Available both as Web pages (click the title) and, in a few cases) as PDF files for easier printing (click PDF)

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...

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...

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