abound. Square-waves burn out tweeters and scratch the soul. Tube amplifiers, in general, produce far less nasty clipped output signals than solid-state amplifiers; thus, the tube amplifier’s reputation for better sound was born....

Sliding Bias
What if you do not own a hard-drive-based music system? What if you spin LPs or listen to the radio or tapes? Then no proactive arrangement is possible, as the amplifier can never know what is coming it way. (Even when the music data exists on the LP or tape, we cannot access it ahead of its being played. And with records, the ticks and pops would throw a random element into the equation, ruining any chance of anticipating the signal before it happens.) When presented with the unknown, the best we can hope for is a fairly good reactive system....

MCD908
The following block-quote holds an interesting clump of prose. See if you can guess where it came from and what is being described.

    Hi-Fi Tube Sound from...

 

Why do I need it?
You deserve an immersive sound experience like
an audiophile.

The built-in vacuum tube preamplifier enables
you to experience the highest quality sound
normally you can only find in premium Hi-Fi
audio equipment....

22 Nov 2007

Old Zune, New Zun, and Classical Zune
Microsoft hasn’t toppled Apple’s iPod, but Microsoft’s Zune has pushed all the other makers of MP3 players out of the way, as the Zune is now the second biggest seller, which some might deem an amazing accomplishment, considering the Zune’s one year anniversary was just a few weeks ago. Unfortunately, in this modern world, to be second is to be seen as a huge failure. For example, once the average consumer realizes that he cannot buy porn on Blu-Ray DVD, the BD will go the way of the Betamax tape, as the lower-capacity, porn-friendly HD-DVD sales will then dwarf those of the BD. It doesn’t pay to be too good. Beta was certainly better, but being better is seldom a critical purchasing choice. Want another example? How about the vacuum tube? Tubes are better than transistors at reproducing music, but solid-state owns 99.9999999% of the consumer audio-electronics market....

SE MOSFET Buffers/Amplifiers
The idea is that since the Aikido line stage amplifier is so good at amplifying signal voltages, why not pair it up with a unity-gain power buffer? The best of both worlds, as they say. When it comes to power output devices, the power MOSFET is an obvious first choice, as it offers a very high input impedance and it is rumored to sound closer to tubes than transistors do. Certainly, the Moskido and Zenkido are creating a commotion and it these hybrid efforts sound just half as good as the e-mail indicate that they do, then this plan has a lot of merit. So, let’s began with single ended circuits.
        14 Nov 2007

MJ Audio Technology OTL Design
This year's tenth issue of MJ Audio Technology held an interesting OTL power amplifier circuit design by Mr. Kadou Teppei that I examined in blog number 121. Based on two e-mails that I have sinced recieved, I feel that more must be said about this and other OTL designs....

The Partial-Regulation Problem
High-voltage regulators are neither cheap to make or easy to implement; thus, their rarity. If we add the ability to deliver high current as well as high voltage, as an OTL power amplifier would require, then such a regulator becomes rare to the point of nonexistence. In contrast, many of an OTL amplifier’s reference voltage are such low-current affairs that voltage regulator is not only cheap, but easy to implement; unfortunately. A little bit of regulation, like knowledge, can be a dangerous thing.

A 6AS7-Based OTL
The 6AS7 is a popular tube. Reasons are not hard to find, as it looks cool and it is used in many OTL power amplifiers. This twin triode can draw a huge amount of current with little cathode-to-plate voltage. Unfortunately, its mu is only 2, its heater current draw is 2.5A, and it is not that linear. This tube is so popular that I should always preemptively announce whether the circuit at hand can use the 6AS7. I failed to make such a proclamation during blog 121’s scrutiny of the MJ OTL. The answer is that while the 6AS7 could be forced to work in that topology, lower rail voltages would better suit the 6AS7.
       05 Nov 2007

The Other John Broskie
I recently attended my cousin's 50th birthday party, where my brother told me that a high school buddy of his had tried to find him (i.e. my brother) by typing “Broskie” in Google, but all the results pointed to me...and some baseball player of the same name. If two famous (or infamous) John Broskies aren't enough for the world, our cousin is also named John Broski* and he is a gifted, award-winning photographer, with a comprehensive website of his own. So if you got here by mistake, here’s the link for my cousin: www.finelight.biz. (You're on your own if you were searching for the athlete.)...

Nelson Pass's Zen
Meets John Broskie's Aikido

First there was Nelson Pass’s Zen power amplifier, which answered the question, "What is the sound of just one power MOSFET amplifying?" Then my Aikido line-voltage amplifier, which delivers low-distortion, low-noise, and low output impedance—without a negative feedback loop. Then Bob Prangnell’s Moskido hybrid push-pull power amplifier, which united the Aikido and a MOSFET push-pull, source-follower output stage. And now, we have the single-ended Zenkido hybrid power amplifier by Stephen W. Moore. In the November 2007 issue of audioXpress, we find part two of Mr. Moore’s article on his hybrid power amplifier, which weds the Aikido PCB to a Zen-like unity-gain power buffer: single-ended in, single-ended out....

Aikido Long-Tail Phase Splitters
Finally we arrive at Aikido-influenced phase splitters. Actually, blog number 73 held a section titled “Aikido-inspired amplifier for Einstein,” which reveals what Aikido magic can be applied to a long-tail phase splitter. First, let’s look at how a conventional long-tail phase splitter is configured in a transformer-coupled power amplifier....
       29 Oct 2007

How to Design an SE Power Amplifier
I get many e-mails that ask "How do I design an SE amplifier for a...?" The tube is usually some rare item that the correspondent was lucky enough to find at a garage sale or inherit, but which has never been used in a power amplifier. First of all, I must say that asking for a design procedure is in itself something of an accomplishment, as most solder slingers go about in a strikingly different way. They bypass all the boring math stuff and race towards the prize. Their first step is to ask their friends or look on the 'Net for the best tube to use and, for example, decide on a 211. The next step is to search for the best output transformer, say a Bartolucci 23, whose 26W output limit seems perfect for the 211. The next search is for the best power supply and, for example, an Ultra-Mega VoltMaster 240V power supply is chosen, as it seems to be getting the best reviews at DIY sites. The last step is to find a frontend to the drive the output tube; thus, I will get an e-mail asking if a 6SN7-based Aikido line stage amplifier will drive the 211 to full power. I then reply that it cannot, but it is used nonetheless...
      18 Oct 2007

Interesting OTL Design from MJ
The MJ Audio Technology’s tenth issue for this year arrived at my doorstep and it holds an interesting OTL power amplifier circuit. Where audioXpress is passionate about tube/solid-state hybrid power amplifiers, MJ favors tube OTL amplifiers. The design is by a Mr. Kadou Teppei (I assume a Mr. and that is the right name) and it makes use of many SRPP stages and 26HU5 output tubes—neither of which interest me in the least. (Although I have to congratulate anyone who finds a good use for any of the many fine oddball tubes that languish on dusty shelves in surplus stores. For far too many solder-slingers ignore any tube with a heater voltage that isn’t 6.3V, with a few exceptions made for 12.6V.) No, what interests in the circuit is the means by which the output stage is brought into balance...
    10 Oct 2007

Still more phono preamp circuits.
Why? If you have given up on LPs or if you are too young to remember them, then I can understand your bewilderment and I fear that nothing short of sitting you down and playing an LP is going to succeed in changing your perspective. Well, I just listened to a 47-year-old LP: Harry Belafonte’s My Lord What a Mornin’ on RCA. I am amazed by how good LPs can sound....

Differential Input
Blog number 75 demonstrated how a balanced-output DAC could differentially drive a single-ended grounded-cathode amplifier. (Turning balanced into unbalanced is easier than you might presume.) Of course, DACs are not the only balanced-output signal sources, as many new pieces of stereo gear sport XLR output jacks. And any signal generating device, such as phono cartridge or microphone, can be configured to yield a balanced output, as long as no lead is directly grounded. Well, my last entry ended with a teaser: a schematic of a balanced connection from a phono cartridge to the front end of an OpAmp-based phono pre-preamp that differentially drove a single-ended tube Aikido gain stage....

Balance In and balance Out
Some run their entire systems in balanced mode. Making a balanced phono preamp is not difficult, although a few traps await the unsuspecting. For example, in the circuit below, the pass RIAA equalization network works both signal phases against each other, which means that any departure from perfect balance will trip up the equalization....

Single-Gain-Stage MC Phono Preamp
The following preamp builds on the previous design. Two step-up transformers provide a ton of gain and the 12AX7 provides x70 worth of gain, so the total gain would be +60dB at least. Note that there are no cathode resistors on the 12AX7 and that the input transformer is floating, with no ground connection! (If less gain is needed, the input transformer can be removed and the cartridge coil would be left floating. Assuming a gain of about x70 from the 12AX7, the output transformer’s step-up ratio need only be 1:14 to get to a final gain of +40dB.) The output transformer can be a high-quality, nickel-core, as no DC current flows through its primary....
      30 Sep 2007

Hybrid Cascode FET-Tube Phono Stage
Using a low noise FET to help the poor tube along in the noise and gain department is an old trick. Unfortunately, it is seldom done right. For example, below is a hybrid cascode phono stage that uses a FET input device and 6CG7s throughout the rest of the circuit. Wrong? What's wrong with the design? Well, before looking into its failures, let's first review its theory of operation...
    23 Sep 2007

Phono Preamps at Last
Despite predictions to the contrary, spinning black vinyl by the warm glow of vacuum tubes persists. Indeed, both grow more popular with each coming day: Marantz once again sells turntables and new records are pressed daily. And tubes refuse to fade to black and solid-state audio gear is still embarrassingly being advertised as sounding tube-like. Yet, both LPs and tubes are—at least in the public’s view—dead, long dead...

Alternate Approach to RIAA EQ
When breaking up the required inverse RIAA curve into two, we usually combine the first two time constants, 3180µs and 318µs, into one shelving network and the last time constant, 75µs into a single low-pass filter....But we could just as easily split the equalization task up differently, by giving the 3180µs time constant to a 50Hz low-pass filter and combine the 318µs and 75µs time constants into one shelving network that transitions at 500Hz and 2122Hz...

Hybrid Grounded-Grid Phono Stage
This circuit uses a mix of both active and passive equalization and a mix of solid-state and vacuum-state technologies to make an interesting low-voltage, low-noise phono preamp. Today, there are shortages of ultra-low-noise (< 1nV/√Hz at 1kHz), high-performance, high-slew (>15V/μs), low distortion OpAmps; the AD797, LMH6624, LT1115 for example....
      09 Sep 2007

6H30Pi EH Octals
Since reading about the 6H30 being available in an octal envelope, I have been eager to get hold of a few to try out. Why? The sad situation for an octal partisan like myself is that 9-pin twin triodes greatly outnumber octal twin triodes. The only real choices for octal twins are the 6AS7, 6BL7, 6BX7, 6SL7, 6SN7, 6SU7, 12SL7, 12SN7, 12SX7, 5691, 5692, 6080, 6082, B65, and ECC32. In contrast, the list of 9-pin twin triodes is too long to list....

Janus Shunt Regulator Update
Well, I understand that congress critters have a simple formula: for every angry or praise-filled phone call , e-mail, or letter that they receive, there are one hundred voters who felt the same way, but didn't bother to make the effort. So if the same ratio holds for TCJ readers, then there are about 400 to 500 who are confused about how the Janus shunt regulator works. Thus, a review and tuneup are required...

Pentode Driver Tube in the Janus Regulator
An interesting variation on the Janus shunt regulator would be to use a pentode instead of the a triode as the driver tube (the rightmost tube). Pentodes offer some interesting benefits, for example, high gain (much higher than the comparable triode could summon). This is due to the fact that the pentode’s transconductance, unlike the triode’s, is not shunted away by the plate resistor, as the screen shields the grid from the plate’s movements. Second, the pentode’s grid number 2 can see a voltage higher than the plate. For example, in an ultra-linear power amplifier, because of the secondary’s DCR, the plate sees a lower voltage than grid number 2 does. This small oddity can be exploited by using the pass tube’s cathode voltage to drive the pentode’s grid number 2, thereby adding a DC feedback loop, which will help keep the DC operating points in line, as the tubes age or are replaced....
     19 Aug 2007

MC Phono Pre-preamp
For the past three years, I have been subscribing to the superb Japanese magazine MJ Audio Technology, truly the premier audio magazine in the world. This magazine sets a standard that no glossy audio magazine in the West could hope to match. Each issue reviews both tube and solid-state audio gear with a greater depth than a Western audio magazine could ever hope to muster, as each article holds, graphs, interior photos, and usually the schematic for the gear under review....

Grounded-Grid Amplifier Overview
Many tube lovers falsely believe that the only signal input a triode can accept is at its grid. The truth is that the input can be any of its three elements: cathode, grid, and plate. In fact, a balanced signal can be applied to grid and cathode at once, making a single-triode difference amplifier. So why are 99.9% of tube audio circuits based on using the grid as an input? The easy answer is, apart from laziness, the grid offers a high-input impedance, whereas the cathode’s input impedance can be brutally low, depending on the plate load impedance....
      04 Aug 2007

I'm Back
It was in turn hot, rainy, overcast, and sunny—but always wet. The American East Coast is beautifully green, but unbearably humid. The sharp distinction between land and sea blurs in New Jersey, where insects fly/swim (flim?) in a stew-thick steam bath of moisture. Golden California, in contrast, is hot, sunny, and completely dry in the summer. (Well, at least such is the area where I live, i.e. the Central Valley.) By the way, the Golden State got its nickname not from the gold found there a hundred years ago, but for the burnt gold color of the grassy mountains in the hot summer drought...

Higher-Voltage Feedforward-Shunt Regulator
My last blog entry held some feedforward-shunt regulators based on the IXCY 10M45S high-voltage constant-current source. This device offers a 450-volt limit that is suitable to most line stage amplifiers and phono stage, but is too low for most tube power amplifiers...

Low-Voltage Feedforward-Shunt Regulator
On the other hand, when working with low voltages, say below 30V, the IXCY 10M45S is not the best choice, if for no other reason that its transconductance is quite low, being only something like 300mA/V...

Push-Pull/Series-Shunt
High-Voltage Regulator

If we look at the higher-voltage feedforward-shunt regulator schematic shown at the top of the page, we will see that we have all the parts to make an SRPP-like higher-voltage regulator, whose output can draw current in both directions...
       30 Jul 2007

Wrong Turn
I have been thinking about the passive feedforward shunt circuits and I am not happy with the last variation I offered. The more I thought about it, the less I liked the following circuit.

Mésalliance* or Hybrid Heaven
Once again, you are invited to a wedding between two dissimilar technologies (solid-state and vacuum-state) and two different shunt techniques (feedback and feedforward) into one high-voltage regulator. But first, we need to look at a circuit from the last blog.
     17 Jul 2007

Hybrid Feedback/Feedforward Regulators
The idea here is that we can wed two dissimilar technologies (solid-state and vacuum-state) and two different shunt techniques (feedback and feedforward) into one high-voltage regulator. In the schematic below, the vacuum tube does the shunting and it handles the high-voltages and high heat dissipation; the IC OpAmp provides the low-noise and high-gain amplification needed to drive the feedback loop; and the zener diode establishes the OpAmp’s power supply voltage and defines the internal voltage reference used by the regulator to maintain a fixed output voltage....
        04 Jul 2007

Passive Feedforward Shunt
Active versus passive: that's the choice we often face in audio design, and each approach holds its advantages and disadvantages. Passive filters, for example, are usually big, heavy, and expensive; whereas active filters are often smaller, lighter, and cheaper. (Well, this holds true at least at lower frequencies; at higher frequencies, the tables may turn, with the passive filter being smaller, lighter, and cheaper.) In addition, the passive filter is less likely to reach voltage clipping or current saturation, whereas the active filter is likely to be easily overdriven. In other words, there is no clear winner overall, only winners for each set of objectives. Now, the question is, "Which is better for power supply noise elimination a passive feedforward shunting circuit or an active feedforward shunt regulator?" Wait a minute, there’s no such thing as a passive feedforward shunting circuit. Oh, did I forget to mention that I created one?...

Feedforward Shunt Regulator Explication
One thing we tube fanciers do not have to worry about very often is losing too much B+ voltage. In fact, we usually face the opposite problem: too much B+ voltage. For example, the feedforward shunt regulator works on the assumption that the voltage-dropping series resistor should equal the inverse of the shunting device’s transconductance. But what if we were to use a resistor twice as large (say 200 ohms, rather than 100 ohms), with a triode that offered a transconductance of 10mA/Volt?

Janus Shunt Regulator
The feedforward shunt regulator only looks forward, creating a counter noise signal to null the original power-supply-induced noise. Unfortunately, it is blind to what develops on the other side of the series resistor. In contrast, the feedback-based shunt regulator sees only the disturbance on the output side of the series resistor. Now, what would happen if we wed the two approaches together?
     24 Jun 2007

More Moskido
The Moskido amplifier is creating a stir in Tube Land, a petite commotion in absolute terms, but relative to the tiny size of Tube Land, it’s a fairly big stirring. Well, at least that is what my e-mail is telling me. Considering that no topic is more beaten-to-death than tube-solid-state hybrid amplifiers—doesn’t seem as if every issue of every DIY audio magazine drags along some new hybrid amplifier design? I can remember seeing at least twenty designs and always it’s the same SRPP or grounded-cathode amplifier driving some MOSFETs. There have been some notable exceptions, but most have a dreary sameness to them. In fact, shouldn’t we have a new acronym, DGNNAHA, for Dear God No, Not Another Hybrid Amplifier?

Small Moskidos
I am not sure whether I have posted the following schematics. (I have just drawn and posted too many schematics.) The idea behind these circuits is that a single power supply is better than three power supplies—well at least in terms of hassle. So, rather than having to provide a heater, tube, and solid-state output stage power supplies, having a single power supply all three circuit elements.
       21 Jun 2007

MOSFET + Aikido = Moskido
Bob Prangnell, long-time TCJ reader (a “tcjer,” what do you think of the term?) in New Zealand, has coined a new word, MOSKIDO," and I like it a great deal. In his own words: BTW I thought of a good name for the hybrid....MOSFET-aikido....mos-aikido... MOSKIDO! (without the annoying high pitched buzzing)"...

Audio Note's Feedforward Shunt Regulator
Kamon, a tcjer in Thailand, was kind enough to send me some information on Audio Note's patented feedforward shunt regulator. I didn't have any luck at the Google Patent Search website for this patent, no doubt because it was originally patented in England. And no doubt one of Great Britain’s tcjers will find the link to a PDF of the complete patent and relay it on to the rest of us...
       10 Jun 2007

Shunt Regulators
Two events force me to take up the subject of shunt regulators again. The first is that audioXpress magazine has just discovered the shunt regulator; or rather, they have re-discovered it, as the title to the article by Ed Simon in the June 2007 issue is “Shunt Regulator: The Almost Forgotten Circuit.” The article is interesting, although not especially well written...
       03 June 2007 PDF

PDF Version
Welcome to the PDF version of the Tube CAD Journal. Yes, it’s true, I am back to posting both a PDF and a HTML version. Why? (Believe me, I as ask myself the same question.) The answer is better printing. As I have mentioned before, this website gets printed by many of its readers. (Now, if only I could get a kickback from the ink and toner manufacturers.) I have read that a 6- inch ream of paper is not enough to print the whole site! That’s a lot of paper, a lot of schematics, and a lot of words...

Schematic Typo
An observant reader spotted a schematic error I had posted in Blog 107 (thanks Steve). I was trying to illustrate the distortion shape for the cathode follower, the grounded-grid amplifier, and the wedding of the two circuits, the cathode-coupled amplifier. I got the distortion shape right, but not the phase; both the cathode follower and the grounded-grid amplifier preserve the phase of the input signal at their outputs...

Zune Survey
Recently I received via e-mail an invitation to tell Microsoft how they could improve their Zune MP3 player. Great I thought, as I had a list of improvements and Microsoft’s reward of a $10 Amazon gift card for completing the questionnaire didn’t hurt either. Well, I answered a few questions and I was kicked out of the survey. Why? My best guess is that I had established that I knew what I was talking about and that is not what Microsoft wanted to hear...

Recommended
Cathode-Coupled Amplifier Circuits

I know that many who find this website through Google searches quickly get lost in it; there is just an enormous amount of material. I can usually spot e-mail from these readers. It isn’t hard, as they often begin with asking which guitar amplifiers or microphone preamps I sell. Or they ask a question that the previous blog was entirely devoted to answering.

But then there are a few readers—who might have seen my name mentioned at another website or forum—that read this blog in hopes of finding the elusive solution to their audio problem, the problem of acquiring the best. They rapidly lose patience with my flood of circuit variations; they long for one circuit, and only one circuit, the Absolute Circuit (hey, what a catchy name for a website, www.TAC.com too bad it is already taken). Why would they want to see anything but the best circuit. They want the answer—not more questions—and they want it now...
      21 May 2007    PDF

Wrong turn
Blog 105 dispirits—well, at least me. First there was the uproar over my seeming endorsement of Bruce Rozenblit’s renaming of the cathode-coupled amplifier the “grounded-grid amplifier,” which blog 106 addressed, with my blunt condemnation of Bruce’s taxonomic blunder. Upon thinking that I had smoothed enough ruffled feathers and that I had proved my tube-topology systematics* commitment, I went on to describe several variations on the cathode-coupled amplifier. I wasn’t done undoing problems with blog 105, however...

Cathode-Coupled Amplifier & Low Distortion
When we read that the cathode-coupled amplifier yields low distortion, the first question we need to ask is: "Compared to what? a grounded-cathode amplifier, a cathode follower, a grounded-grid amplifier, a plate follower, a White cathode follower, an SRPP, a cascode amplifier…?" The answer is that it offers lower distortion than a comparably arranged grounded-grid amplifier, with the same B+ voltage, plate resistor, triode, idle current, and output voltage swing...

Aikido cathode-coupled amplifier
The input is AC coupled and a two-resistor voltage divider creates a 50Vdc reference voltage for the cathode-coupled amplifier, which means that the two-resistor feedback network will ensure a matching 50Vdc on the other grid, which in turn ensures that the output stage’s output is centered at 150Vdc. The Aikido cathode follower works to undo the near 100% power supply noise that appears at its input by injecting all the power supply into the bottom triode...
       14 May 2007

Turmoil in Tube Land
It seems that I have caused turmoil in tube land; the angry towns people are taking up pitchforks and torches, and soon they will break down my door. What was my offense? Surely it would have to be something spectacular, like describing a D-getter as a halo or confusing the 12AV7 for the 5965 or by gratuitously quoting Shakespeare. No, much worse than any of these, my sin was not being sarcastic enough...

Cathode-Coupled Amplifiers
Posting a blog entry without a schematic would be almost unthinkable, so let’s have some more fun with this topology...

Now, what if we didn't need any gain, but we did want the widest bandwidth possible. The great feature of a cathode follower is its low-input capacitance. This makes sense, as the grid-to-plate capacitance is small and the larger grid-to-cathode capacitance is mitigated by the cathode following the grid in phase. Well, what if the plate also followed the grid? Such an arrangement would effectively nullify the grid-to-plate capacitance. The following circuit shows how this can be realized...
       08 May 2007

Common-Cathode Amplifier
I am a big fan of the common-cathode amplifier (also known as the “cathode-coupled” or “grounded-grid” amplifier—thanks Bruce). This topology offers low distortion, no phase inversion at the output, and a wide high-frequency bandwidth. This last characteristic is due to a very low input capacitance for the input, as no Miller-effect capacitance is created. Additionally, a negative feedback loop can readily be applied, because of the inverting, high-impedance input presented by the rightmost triode...

Slew rate
Before moving on to plate-follower topology, let’s look into an important design issue when designing any follower circuit. Yes, it sounds like heresy, but a cathode follower often stumbles when asked to drive a large capacitance. Quickly charging and discharging capacitance requires current, not low output impedance. For example, a 12AX7, when used in a cathode-follower circuit, presents an output impedance of about 600 ohms, which seems plenty low for driving a 1,000pF load, as these two values seem to imply a -3dB frequency of 265kHz...

Plate Followers
The plate follower (a.k.a. anode follower), like the common-cathode amplifier, is a much-too-overlooked circuit. In many ways it is the inversion of a cathode follower: where the cathode follower takes its output at its cathode, the plate follower takes its output at is plate; where the cathode follower preserves the input signal’s phase, the plate follower inverts; where the cathode follower delivers an output signal close to, but never equal to or greater than unity, the plate follower can readily impart a gain of unity or greater than unity; and where the cathode follower presents an ultra-high input impedance, the plate follower offers an impedance equal to its series input resistor, which is usually lower rather than higher, so as to ensure a greater high-frequency bandwidth, due to Miller-effect capacitance...
         06 May 2007

More Cathode-Follower Stuff
(I hope that I do not hear from Bob Pease's lawyer for using the word "stuff" in the title. Speaking of Bob Pease, aka RAP and a long-time contributor for Electron Design magazine, I am a long-time fan of his...

Back to tubes, last time we took a large back step, reviewing cathode follower topologies, in preparation for moving forward to hybrid, DC-coupled throughout, tube-based, unity-gain buffers—which is much harder to say quickly than “No-Gain—No-Pain” line-stage...

Pentode-Based Followers
Pentodes do not work that differently from triodes in a cathode-follower circuit, unlike grounded-cathode amplifiers that use pentodes to realize high gain, but at the cost of higher distortion and output impedance than a triode would give in the pentode’s place...

Super Symmetrical Cathode Follower
This modified cathode follower both rejects power supply noise and produces less distortion. The guiding principle behind the circuit is that a simple cathode follower is incased in a complex circuit, with the topmost triode working to provide a constant cathode-to-plate voltage across the simple cathode follower’s triode, while the two bottommost triodes work to define a constant-current source in place of a cathode resistor...
      28 Apr 2007

Teflon Coupling Capacitors
I own three different Teflon-based coupling capacitors. I have tried them all, but I do not use them. Why? Although they each can do certain things better than any other coupling capacitor, overall they sound wrong. I mentioned this to my friend Chris and he told me that I probably hadn’t given them enough of a break in period. I asked how that should take; his reply was sometimes months. To make things even worse, he pointed out that each time they are energized, they must undergo a mini-break-in period, sometimes lasting an hour or so. Well, I do not have the time or patience for such recalcitrant capacitors, so they languish in my part pin...

More No-Gain—No-Pain
Before moving forward with the last blog entry's circuit, let's back up a bit first. Tube-based buffer line stages that provide no voltage gain are rare. As far as I know, no commercially-offered, unity-gain, tube-based buffer exists. This is an odd situation, as passive line-stages are popular, which proves that extra signal gain isn’t always required. Yet passive line stages often prove inadequate, incapable of driving high-capacitance cables or low-input impedances. (Additionally, active line-stage amplifiers can often impart the missing heft and solidity that is missing in many passive setups, even when the load is wimpy, but at the cost of some added noise and distortion.)..
       22 Apr 2007

Passive Line Stages
and the TCJ Stepped Attenuator

Passive line stages are popular, with good reason. Many CD players and stand-alone DACs deliver a healthy 2V to 3V of output voltage and the average amplifier can be driven to full output with only 1V of peak output signal. If there is no gain, there cannot be much distortion. No line amplifier is distortionless, whereas a quality stepped attenuator’s distortion cannot be readily measured. Well, in practice, things can dirty this clear solution. Still, it is hard to argue against not having to spend a bundle on an active line stage amplifier when no extra voltage gain is needed...

NO GAIN, NO PAIN
Speaking of no gain—no pain, back in October of 1998, at the GlassWare website, in the Tube Circuit of the Month section, I described a tube-based, unity-gain buffer circuit that named the NO-GAIN—NO-PAIN line stage amplifier The circuit was simple enough: a triode-based cathode follower (with a small unbypassed cathode resistor) terminates into a complaint constant-current source, which terminates into a -12V power-supply rail...
     16 Apr 2007

Patent number: 3,184,687
AKA, SRPP meets Ultrapath

Here is an interesting patent by Charles A. Wilkins, a fellow who worked for Amperex and upon whom I will defineatly do more patent searches. Interesting because I came up with the same circuit on my own about 20 years ago, but I never could get it to stop oscillating (big, nasty 50Vpk swings at >500kHz)...
      01 Apr 2007

A Phone Call to Bill Perkins
“Let me ruin your life,” is what I said on the phone the other day to Bill Perkins of PEARL Audio fame. Stop and think about it: What would it take ruin someone’s life? What could I tell him that would be so devastating? Something big and really nasty? Often, yes, but not always...
     25 Mar 2007

Tube Tidbits
   Make magazine delivers once again. No, not a new tube amplifier, but a...

   I recently picked up a Slim Devices Squeezebox and I love it...

     David Lin, of Firestone Audio, has created a gem of a hybrid headphone amplifier, the “Little Country” headphone amplifier...

Current-Output Amplifiers
Since my last posts on this topic, I have received a few e-mails from readers interested in building a current-output tube power amplifier. My recommendation to all of them was to start small; for example, build a current-output headphone amplifier before you build a 100W power amplifier, as the headphone amplifier will prove both cheaper and easier to build, yet it will give a good sampling of what a current amplifier sounds like. In addition, with their single drivers and no crossovers, headphones make an excellent load for a current-output amplifier. Moreover, it would take little effort to make a tube-based, OTL, single-ended or push-pull current-output headphone amplifier; whereas the equivalent power amplifier would require a great deal of effort and care...

XPP Amplifier
Like the duck-billed platypus, the XPP amplifier does not easily fit in any category. It is either a voltage-output amplifier with a high-output impedance or a current-output amplifier with a low-output impedance, depending on your perspective...

Zen-like Current-Output Power Amplifiers
Here is a Zen koan, "What is the sound of one MOSFET amplifying?" Nelson Pass’s Zen amplifier uses a single power MOSFET, loaded by a MOSFET-based constant-current source, to create a single-ended power amplifier. I have scrutinized and offered some variations on the Zen amplifier back in the early days of this blog (number 5). Well, could a current-output Zen-like power amplifier be created? The answer is, of course, yes. In fact, the current-output version offers a few advantages over its voltage-output brother...
      08 Mar 2007

Current-Output Tube Amplifiers
Current-output amplifiers, although exotic and rare, are no more difficult to design than voltage-output amplifier; they just run on different assumptions. A good starting point is to take all that you know about voltage amplifiers and stand it on its head. Where a voltage amplifier strives to deliver a minuscule output impedance (a high damping factor, in other words), the current amplifier delivers an ultra-high impedance output. Where a voltage amplifier runs out of output voltage, the current amplifier runs out of current. Thus, a near-ideal voltage amplifier can dump near-infinite current, but finite voltage; whereas the current amplifier can spew near-infinite voltage, but limited current...

Current-Output Amplifier Design Issues
The unspoken assumption here has been that the current amplifier, like the conventional voltage amplifier, would receive an input voltage signal, but a current amplifier could be designed to accept a current input signal instead; thus, creating a current-to-current amplifier, not a voltage-to-current amplifier. Why would someone want such a thing? One possibility is those who have current-output DACs as their frontend. Current-output DACs, such as the TDA1541 and PCM1704, source and sink 1mA at full output, which a current-to-current amplifier could magnify to 1A to 5A. In other words, such an amplifier would need a current gain of at least 1,000...

Living in a Voltage-Centric World
Not only do most of us primarily think in terms of voltage, but we have set up our electrical power stations and houses and electrical appliances to match that preference. Power stations put out a fixed voltage, 120v in the USA, 100v in Japan, and 230v in most of Europe. No matter were you live, all the wall sockets in your house are wired in parallel, so that all of your appliances will see that same fixed voltage that the power station went to such great lengths to ensure...
       18 Feb 2007

More Feedforward Shunt Regulators
I have been thinking about the feedforward shunt regulator (FSR) and I thought it would prove interesting to convert the solid-state circuit from EE Times's Website www.planetanalog.com—shown below—into a tube circuit. In other words, create a tube-based OpAmp to drive a power triode’s cathode to follow its input signal...

First-Watt Amplifier
The justly famous and well respected Nelson Pass has a new amplifier design: it is a current amplifier rather than a voltage amplifier. Where a voltage amplifier offers a low output impedance and steady output voltage but a varying output current, the current amplifier offers a high output impedance and fixed current but a variable output voltage. This trick is accomplished by the feedback loop monitoring the current out of the amplifier, not the voltage. Interestingly enough, the worst thing that can happen to a voltage amplifier is a dead short on the output, as it implies an infinite current draw; inversely, the worst thing that can happen to a current amplifier is an open circuit at the output, as it implies an infinite voltage. In other words, the current amplifier is the alternative universe reflection of the voltage amplifier.
05 Feb 2007

Broskie OTL Update
Soon after creating the Broskie OTL, my first thought was, How good a headphone amplifier would it be? Well, I finally got around to running some SPICE simulations on the Broskie OTL circuit. Interesting, indeed. The distortion is quite low, as we would expect from the higher gain that it realizes compared to the simpler Broskie cathode follower, for example. What I didn’t expect was the harmonics to reveal a single-ended influence. Usually, a push-pull amplifier’s harmonics look like a saw’s teeth, with the odd-order harmonics peaking high above the even-order harmonics...

Feedforward Shunt Regulators Update
I know it has been only a week since I posted my last entry on feedforward shunt regulators, but I expected more than just three e-mails on the subject. How do you setup an adjustable-idle-current feedforward shunt regulator? The answer is easy. Just take some resistance from the top voltage-dropping resistor and add it to the bottom series resistor; then add a potentiometer and a few resistors, as shown below...
       31 Jan 2007

Shunt & Feedforward Shunt Regulators
Let’s start with some perspective. If you were a high-voltage regulator manufacturer, and if your adjustable voltage regulators had to provide a wide output voltage range, say 100V to 500V, and sustain a current draw up to 300mA, the last regulator topology you would chose is the shunt regulator’s. Why? Isn’t the shunt regulator the new, hot, must-have topology in high-voltage regulators?...

Feedforward Shunt Regulator
A related regulator design is the feedforward noise canceling circuit. Like a conventional shunt regulator, the feedforward shunt regulator employs a series voltage-dropping resistor. But unlike the conventional shunt regulator, the feedforward shunt regulator receives its error signal from the other side of the series resistor. The theory is that if the exact same signal is imposed on two identically-valued resistors, then the signal will null at the output of the series resistor, as precisely the required current fluctuations needed to counter the raw power supply noise will have been generated by superimposing the power supply noise across the bottom resistor. Brilliant, don’t you think? Very Aikido, indeed...
      23 Jan 2007

Hybrid OTLs
The Broskie cathode follower uses only two triodes and converts a balanced input signal into a single-phase output signal. Surprisingly, a single 6DJ8 used in this topology can beautifully drive a 300-ohm headphone, such as the Sennheiser and AKG models. Driving 8-ohm loudspeakers, or even Grado 32-ohm headphones, will take much more muscle. The 6DJ8 can be replaced by a beefier tube, such as the 6AS7, 6H30, 6C33, 12B4, 5687...but none of these tubes is up to the task of driving inefficient loudspeakers directly. Compared to modern-production, poor-quality vacuum tubes, even the best MOSFETs are dirt-cheap; for example, the excellent BUZ901 cost less than $10 each and mediocre IR HEXFETs cost only a few dollars each.
        07 Dec 2007

OTL Amplifier Design Revisited
The last schematic I posted held a few typos, which have been corrected. In redrawing the schematic, however, I realized that it was too big a jump for many, and that I needed to work my way up to this schematic in stages, starting with the underlying OTL topology without the crossover-notch-eliminating circuitry. At the same time, I saw that the schematic was too simple, that I had left out too many subtle, but important features for actual construction. So, let’s start anew. Below is the new OTL topology in its simplest terms...
      30 Dec 2006

More Zune Thoughts
Yes, I still love my Zune. I have filled its hard drive with 27.5 Gig of music, hundreds of albums, and 4,273 songs. Rock and Popular Vocal predominate, followed by World, Blues, then Jazz, and, finally, classical. Interestingly, this is the exact inverse ratio of my music collection. How or why did this happen? An MP3 player’s music library, like the books and magazines one takes aboard an airplane, should be a light, frothy, insubstantial mix, as the many short listening sessions and high background noises that accompany moving about in the world do not befit anything as grand or long as Mahler’s 3rd Symphony (97 minutes), just as Tolstoy’s novel, War and Peace, deserves more than 40% of one’s attention. On the other hand, a Zune or iPod is perfect for harvesting interesting snippets from old familiar albums...
     27 Dec 2006

Triode Centennial
Held at Beukenhof, in the Netherlands, from November 30th to December 3rd of this year, the European Triode Festival (ETF) has come and gone. Sadly, unlike last year, I was not able to attend the event. Nonetheless during the end of November and the beginning of December, as each day of the festival passed, my thoughts were drawn to all that I was missing: lively conversations, great beer, tube-related presentations, tube equipment shootouts, and the chance to see again so many tube enthusiasts and friends who made last year’s ETF so memorable for me...

Crossover-Notch Distortion (continued)
I know that many ardent tube lovers tuned out on the topic of crossover-notch distortion and mixed-mode amplification. Admittedly, the topic is a technical one (and the many transistor-based examples didn’t help). Nonetheless, unless an amplifier is running in pure class-A, all push-pull amplifiers will face either the notch or gm doubling distortion. So what is the big deal? Shouldn't we just run the amplifier in class-A and be done with it? Well, the big deal consists of expense, weight, bulk, and heat—lots of heat...
     20 Dec 2006

"What? No iPod?"
No, I do not own an iPod, but twice I have been tempted to buy one. Indeed, twice I have gone to the electronics store, money in hand, ready to buy an iPod; and twice I have walked away with the cash returned to my wallet. I just couldn’t do it. I couldn't actually purchase one, because while I liked the iPod’s sleek design and hand-feel, I didn’t like the sound. I have never heard an iPod better my Sony Minidisk player which, although not perfect, holds some nice features... Still, I longed for more storage capacity and a bigger display, the latter feature being more important than the first...
      13 Dec 2006

More Class-XD™
In general, a sharp divide separates solid-state from vacuum-state electronics, but only on a physical level. Topologically, the division is much more rounded and shallower, if not totally nonexistent. Indeed, the only sharp, unbridgeable divide is the P-channel versions of the MOSFET and FET, and the PNP version of the transistor that finds no equivalent in the vacuum tube world...

In other words, just about any technique or topology can be translated from one device technology to another. For example, I have performed the electronic-alchemist’s trick of converting glass into silicon and solid-state into vacuum-state many times in this journal: the tube-based White cathode follower, SRPP, Broskie cathode follower, and Aikido amplifier into solid-state circuits; the solid-state-based Taylor and the Macaulay amplifiers into tube-based amplifiers.
      09 Dec 2006

Amplifier Class XD™
Although a new product, the 840A has already won many recommendations and awards, including the CES Innovations 2007 Award for Design and Engineering. (Which is truly impressive considering that we are still living in the year 2006.) So, what is class XD™ and how did they do it? The white paper at Cambridge Audio's website reveals the important details, including that Douglass Self, whom I much admire, now works for Cambridge Audio and that the XD™ project took two years to come to fruition. But before you read the whitepaper’s many pages, you should read just one page I wrote almost five years ago in the Tube CAD Journal.
       02 Dec 2006

A New High-Voltage Regulator
Ah, the thorny topic of high-voltage regulation. There are so many options: all-tube, all solid-state, some hybrid of the two technologies, series, shunt, DC regulation, AC regulation, linear, switching… Then there is the danger: a mis-wired high-voltage regulator often smokes, if not catches on fire, or—failing such a dramatic exit--when poorly laid out, oscillates wildly. So where does one begin? Or should one begin? Voltage regulators are high-gain, unity-gain amplifiers, which means that they are also high-feedback amplifiers...
     15 Nov 2006

OTL headphone amplifier circuit revisited
In spite of the little time spent writing the blog entry, the 6BQ5-based headphone amplifier was actually carefully designed, with the part values not casually chosen. The result is that the headphone amplifier acknowledges and anticipates the power supply’s failings, presenting the requisite amount of injected power-supply noise into the output stage to cancel the power-supply noise from the output. Below is the bad schematic that I had posted...

6BQ5 issues
John Atwood has set me straight about the old Svetlana SV83. They are relabeled 6P15P pentodes, a Russian equivalent of an EL83, whereas the 6P14Ps are the Russian equivalent to the EL84...

6N1P Input Tube
One reader asked if the 6N1P could be used instead of the 12AX7. It can, but at a price. First of all, it will offer less gain, which means less feedback will be available, which might be important to those seeking the lowest output impedance and distortion. Second, the 6N1P should be run much hotter than a 12AX7, 5mA per triode, not the 1mA that a 12AX7 can get away with. So what’s the big deal, 10mA versus 2mA?...
      04 Nov 2006

Non-Aikido Headphone Amplifier
I greatly like the 6BQ5 (aka 7189, EL84), as it seldom disappoints. (I am not alone. The Beatles loved this tube and their old Vox equipment was filled with them.) The only real downside to this tube is that NOS (new old stock) 6BQ5s are becoming quite expensive, many of which sell for over $50 each. (I understand, however, that the current production JJ 6BQ5s sound decent and I remember liking the sound of the Russian SV83.) So, let’s use the 6BQ5/7189/EL84/SV83 as the output tube. Like the 6BQ5, the 12AX7 is a famous and ubiquitous. It is a low-current, high-mu, dual triode that is still in production. Since its mu is 100 and since one of the design goals was low-gain, this tube would seem to be out of the running, but in a feedback-based headphone amplifier, the extra gain will power the feedback loop, so we can still use it as the input/driver tube.
     31 Oct 2006

How Do I Build...
“How do I build ____?” is the first line of many e-mails that I receive, whereas only few have asked, “How do I understand ____?” Nothing exceptional here. My guess is that for every tube fancier who seeks to understand how tube circuits work, there are at least one hundred other tube aficionados who just want tube gear, preferably without the conceptual overhead. (Imagine if one had to understand how a microwave oven worked before being able to buy one.)...

ThermalTrak Output Devices
(Yes, this is entirely solid-state affair, but elements can be profitably lifted for hybrid-amplifier use.) The schematic came from a flyer that I was given at the 2005 CES at the ON Semiconductor room. At first, I thought I was looking at a new brilliant amplifier frontend, but soon that appraisal was replaced with the observation, “Can’t work, the input stage is totally botched...
     22 Oct 2006

New Mono Octal PCBs
A new, shiny, revised batch of mono octal Aikido PCBs has been sitting in my lap for over a week now, but I only just finished the user guide for the boards today. The boards are in revision A, which includes the same White-cathode-follower output-stage option as the new 9-pin mono boards. And like the 9-pin boards, they now sport breathing holes around the tube sockets, so that brutal and hot-running tubes like the 6BL7, 6BX7 and 6AS7 can more readily be used. In other words, the new mono octal PCBs are headphone-amplifier friendly, which leads to the following design techniques...

Optimal White Cathode Followers Revisited
I coined the phrase “optimal White cathode follower” to describe a method to find the optimal value for a White cathode follower’s plate resistor, Ra. The optimal value is the one that yields the largest, most symmetrical voltage and current swing from both top and bottom triode, in other words, an optimally adjusted push-pull follower. In my article on follower circuits, I determined that the optimal plate resistor equaled the inverse of the triode’s transconductance, or rp/mu..

Improved Followers (line stage and headphone amplifier emphasis)
I don’t like to attach my cathode followers naked, preferring to use a small-valued cathode resistor as a buffer instead. This extra resistor greatly troubles many readers; isn’t less more? No, not always; sometimes more is, indeed, more. The added cathode resistor further linearizes the cathode follower and buffers it from excessive capacitance. In fact, the Aikido amplifier uses this technique as part of its ear-pleasing arsenal. In the schematic below, we see both a cathode follower and a White cathode follower with extra cathode resistors...
      13 Oct 2006

Genius Grants
In giving my review of Morgan Jones’s new book, Building Valve Amplifiers, I have somehow given the impression to several readers that I sell his book(s) at my Yahoo! Store. I do not; in fact, I do not sell any books. Why not? The sad fact is that I am not even smart enough provide one of those paying links to Amazon.com, which provides a small kickback every time someone follows the link to Amazon's site and buys a book. Oh well. Speaking of not being intelligent enough, a good friend of my mine, Glenn, called the other day to offer solace for our not receiving a MacArthur Foundation (Genius) grant—once again...

Tube-based voltage regulators
Given the choice between series and shunt regulators, I like shunt regulators best, but they are not perfect. One topology I would like to experiment with is an SRPP-based regulator that I have mentioned before. Such a regulator could pull the output back in line in both directions, whereas the series can only pull up and the shunt can only pull down...

Aikido Headphone Amplifier Recipe
I knew that my last entry would not be enough, that readers eager to build a tube-based headphone amplifier would want to see a specific circuit, with all the part values and tube types inviolably carved on a titanium reliquary. Only a 6CG7-input tube with 470-ohm cathode resistors, only a 6922-output tube with a 10mA idle current, only a 47µF coupling capacitor… Anyone who has read the Tube CAD Journal even casually will know that I am big on circuit topology and hesitant to proscribe any single fixed schematic, whose values are predetermined and immaculate, preferring to array many different possible circuits. Why? Two reasons: the first is that I am trying to expand the universe of possible tube circuits and the second is that I have no faith in any circuit configuration absolutes...

Reiffin's SE Cathode-Follower Power Amplifier
Martin Reiffin, who holds many patents in electronic circuit topology, has a patent (United States patent 6,265,938) that describes a "Linear high-voltage drive stage and cathode-follower high-fidelity power amplifier implementing same." ...
     28 Sep 2006

They're Back and They're New!
There are three topics that that provoke a big e-mail response: tube phono stages, tube headphone amplifiers, and tube shunt regulators. This website could devote every word and schematic to these three topics (and only these three) and 80% of its readers would be ecstatic. Unfortunately, an Aikido-based phono preamp would require two Aikido amplifiers (four tubes per channel) and a shunt regulator could never be built from the Aikido topology. That leaves an Aikido-based tube headphone amplifier. Unlike the other two, this project is easily doable; in fact, many have built headphone amplifiers with the existing Aikido PCBs. So I started thinking about what would need to change to make headphone-amplifier friendly Aikido boards...

Cathode-follower push-pull
power amplifier design

Although much of what follows was covered in the blog entries on single-ended, cathode-follower power amplifiers, it is worth hammering home again. First the big fallacy: since a cathode-follower-configured amplifier presents a low output impedance (Zo), the output transformer’s primary impedance must be equally reduced, which would allow the use of 300-ohm primary instead of the 3,000-ohm primary usually needed for a 2A3-based push-pull amplifier, for example. This false conclusion is as seductive as it is false...
     19 Sep 2006

Actually Building Projects
Speaking of actually building a tube Hi-Fi project, instead of just storing parts in your closet, my enthusiastic recommendation of Morgan Jones’ book, Building Valve Amplifiers, becomes even more persuasive. I get e-mails all the time that outline the most amazingly majestic tube-project that each writer hopes to build...

Cathode-Follower Output Stages Once Again
It has been a while since I last touched on the topic of using the cathode-follower topology in an output stage. I had covered mono-polar power supplies and positive-and-negative bipolar power supplies, which left out the bipolar power supply that holds two positive voltages. I had saved the best for last, but I never got to the end...
    13 Sep 2006

Book Review: Building Valve Amplifiers
The title of Morgan Jones’ 2004 book, Building Valve Amplifiers, reveals it all. The title isn’t Understanding Valve Amplifiers nor is it Designing Valve Amplifiers. The operative word in the title is “Building,” which includes both construction and testing. Jones’s previous work, Valve Amplifiers, quite satisfactorily covers understanding and designing tube amplifiers, but is thin on building practice; in contrast, Building Valve Amplifiers is a practical guidebook that carefully explains the ins and outs of constructing, modifying, repairing, and testing of tube amplifiers...
    05 Sep 2006

Mystery Solved
The mystery is solved. This whodunit involved many hits, mobsters, missing links, and much head scratching on my part...

More Balanced-Output DACs and Tubes
Last time we left off, I was extolling the virtues of un-bypassed cathode resistors in a grounded-cathode (and differential) amplifier for their ability to linearize the output. I should also mention that these resistors have downsides, as they increase the output impedance, reduce the gain realized, and worsen the PSRR...
        28 Aug 2006

Death, Taxes, and Spam
It seems that that the Tube CAD Journal may have more readers in Russia than I thought. Last year, I posted a blog entry that made the modest proposal that Mafia types should perform "good" works for the general public by eliminating spam-kings pro bono...

Balanced-Ouput DACs & Diff Amps
When presented with a balanced input signal source, most tube fanciers’ first response is to turn to the classic long-tailed differential amplifier. The reason is not hard to find. The differential amplifier accepts a balanced input signal and delivers a balanced output signal. In addition, it offers an excellent CMRR figure, which means that common-mode noise will be dropped from its output...
        21 Aug 2006

Balanced-Ouput DACs and Tubes
Back to DACs. Last time we looked at this topic, we saw tube circuits picking up their input signal from DACs with a single output, which was either a voltage or a current output. So far, everything has been fairly straightforward; however, when we come to DACs with balanced outputs, we face a few twists. For example, most CD players and stand-alone DAC units that sport balanced-output DACs are not balanced themselves at their outputs, with a single RCA jack per channel as an output. Here is another oddity: the output of a non-balanced DAC is usually referenced to ground, with an output that expects to see 0V. On the other hand, balanced outputs are usually referenced to some positive voltage, say 2.5V...
       12 Aug 2006

Transformer-Coupled Aikido
Capacitors stink. And who—other than capacitor manufacturers and vendors—would argue otherwise? Yes, I know that resistors, inductors, transformers, and all active devices, such as transistors and, yes even, tubes also fall short in their own way. So, a more accurate phrasing might be: capacitors, relative to other electronic components, stink...
        05 Aug 2006

Einstein's amplifier
No, Albert didn't design this amplifier; he enjoyed it. Albert Einstein was a dedicated, even ardent, music lover and he played a mean violin as well. Music was easily as important to him as math and physics. (Einstein also loved to sail.) Einstein had excellent taste; for example, he was a huge Mozart and Bach aficionado and, like all right-thinking music lovers, he couldn't tolerate Wagner...

Aikido-inspired amplifier for Einstein
If I were asked to build an amplifier for Einstein—what a great honor it would be (the only person alive today who would justify a similar request and prove such an honor is physicist Craver Mead)—it would look something like this...
       28 July 2006

NOS
"NOS" used to mean "New Old Stock;" today it also means "No Over-Sampling." Once again, we boldly lunge into the past. Audio Note and Zanden Audio and others are making 1X, no oversampling, no digital filters, standalone DACs. Why? They are said to sound much better than the modern alternative. Do they? I don’t know, as I have yet to give them a listen. Certainly, one real advantage to this approach is that it is a breeze to design such a DAC, as so much has been removed...
       07 July 2006

Low Output Impedance
I had planned to dive into the subject of balanced output DACs, but a reader sent me a troubling e-mail. He asked why I keep recommending using the Aikido amplifier as the front end of an SE power amplifier, when the Aikido holds such a high output impedance.

What!!!!!

Aikido Cathode Follower
Here is a new circuit born from the rib of the Aikido amplifier topology. In other words, it is basically the last half of the Aikido amplifier, but with a subtle twist. A simple, textbook cathode follower consists of only two key components: a triode and a cathode resistor. The cathode follower’s gain is always less than unity, its output impedance is roughly the reciprocal of the transconductance, and its PSRR is roughly equal to the inverse of the triode’s mu.
       01 July 2006

DACs & Tubes / Bodies & Souls
Digital-to-analog converters (DACs) and vacuum tubes: it’s odd that I haven’t covered this topic before. This topic is so big that several blog entries will be needed no doubt. When I mentioned that I was going to cover this topic, I was asked, "What’s a DAC?" My answer tickled me so much that we will take a small detour on the way to DACs and tubes.
       27 Jun 2006

Ultra-Linear Aikido
I do not know why I did not think of creating an ultra-linear Aikido amplifier earlier. I must be slacking or it might my triode-centric worldview blinded me to the possibility. In any case, the idea is an interesting one: a two-ganged potentiometers allows us to vary the pentode’s mode of operation from 100% triode to 100% pentode, with any ultra-linear ratio in between also selectable; at the same time, it allows us to maintain the optimal power-supply noise injection into the noise nulling output stage. In other words, one potentiometer adjusts the triode-pentode ratio, while the other adjusts the power-supply noise ratio...
       15 Jun 2006

Transformer-Coupled Power Buffers
In the schematic above, we see a power pentode configured in a simple single-ended topology, with the only interesting twist being the pentode’s cathode direct connection to the transformer’s secondary. This connection eliminates the pentode functioning as an amplifier, as the cathode follows the grid, thus providing 100% degenerative feedback. This short feedback loop gives the pentode’s output stage a low impedance output that it would otherwise be altogether lacking. It also lowers the distortion, improves the PSRR, and extends the bandwidth. Actually, the PSRR is fairly good in a pentode-based single ended amplifier, even without the feedback loop, as the pentode’s high rp, unlike the triode’s low rp, makes a poor voltage divider against the output transformer’s primary impedance...

Complex Tube Power Buffers
Just adding one extra tube gain stage will greatly increase the effective transconductance of the output pentode. In other words, if the gain from the added tube equals 10, then the output tube’s effective transconductance will be 10 times greater: a 10mA/V output tube would now equal transconductance of 0.1A/V. Thus, the output impedance will also drop by tenfold...
         07 Jun 2006

Buffers and More Buffers
For our purposes, namely, using a solid-state power buffer after a vacuum tube voltage amplifier, low distortion is primary; with low-output impedance and freedom from feedback coming second and third. However, a low distortion buffer comes at a price: high idle current. Ideally, our buffers should only be run under class-A operation. Push-pull class-A output stages are naturally clean, as sharp on-off transitions are excluded and the two output devices’ transfer curves combine in single flatter curve; additionally, class-A operation prevents the problems of switching distortion, gm doubling, and tweaky bias adjustments. Class-A operation, however, is far from efficient. In spite of what the glossy ads say, class-A is brutal, requiring a heavy current draw and lots of heat, requiring massive heatsinks and heavy transformers. This isn’t news and everyone knows this, except for those who work in the high-end audio industry...

BUF634
So you want to build a hybrid amplifier, maybe a small headphone amplifier. But how to proceed? Which tube front end? (Well, that’s an easy question to answer: Aikido amplifier. Of course, it doesn’t have to be an Aikido front end.) Which solid-state output stage? Which technology? Which topology?

Ken, a reader from Australia, wrote asking about the suitability of using the Bur-Brown BUF634 with a 6GM8-based Aikido amplifier that used a 24-volt power supply. The buffer and Aikido amplifier would run of the same low-voltage power supply (+24V)...

Power Buffers for Loudspeakers
With the headphone amplifier, we were lucky to find a pre-built and tested solution in a buffer IC. But when it comes to driving loudspeakers, this easy option is not really available. Not that good power ICs do not exist, they do; however, only the LM12 is unity-gain stable, as far as I know, which greatly limits our choices. The LM12 is complete power amplifier with its own input and driver stage and feedback loop.

Crazy but Good Idea
Now what if we think just a little bit bigger than an IC. How about using a complete power amplifier, for example? The idea here is that an existing power amplifier can have its output stage highjacked. Very little of the space inside a solid-state power amplifier is taken up by its input and driver stage, the majority belonging to the power supply and the output stage.

In other words, why not buy a decent solid-state push-pull amplifier on eBay or at a garage sale and convert it into a unity-gain power buffer. No, we do not strip away the input stage, we just modify it so that it only controls the DC offset at the output...
        04 June 2006

Hybrid Promise
Tubes amplify sweetly; solid-state output devices slam and punch. Combined, the tube input stage bestows the delicate signal to the solid-state output stage, which then robustly relays the signal to the loudspeakers or headphones. So the proposition compels us: the notion of using vacuum tubes only for voltage gain, and solid-state devices only for current gain. It’s a division of labor that lets each device do what it does best. At least that’s the seductive promise. But can it be fulfilled?...

Soft-Clipping Circuit
First of all, not all solid-state output stages clip in the same way. Some clip much more gracefully. How so? 99.9% of all solid-state amplifiers use three stages: an input stage that provides gain and a feedback input, a second stage that provides gain (VAS, voltage amplification stage) and current gain, and a unity-gain buff output stage. Now, if the second stage holds an output-referenced cascode topology, this stage will help make for much softer clipping, as it runs out of gain as the output approaches clipping...
        27 May 2006

Spoiled
So, what was I soldering together? I was so tickled by my Aikido as single-ended amplifier front-end experiments that I modified my single ended amplifiers to hold an Aikido circuit, using a 12SL7 as the input tube and a 12SN7 as the buffer tube, followed by two EL34s in parallel. How did it turn out? Much nicer than I expected, to be honest. Paradoxically, the Aikido’s four triodes sound much purer, less cluttered, than the single triode they replaced, which sounded diffused and slightly blurred by comparison...

The 6082
One reader wanted to know if he could use the 6082 with his octal Aikido PCB (the stereo version). My first guess was no, as the 6082’s 26.5V heater would preclude using any typical octal input tube, like the 6SN7. But then I saw the answer was in fact, yes. The 6082 heaters can be wired in parallel (12V on the PCB, jumpers, J2 and J3 only) and the two 12SN7/12SX7 input tubes can have their heaters wired in series (6V on the PCB, jumper, J4 only). Now all the heaters can be run off a 26.5V power supply, which would greatly reduce the current draw and the power supply losses (if nothing else, the diode voltage drops are a much smaller ratio than in a 6.3V power supply, for example, as 1.4V/[6.3V + 1.4V] = 18%, whereas 1.4V/[26.5V + 1.4V] = 5%)...

Free DC Voltage for Heaters
Thinking about the 6082’s 26.5V heater led to my thinking about the trick I mentioned long ago: using the input and driver tubes’ heaters as a cathode resistor for the output stage in a power amplifier. On the mono Aikido PCBs, the two tube heaters can be placed in parallel or in series, so this trick would be easy to accomplish. For example, a 12AX7 as input tube and 12AU7 as buffer tube have 12.6V heaters, which placed in series yield 25.2V. (Fortunately, these two tubes share the same heater current draw: 150mA.) Given that 25.2V divided by 150mA equals 168-ohms of resistance, we can use the heater string as part of a cathode resistor’s total resistance, as long as the current flowing through the cathode resistor is equal to or greater than 150mA. Now 150mA is far too much current for a single 300B or 6550/KT88, but not for a 6AS7 or 6C33 nor for parallel 300Bs and EL34s and 6550/KT88s...
        22 May 2006

Aikido Tubes: Results and Values
I keep getting requests for more help in selecting tubes and cathode resistor values for the Aikido line stage amplifier. As many of you can guess, I am not about to say that one tube and set of resistor values are perfect. (What separates me from so many other tube gurus, aside from my not considering myself to be a guru, is my steadfast refusal to believe in one tube, one resistor value, one topology, one capacitor type…) So, instead of one quintessential archetype, I’ll give many design examples. I have compiled a table of tubes and values and results.

Aikido and 300B Amplifiers
Once again I was overly pessimistic. Many want to build a single-ended, 300B-based amplifier that uses the Aikido amplifier as the front end. My guess was that the 300B would require too much voltage swing from the Aikido stage. Well, I now think that I was wrong. I performed a quick experiment: I threw together an Aikido amplifier (nothing like having a stack of PCBs to play with) that used a 12AX7 as the input tube and a 12AU7 as the output tube. I then fed the Aikido a 1kHz signal...
        13 May 2006


Aikido Hybrid Headphone Amplifiers
In a previous blog entry, several hybrid, power amplifiers based on the Aikido were shown. The idea behind these amplifiers was that the Aikido circuit could provide clean gain and that the power MOSFETs, configured in single-ended output stages, could provide the needed heavy current swings...
       07 May 2006

Aikido Low-Impedance Headphone Amplifier
The design goal here is to build an Aikido headphone amplifier that can drive Grado 32-ohm headphones (or the 55-ohm version of the AKG K 240M), without resorting to a global feedback loop. How much voltage and current does a 32-ohm headphone demand? Well, I remember reading that the iPod puts out a peak voltage of about 1 volt into a 32-ohm load, which equals 31mA of peak current. To comfortably match and, even, exceed this value, our tube-based output stage must draw something like 40mA at idle. No, the 6DJ8 will not work, as it only draws 40mA with its grid at the same voltage as its cathode and we need to draw twice this amount of current (80mA) at peaks. This is the point where I lose half the readers. I can almost hear all those heads being scratched...
       27 Apr 2006

Aikido Headphone Amplifiers
So is the Aikido amplifier, with its low distortion, low output impedance, feedback-free design and excellent PSRR figure, the best choice for driving headphones? No, I don’t think so. In fact, I would argue against its use as a headphone amplifier, as its single-ended output stage greatly limits the potential current swings from the triodes used, and thus a push-pull circuit would be—regrettably— essential. But then there are the e-mails from readers who tell me that they are using an Aikido amplifier as a headphone amplifier and that they love the sound...
       21 Apr 2006

Getting R15 & R16 Straight
The tipoff came from longtime reader, Paul, who told me that he measured more noise at the output of his Aikido after following the noise voltage divider formula,
    Resistor ratio = 1/mu + ½
or (for those with my Aikido PCBs)
    R16 = R15(mu + 2)/(mu - 2)...
       17 Apr 2006

24-Volt Aikido Amplifier
The 6Gm8/6N27P/ECC86 dual triode was designed to be used as an RF amplifier and as a self-oscillator mixer in a car radio, back when car radios held tubes. Unlike most triodes, it works quite well with only 12 volts on its plate. Additionally, like the 6DJ8/6922 this little triode stows a grid frame, making it doubly rare. Fortunately, it shares the same pinout as the 6DJ8/6922, so using it with the 9-pin Aikido PCB is no problem...
       11 Apr 2006


Hybrid Aikido Amplifiers
Remember the MOSCODE amplifiers from New York Audio Labs? These hybrid amplifiers used two stages of tube amplification and buffering to provide the front end to a push-pull, class-AB MOSFET output stage, which delivered no voltage gain but lots of current gain. Not a bad idea, actually. Well, with the Aikido amplifier the input and buffer stages of a hybrid amplifier are nicely taken care of. Using a 12AX7 (or 5751 or 6072) as the input tube and a 6N1P (or 6FQ7 or 12BH7) as an output tube in the Aikido amplifier would provide both the voltage gain and the drive current needed to drive a pair of MOSFETs to full power in a quiet and clean fashion...
       01 Apr 2006

Three-Switch Stepped Attenuators
If you don’t know what the attenuator is all about, you didn’t follow the link to the GlassWare Yahoo! Store. The attenuator is a hybrid design that uses both series and ladder attenuators and three rotary switches to yield 36 positions of attenuation in -2dB decrements. In the first six positions, the attenuator is just a ladder attenuator, with no more than two resistors in the signal path; thereafter, the attenuator uses both a ladder and series attenuator configurations, with never more than eight resistors in the signal path. With -2dB decrements, a maximum of -70dB of attenuation is accomplished. ...
       26 Mar 2006

Printed Circuit Boards for the Aikido Amplifier
Dear Readers, I’ve got good news and bad news. First the good news: the rumored Aikido printed circuit boards do exist, and they are beautiful. They look fabulous and feel solid in the hand. They are extra thick, 0.093" (inserting and pulling tubes from their sockets won’t bend or break this board), double-sided, with plated 2oz copper traces, clean silkscreen and solder mask. (The comment was made repeatedly that they look “military grade,” as if their intended use was inside a spy satellite, not a line stage.)...
       18 Mar 2006

European Triode Festival &
Aikido amplifier PCBs

Well, it’s now December and I have just returned from Switzerland and Germany, where I attended the European Triode Festival (ETF). Actually, they paid me the great complement of inviting as this year’s special guest. I am not sure that they got their money’s worth, as I got much more than I gave. I met some truly wonderful men and women. (But then, tube lovers are always an interesting lot.)...
       10 Dec 2005

Jeb’s Amplifier
Before leaving the topic of cathode-follower output stages with bipolar power supplies, let’s look into a circuit that reader, Jeb, designed. His single-ended amplifier uses a single coupling capacitor at the input and DC couples the input stage to the output tube’s grid. In addition, its output stage is configured in a parallel-feed arrangement, with the cathode connected to an output transformer and to a solid-state constant-current source.
       09 Sept 2005

Radiotron Designer’s Handbook
I’ve been asked to provide some reference works on the cathode-follower output stage. This isn’t easy to do, as very little has been written on the topic. Even the expansive and exhaustive Radiotron Designer’s Handbook is startlingly thin and terse on this topic... PDF
       14 Aug 2005

Paying up front
Let’s say that we are willing to fight a fair fight and try to give the cathode follower output stage just what it needs: a larger driver B+ voltage than the output stage’s B+ voltage. How do we proceed? PDF
       04 Aug 2005

Aikido Amplifier Once Again
However, I believe the Aikido circuit needs a bit finer stroke now that it seems more familiar. The broad-stroke explanation of the Aikido amplifier was that the circuit eliminated power-supply noise from the output, by injecting the same amount of power-supply noise into the top and bottom of the two-tube cathode follower circuit... PDF
       15 July 2005

A Wrong Turn
A cathode-follower output stage can excel, but only if it is given a pure input signal—a very-large pure input signal. Yes, indeed, the problem of giving the output stage the required drive signal is a big one: for what shall it profit a man, if he shall gain the lowest distortion output stage, but loses the driver stage’s integrity? Verily. Remember the output tube(s) grid must see a signal larger than the cathode will swing: the inverse of the cathode follower’s gain, in fact... PDF
       07 July 2005

Cathode-Follower Power Amplifier Design
Here’s the usual cathode-follower power-amplifier design thought process: since—as compared to a grounded-cathode amplifier—the cathode follower offers a much lower output impedance (Zo), the output transformer’s winding ratio can be equally reduced, allowing the use of 500-ohm primary, where a 5,000-ohm primary impedance might have been needed in a grounded-cathode amplifier. Right? Absolutely dead wrong!...PDF
       29 June 2005

Cathode-Follower Power Amplifiers
Cathode-follower-based output stages are rare—surprisingly rare. A conservative guess would be that 99.9% of tube power amplifiers use the grounded-cathode amplifier configuration, with the load attaching at the plate, not the cathode; the output tubes working as amplifiers, not buffers. Yet, the cathode follower produces far less distortion and a much lower output impedance than the comparable ground-cathode amplifier does when working into the same load. Furthermore, the cathode follower is not straddled with the Miller-effect capacitance that the grounded-cathode amplifier is burdened by. So, the big question is Why isn’t the cathode-follower configuration used more often?...PDF
       22 June 2005

Auto-Bias and the Circlotron OTL Amplifier
Back in 2001, I covered a circuit to balance the idle current (and thus to eliminate the DC offset) of a circlotron OTL amplifier, which easily could have been part of the auto-bias series of blog entries. Here's the link. The title is "Subject: active DC offset correction."
      09 June 2005

More Auto Bias Circuits
One reader asked if the clipping or Broskie auto-bias circuits were suitable for single-ended amplifiers. The answer is yes...
Alan Blumlein came up with an auto-bias matching circuit that does not aim to set a specific idle current, but works to keep both output tubes conducting the same idle current; a current balancer, in other words... PDF
      21 May 2005

Broskie Auto-Bias Circuit
What if we don’t clip the error signal? What if, instead, we use it to prevent the DC servo's capacitor's excessive charging from throwing the bias voltage off? Well, that’s the idea behind the Broskie auto-bias circuit. PDF
      09 May 2005

Class-AB Auto-Bias Circuits
If the usual DC-servo auto-bias circuit did not work perfectly with a class-A amplifier, an amplifier operating mode that seems a perfect candidate for a simple DC servo circuit, what hope do we have for an auto-bias circuit for class-AB amplifiers? Well, a few techniques have been tried. PDF  Updated 04 May 2005
      29 Apr 2005

Amplifier Auto-Bias Circuits
Regardless whether the amplifier is single ended or push pull, solid-state or vacuum tube, class-A or class-AB—to achieve the best performance—all power amplifiers require a precisely set idle current for their output stages. In a single-ended amplifier, too much or too little idle current will limit the potential power output. In a class-A push-pull amplifier, too much idle current will needlessly overheat the output devices and too little will force the amplifier out of class-A into class-AB operation... PDF
      16 Apr 2005

Common-Cathode Amplifier Design Ideas
Although not that popular, the common-cathode amplifier is one of my favorite tube circuits—with good cause, as it offers low input capacitance, low distortion, no phase inversion, and two inputs, the second of which can readily accept feedback from the output or a sampling of power supply noise, to null the noise at the output... PDF
      09 Apr 2005

Big Amplifiers
While at the CES in January, I was surprised to find so many gargantuan tube amplifiers: huge, massive, expensive amplifiers, many the size of small refrigerators or big air conditioners. These amplifiers held more glass than my wallet could bear to contemplate. The motives are obvious: these days power and overindulgence are both desired and marketable... PDF
      19 Mar 2005

"Which Tube Should I Use?" Typos
A few—quite a few—typos made it into my OCR translation of Cooper’s article, “Which Tube Should I Use?” It turns out that a Word document that is heavily stuffed with Greek characters does not readily translate into a PDF, as the Adobe PDF Writer (version 5) will not imbed all the extended set characters in a typeface. Fortunately, Bjørn, a reader in Norway, sent me a list of errors. Thanks. The newly corrected PDF is up now, so you should download it if you had a hard time following the math... PDF
      10 Mar 2005

Which Tube Should I Use?
& Unbypassed Cathode Resistors

Since mentioning the results from the European Triode Festival wherein an unbypassed cathode resistor beat out alternate cathode-bias devices, such as an LED or a battery, a few readers have asked how to remove all cathode bypass capacitors. From one extreme to another…
      02 Mar 2005

All-Tube SRPP on Steroids
In the last blog entry ended with an all-solid-state power amplifier based on the SRPP topology. Two OpAmps controlled two N-channel MOSFET output devices. Now, the question is How do we translate this circuit into an all-tube one? First, whenever you see an OpAmp, think of replacing it with a single triode, as the triode is analogous to a current-feedback OpAmp in that offers both a high-impedance and a low-impedance inputs... PDF
      16 Feb 2005

SRPP translated again
Where have these blogs been going? We started with the White cathode follower, then the Taylor amplifier, then the Taylor source follower, then the Macaulay amplifier, and then the Broskie-Macaulay amplifier, each defining a signpost on the same road. The idea behind all these seemingly different circuits is that a push-pull amplifier can be made that uses the same polarity output devices, but dispenses with the need for a phase splitter by sensing the current flow through a primary output device to derive the needed drive signal for the other output device... PDF
      12 Feb 2005

Mixed-Technology Hybrid
I have been evaluating the mixed-technology hybrid amplifier, wherein the output stage holds both tube and solid-state output devices. First of all, what a curious mix this is. In the typical hybrid amplifier, the front-end is tube-based and the output stage solid-state, or vice versa. But not always, sometimes dissimilar technologies are used within a given amplifier stage, say the use of the solid-state constant-current sources to load a tube’s plate. So too, in a power amplifier, when the output devices differ in fundamental technology, one device plays a relatively passive role, acting as a constant-current source... PDF
      09 Feb 2005

Broskie-Macaulay Amplifier
Something about Jeff Macaulay’s amplifier always troubled me—beyond the claims of single-ended class-A operation. That something lies in an asymmetry in output devices’ transfer curves; this is a mismatch that cannot be eradicated with tightly matched output devices, as the unevenness is imposed externally to the devices. In essence, the bottom output device benefits from the 1-ohm resistor at its emitter/source/cathode, as it provides local degeneration (feedback), straightening the device’s transfer curve. PDF
       04 Feb 2005

Taylor Source Follower on Steroids
I have just broken the unspoken, but well understood rule: always mention tubes in the title. Not to fear, tubes are on their way, but first we must go through the steps.

First of all, I am a little fearful to describe any circuit that can be exploited by a marketing department. Hype sells and today’s hot audio-gear adjectives include “single ended” and “class-A.”  PDF
      28 Jan 2005

Aikido enhancement
The aikido amplifier allows for an easy sound enhancing control: a sonic width control...Neither recordings nor listening rooms are created equally. Many of the classic recordings from the fifties and sixties were purposely mixed with too much separation between the channels; the motive is easy to surmise: stereo was new (and expensive) and the customer expected to hear separation and separation is what they got... PDF
    25 Jan 2005

More Aikido Testing
Power supply noise reduction is only half of the Aikido’s virtues—possibly the smaller half. This circuit offers an amazing amount of flexibility, as different input and output tubes can be easily interchanged, as long as they share the socket pin-out and heater voltages...
    22 Jan 2005 PDF

T-Rex & Shunt Regulators
Dick Olsher has created a mighty 300B amplifier: the T-Rex (Transformer King, in other words, as the amplifier uses Plitron toroidal output transformers, power transformers, and chokes). Steve Bench and I have lent a helping hand and the DIY article is available at Enjoy the Music.com. The T-Rex is big, complex, and expensive. It certainly isn’t your father’s 300B amplifier...

I must get at least five e-mails a week regarding the shunt regulator. It is popular, but little understood (hence its popularity?). One reason behind its new-found acceptance is the TL431 shunt regulator IC. This small three-pin IC is almost perfect in the eyes of many tube fanciers. Now, if it held only two leads it would be perfect...  PDF
    18 Jan 2005

Aikido Amplifier Revisited
Bruce has published a review of the Aikido amplifier he built at audioasylum.com. So far no bad reviews for the Aikido topology, only good reviews. (Actually, prior to the circuit having been built, it was panned at some DIY website. If only I held such extrasensory powers; why, I might even be able to adjust the cartridge tracking at a distance.) Even at the CES show, I met one TCJ reader who has probably the most expensive system I have ever heard of —my wallet aches from thinking about it. He too has built and tested the Aikido circuit and he is quite impressed with the results...  PDF
    16 Jan 2005

Settling dust
It’s been one week since I first stepped on the CES floor. Since then, I have marveled at the difficulty of accumulating any substantive snippets from the CES show fitting for this journal. The show’s emphasis is the surface: the eye and ear candy, the shiny, blinking, glowing, thumping, sizzling aspects of consumer electronics; this journal’s emphasis is the internals, the understanding and the discernment of topology and technique, things that are only mentioned when the advertising department deems it advisable…
    13 Jan 2005

Making Sense of the CES
I have just returned from Las Vegas and I still haven’t fully digested what I have seen. Maybe I never will. Although on at least a half dozen occasions I have visited Reno, Nevada’s smaller city to the north, this was my first trip to Vegas (locals drop the “Las”) and John Atwood of One Electron was kind enough to lead me through the maze (if you haven’t been to a CES show, you can’t imagine how big it is; it was at least four times bigger than I expected)... PDF
    10 Jan 2005

E-mail from Australia & Curry amplifier
Long-time reader Bart S. writes:
"Is the negative power supply essential? Couldn't you earth the 20K resistor and increase the value of the output cathode resistor. This would waste some heat but allow a smaller bypass cap also."
Bart is referring to this current-mirror-based amplifier... PDF
    05 Jan 2005

Reflections on perfect reflections
In the post script to the last blog, "Perfect amplifier & perfect reflections," I gave a closer approximation to a complete formula for calculating the reflected plate resistor. Much is still missing, as the transistor-based current mirror entails its own set of formulas. As a quick quasi-reality check, I ran a SPICE simulation on just the current mirror, and then, a check on the entire circuit... PDF
    04 Jan 2005

Perfect amplifier & perfect reflections
The constant-current source loads the input triode, isolating the triode from the positive power supply rail’s noise. It also allows the greatest gain from the triode (close to the amplification factor). And it also keeps the distortion low, although not nonexistent. At the output, the constant-current source that loads the cathode follower does wonderful things for this buffer. It allows the greatest gain from the cathode follower and it buffers the cathode follower from the negative power supply noise. However, it does not lower the cathode follower’s output impedance or make the cathode follower perfectly noise free...  PDF
    01 Jan 2005

The Einstein amplifier
Looking at the fixed version of the “bomb” amplifier, we see a problem. Noise. Here's the paradox: a feedback loop that terminates at the plate will reduce the noise at the plate, but not at the speaker terminals, as all the power supply noise will be superimposed across the output transformer's primary...
    30 Dec 2004

A second helping of crumbs
I’m back with a second helping of crumbs. Yesterday’s blog illustrated how an LM337 could be used to set the bias of a cathode-biased tube output stage. One circuit was described as being a potential bomb, as it paired two feedback loops in opposition with each other and suffered from a worse PSRR figure, in spite of a lower distortion figure and lower output impedance. The first problem can be eliminated by doing away with one of the feedback loops...
    29 Dec 2004

A Feast of Crumbs
“The sad fact is that very few active devices were actually designed with audio in mind. The 300B and the 12AY7 were. As were the Hitachi lateral MOSFETS. But most discrete electronic devices whether tube or solid-state were designed for power supply, computer, motor, or radio use. Audio must live on the crumbs that fall off the technology table.”...
    28 Dec 2004

Murray Amplifier
There it was, lodged at the back of my mind for while—ever since I wrote on the Gomes and Sandman circuits, in fact. Then, the Taylor amplifier was finally able to push the memory forward: the Murray amplifier. Both the Gomes and the Taylor amplifiers are push pull in operation; neither uses a separate phase splitter, relying on the phase inversion within the circuits to supply the inverted drive signal for one output tube. Back in the fifties, Cyril Murray from Australian came up with related topology, one that fits with the Gomes amplifier in the category of asymmetrical amplifiers (or rather, half-asymmetrical amplifiers; “half-asym” for short).
    23 Dec 2004

More Translating Glass to Silicon
I am as often wrong as I am right. I expected to get many e-mails dealing with the Aikido amplifier, particularly its use in an SE amplifier; and I did. But I also expected to get many e-mails dealing with the SS SRPP (the transistor-based SRPP amplifier). None were forthcoming. But a few wrote asking about the MOSFET-based White cathode follower (if only to find out where I had mentioned the topology before). My guess is that transistors are just too creepy for most tube fanciers...
    20 Dec 2004

OTL E-Mail
I will think about the PDFs, as you are at least the third reader to ask for them. I wondered how long it would take before someone asked for a coupling-capacitor-less version of the computer amplifier. Below is a such a remake...
    18 Dec 2004

Error in Power-Supply Schematic
The power supply schematic posted yesterday held a miswiring: the negative power supply rail would put out –160 volts, not the desired –80 volts. The schematic has been corrected.

More OTL
Before anyone asks, let me say that the amplifier can be restored to a higher output wattage by three means. The first is to use a much high power supply voltage for the output stage. By returning to the original 160 volts of B+ voltage, the amplifier can force a lot more current out of the output tubes before hitting positive grid current conduction. The distortion will be quite a bit higher, as the idle current will have to be reduced to keep within the 6AS7’s plate dissipation limit, but this may not be a liability in all situations, as musicians may prefer a little grunge in the mix...
    10 Dec 2004

Gomes & SE+ & Error Take Off
It turns out that I got quite a few things wrong: first of all, Jose Gomes's name is not spelled "Gomez." A thousand apologies. Second, his circuit uses a single bypassed common cathode resistor for the bottom triodes...
    04 Dec 2004

OTL Regrouping
Where are the computer amplifier’s parts values? Two events have forced me to do a major rethink...
    03 Dec 2004

OTL Amplifier Design
Having already specified the load impedance (32 ohms), the B+ voltage (330V), the idle current (40mA per triode), and the number of output tubes (two 6AS7s per channel), we can now proceed to the details of the amplifier’s design...
    28 Nov 2004

32-ohm Speaker Design
A few notes on designing a 32-ohm speaker. The first question to answer is how many drivers to use. Eight 4-ohm loudspeakers can be placed in series, as could four 4-ohm loudspeakers or two 16-ohm loudspeakers. The advantage that the 16-ohm loudspeaker holds over the others ohmages is that it readily lends itself for use in a vertical d'Apolito array with a single tweeter situated between two woofers...
    24 Nov 2004

Tube-Based Computer Amplifier
I’m am not alone in spending far too much time in front of a computer. I wish it were otherwise. Still, if we are to be chained to a keyboard, let’s at least have as good a time as we can. Which brings us to today’s topic: designing a good tube-based amplifier and loudspeaker system for use with a computer...
     23 Nov 2004

Translating Glass to Silicon
Two readers have written asking what would happen if vacuum-tube circuits were translated into solid-state circuits, would they sound similar? Good question...
     19 Nov 2004

Special Use for Gomes Amplifier
I had to cut the comparison between the Gomez and the Aikido amplifier short last time (for the usual reasons: no time and the blog was running far too long). I pointed out that the Gomes amplifier works worst into low-impedance loads and best with an infinite-impedance load...
     17 Nov 2004

More Aikido
Since last Friday, I have been barely able to catch my breath. I don't really grasp the point of blogging: short and often. By nature, I am an essayist, not a diarist Post-It-note scribbler...
     15 Nov 2004

Gomes Vs XPP
I’m sure that few readers thought that the Aikido amplifier bore more than a passing resemblance to the Gomes amplifier circuit. The reasons are obvious enough: both circuits use four triodes, totem-pole configurations cascading into totem-pole configurations....
     
12 Nov 2004

DC Coupled
In the last few days, I’ve received three e-mails with a common theme: DC coupling. Additionally, over the last year, I have been receiving many e-mails from readers eager to build some of the simple hybrid amplifiers described in this journal, particularly the DC coupled designs...
     11 Nov 2004

Aikido Variations
After having read of reader Paul’s predicament with his Antique Sound Labs AQ-2004 line stage, I’m sure that a few readers have wondered how three 12AU7s could be used in the Aikido amplifier, as the circuit seems to require an even number of twin-triode envelopes...
     10 Nov 2004

Solid-State E-Mail
The e-mail I have received lately both encourages and discourages. Many letters have been filled with enthusiasm and joyful experimentation— unfortunately, most of these have also held unadulterated solid-state circuits and come from solid-state partisans...  Email
     09 Nov 2004

Aikido Amplifier
Belonging to my school of Audio Aikido, this amplifier sidesteps power supply noise by incorporating the noise in its normal operation. As a result, in terms of distortion and output impedance and PSRR, the following circuit works at least a magnitude better than the equivalent SRPP or grounded-cathode amplifier...
     08 Nov 2004

Where Have I Been
“The reports of my death are greatly exaggerated.” So said a telegram sent by Mark Twain from London to a newspaper in the United States after it had mistakenly published Twain’s obituary. Well, the same can be said the Tube CAD Journal and myself...
     07 Nov 2004

The Experiment
I wanted to devise an experiment to see if the Tube CAD Journal could support itself, so I could drop another ball and publish once again a full journal each and every month, complete with readers' email, articles, and even a construction project per issue. For those who do not understand why putting out a journal once a month is such a big deal, consider the following...
     01 Dec 2003

Unusual Circuit
If used with a solid-state amplifier, then this circuit probably holds to the tubes-make-nice-distortion school of audio design, using a tube’s euphonic distortion to make a nice-sounding hybrid, a school of thought I adamantly oppose. I do not like tubes for their distortion, but in spite of their distortion...
     21 Oct 2003

Differential SRPP from Poland
The goal of “no capacitors, no feedback” is laudable—within reason. In designing my own equipment, I always strive to use the fewest capacitors and the shortest feedback loops possible or, preferably, no feedback—within reason...
    17 Jun 2003

Tubes and Headphones Once Again
The bad news: my old portable CD player died; the good news: I picked up a new Sony D-FJ401. This new player runs forever on two AA batteries and offers 45 seconds of shock protection (which must be disabled for serious listening) and, most importantly, a line-out (which is increasingly rare these days). The player shares a desirable feature with the best solid-state equipment: its sonic failings are only subtractive.
    03 Apr 2003

Simple PP Amplifiers (Updated 6 Nov 2004)
The key word here is “simple,” not “best,” as simple amplifiers often involve complex problems. Simple amplifiers usually make extra demands of the rest of the system and their performance is usually compromised in a few key specifications, such as output impedance, input impedance, frequency response, and distortion. Still, aside from the quicker assembly and lower cost, there is something special about a simple amplifier’s sound, something right sounding...
    21 Mar 2003 (Updated 6 Nov 2004)

Simple Single-Ended Amplifiers
Few in parts, easy to understand, and quick to build, simple amplifiers have a lot going for them. Yet making a great simple amplifier is probably more difficult than making a great complex amplifier, as each part is asked to do so much...
    22 Feb 2003

LPs Covers Again
& a Letter to the Editor of Electronic Products

This is not going to be one of those letters that decries your lack of sensitivity for failing to hear subtle difference between five “9s” copper versus six “9s” copper. But then it is not going to be one of those letters that heartily slaps you on the back for trashing a group of spoiled snobs with more money than sense. No, this letter’s goal is much more difficult to achieve...
    19 Jan 2003

Zen Amplifiers
The ZEN amplifier is an intriguing design but with many limitations. First, let’s pull way back and think about what is going on in the amplifier: one MOSFET is doing all the work of amplifying the 1-volt input signal to 16 volts and driving 2-amps of current into the speaker and providing a low output impedance; all at once! What fuels this miracle? It cannot be the wimpy 1-volt input signal...
    20 Dec 2002

Letters from Acme Tube Design
Help! Our amplifier sounds kind of dull and lifeless compared to other 2A3 amplifiers and sometimes it even breaks into oscillation; it also hums, so much so that I am almost embarrassed to charge $6,000 for the pair. Maybe the transformers are to blame? Who makes the best output transformers? Which one should we use? I hear that 3k is a good impedance. We are using US Army pulls RCA 2A3s; should we be using 211s instead? Or would a new input circuit be better? What is a White cathode follower? Could it be used...

E-mail from Erno Borbely
Dear Editor,

I have received a copy of pages 20-27 of Vol. 3, Number 2 of your magazine from one of my customers. It is related to the Hybrid Tube/MOSFET amp design, that your contributor attributes to an Italian guy by the name of Generoso C. I am afraid he was criticizing the wrong guy for the design, because I designed the original Hybrid Tube/MOSFET circuit; he just copied my topology 100%. My circuit was published in issue 1/98 of Glass Audio , see copy of the original article on my homepage under ARTICLES (www.borbelyaudio.com) Nothing is wrong copying my circuits for private use, but it is illegal/unethical to produce/sell somebody else's design. I complained to Editor Ed Dell about this and he sent several notes to Mr. C, but received no answer...
    29 Nov 2002

 

 

Cars, Planes, and Circlotron
As for the circlotron circuit, it’s no secret that it perplexes many, if not most tube practitioners. How does the current flow from tube to tube? Why are there two power supplies? Why is it by necessity a class-A amplifier, or must it be? Is it an SE or a PP amplifier...     
    21 Oct 2003  

Grounded-Cathode Amplifier
The grounded-cathode amplifier is approaching its centennial (2007) and it remains the building block of most tube audio equipment. It is simplicity itself, with little more than a few resistors and a triode; yet many are ignorant of its inner workings.   
    19 Jan 2003   PDF
 
RIAA Preamps Part II
The alternative to active equalization is passive equalization. Passive RIAA equalization means brute force equalization...
    30 Nov 2002     PDF
 
RIAA Preamps Part I
What I find amazing is not that vinyl persists, even twenty years after the introduction of the CD ("perfect sound forever"), but rather that it ever became popular in the first place.
    12 June 2002     PDF

SRPP Deconstructed
The SRPP is a controversial circuit. Even its name is not set in stone, as it is called the SRPP, the SEPP, the mu follower, the mu amplifier, the cascoded cathode follower, and the totem-pole amplifier. Interestingly, the name given in the 1940 patent (US 2,310,342) for the circuit...
    27 May 2002    PDF

Mixed-Class and Mixed-Topology Amplifiers
If only we could save our cake and eat it at the same time. The aim of a mixed class amplifier is to provide the high quality sound of Class-A operation with the greater efficiency and power output of Class-B operation...
    29 March 2002   PDF

Power Buffers
Although buffers might be new to many audiophiles, they are a central part of the analog electrical engineering practice. In short, a high-power buffer is a special type of power amplifier...
    05 February 2002    PDF

Missing Sonic Controls
Imagine if you encountered a telescope manufacturer, whose product line embodied a severe minimalism: telescopes built with the fewest lenses possible, telescopes without eye adjustment knobs, telescopes without color or polarizing filters, telescopes without magnification adjustments...
    09 January 2002     PDF

Complementry Inverse Distortion Cancellation
Adding distortion to eliminate distortion seems as contradictory as trying to prevent war by preparing for it. Yet in audio practice, seeming contradictions abound, feedback making an amplifier sound less clean, lower damping factors creating a better bass reproduction, for example. While these examples are controversial, adding negative distortion to distortion to yield no distortion should not be, yet this technique has few adherents...
    19 November 2001   PDF

Accordion Amplifier:
A New Single-Ended Amplifier
It's time to stir things up a bit: how about a single-ended amplifier that doesn't look single-ended...
    17 October 2001     PDF

Active Crossovers and Filters
What are active crossovers? In short, they electronic circuits that divide the audio spectrum up into discrete bands of frequencies and they function in place the passive crossovers found in loudspeakers...
    01 October 2001      PDF

Tube-Based Crossovers
Just behind tube-phono preamps and tube--headphone amplifiers, the largest number of circuit requests I receive is for tube-based crossovers...
    01 October 2001     PDF

300B as Regulator
The 300B has justly won the reputation for being a superb output tube in single-ended amplifiers. Of course, it actually has a much wider application, such as a push-pull amplifier's output tube and possibly as a headphone amplifier tube or super-buffed line stage tube...
    09 September 2001   PDF

Output Stage PSRR Enhancement
In all the push-pull topologies, the goal is the same: to provide an equal drive signal for the output devices. This goal is important, as it ensures a low distortion output signal by forcing each output device to work equally into the load impedance, a task required for low distortion operation. But even when we have labored to ensure an equal drive signal, we may still find the output signal tainted with noise from the power supply...
    26 August 2001      PDF

Email from the Summer of 2001
I have thank reader Morgan Lundberg for the link to Steve's web page.  I almost fell out of chair when I followed the link. Steve Bench has performed a true service for us tubeholics... PDF

April & May 2001
Tube Hybrid Amplifiers, Tube Mixers, E-mail includes Tube Line Stage, Tube Headphone Amplifiers, Hybrid Amplifiers, Small Tube OTL amplifiers, RIAA EQ, “UltraPath”    PDF

March 2001
Partial Feedback Amplifiers (300B design example), Portable Tube Headphone Amplifier Design Part 2, Tube/MOSFET hybrid amplifier  PDF 

January 2001
Inverted Tube Shunt Regulators, Portable Tube Headphone Amplifier Design    PDF

December 2000
Circlotron Relativity: Horizontal SRPP, Horizontal White cathode follower, SE Circlotron, What Are Maximum Ratings?, Lots of e-mail

October 2000
High Frequency Anomalies in Output Transformer Tests, Safer Tube Amplifiers, Lots of heater e-mail

July 2000
Heater Concerns: Free DC Power Supply , Free Transformers, Current Regulation, Using a 6SN7, 8SN7, or 12SN7 Directly Heated Filaments, Voltage regulators for the 2A3, Voltage Regulator for the 300B, Under-voltage Heaters

June 2000
Voltage References, SRPP Once Again , Correction to May's SRPP Article, Design Idea: Constant Power Amplifier

May 2000
SRPP Decoded, Design Idea: A Safe Loftin-White Amplifier

March 2000
Hybrid Voltage Regulator, RIAA EQ as Crossover, (L+R) & (L-R)

February 2000
Solid-State and High Voltage, Solid-State as AC Circuit Support

January 2000
Ultra-Linear Output Stages: Line Stage Amplifier, The Ultra-Linear Cascode, OTL Power Amplifiers, Headphone Amplifier, Differential Input HP Amplifier, Extracting Extra PS voltages, Heater Concerns, The Feedbackloop

December 1999
Electrostatic Headphone Tube Amplifiers (part 2) Design Idea: Polarized Wires

November 1999
Electrostatic headphones Amplifiers (part 1),  Book Review: Audio Reality

October 1999
Cathode Follower, White CF, Plate Follower, Broskie CF, Circular CF, Tube Microphone Preamplifier

September 1999
Circular Amplifier: Class A?, Vacuum Tube Plate Curves, Simplest Tube Amplifier Possible

August 1999
Circular/Bridge/Balanced amplifier , Tube Voltage Regulators (part 4), Design Idea for SE Amplifiers

July 1999
Push-Pull Amplifiers (part 2), Tube Voltage Regulators (part 3), Balanced Phono Stage, Tube Shunt Regulators

June 1999
Push-Pull Amplifiers, Tube Voltage Regulators (part 2), Balanced Line Stage, Unbalancing Acts: From Balanced  to Single-ended

May 1999
Lowering the SE Amp's Noise (part 2), Tube Voltage Regulators, Design Idea: Para-Feed Dynaco ST-70

April 1999
Lowering the SE Amp's Output Noise, Balancing Acts (part 2), Design Idea: Single-ended Transformer Testing

March 1999
Improving the Cascode's PSRR, Unbalanced to Balanced , Variable Feedback amplifiers

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