John Broskie's Guide to Tube Circuit Analysis & Design

26 March 2006   

Aikido PCB update
Well, the Aikido printed circuit boards were a much, much bigger hit than I anticipated. I did expect quite a few to buy one board; I didn’t expect so many to buy two, three, four and more boards at once. Within the fist few days, all the boards were sold out, and so far about forty Aikido fans have placed back orders for the new batch of PCBs (not a bad idea, as its going to be first come, first served). Of course, the demand makes sense now, after the fact. The Aikido topology is popular, if not downright hot. I know there is a big buzz going on in the DIY websites about the circuit’s excellent performance and relatively simple design. And I have received many e-mails from readers who have built an Aikido and, while attending the ETF, I met in person quite a few who had built Aikido amplifiers.*


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.

In other words, the attenuator holds its own balance control of sorts. The center knob controls the volume for both channels, and offers six large -12dB decrements; the flanking knobs offer six fine -2dB decrements for each channel, creating a volume control and balance control in one easy-to-use stepped attenuator. Why did I also offer this attenuator for sale with the Aikido boards? The Aikido amplifier is so good I was loathe to imagine the cheap, bad-sounding volume potentiometers that would be used with it and I knew that few would want to buy an expensive stepped attenuator. So, this attenuator ($49 complete with resistors or $39 without) was an ideal way to help build a great line amplifier for a lot less money.

By the way, speaking of lots less money, my friends tell me that I am charging too little for the boards. In fact, customers, who have the boards in hand, tell me that I am charging too little for the boards. While I know that some believe that the more a piece of audio gear costs, the better it must sound, I am tickled by the idea of being able to build an entire line stage for less than $300 that will make your friends who own the high-priced spread green with envy.


Low-Voltage Aikido
I mentioned last time that certain tubes allowed low-voltage power supply to be used. In terms of the lowest voltage, the winner has to be the 6GM8 (ECC86, 6N27P) dual triode. This tube looks like a 6DJ8, but it works with as little as 12 volts on its plate. So, if this tube filled all four tube sockets on the Aikido PCB (the board holds two channels), the line amplifier could be run with only a 24-volt power supply (maybe even batteries). How does it sound? I don’t know, but I would love to know. I have some 6GM8s on order right now. (I own a few linear +/-12 volt power supplies, which would work perfectly, as I could use them as 0-12 & 0-24 volt power supplies, with 12 volts for the heater string.)

The 12SX7 is a 6SN7-type tube with a difference: it can work at a much lower cathode-to plate voltage, say 40 volts. Still I would prefer to use a B+ voltage of 250 to 300 volts with this tube, as, in general, every extra volt and every extra milliamp of current makes for a better sound. However, there might be a situation where a lower B+ is desirable. In an Aikido headphone amplifier (300-ohm load) that used a 6BX7 as the output tube, the 12SX7 would make a great choice for the input tube, when the B+ voltage was only 140 volts, for example. (The 6BX7 can draw an idle current of 30mA with only 70 volts on its plate, which is more than enough to drive Sennhieser 300-ohm headphones to deafening levels.)


Aikido Power Amplifiers
I know that many will use the Aikido PCB to make single-ended amplifiers, wherein the Aikido drives an output tube, such as the 2A3, EL84, EL34, or the KT88…but not the 300B, as it requires more gain than even the 12AX7 as the input tube can provide.

This amplifier is double Aikido-ized, in that the SE output stage uses a PS noise canceling technique that was laid out long ago in the Tube CAD Journal. Capacitors C1 and C2 inject a portion of the PS noise into the cathode of the output tube, which, in turn, cancels the noise across the transformer's primary. Here is the same idea with a KT88/6550 output tube.

I also know a few will use an Aikido PCB to make hybrid amplifiers, both single-ended and push-pull. The secret is to couple a unity-gain power buffer stage to the Aikido’s output. More details next time; I promise

Next time
I have gone beyond my fingers' limits once again. Next time, expect to see many more uses for the Aikido amplifier.


*A while ago, in Northern California, I ran into an Aikido expert, someone who not only understood circuit in its entirety, but held a more expansive and deeper understanding of the topology than I…well at least that’s what he told me. Reality, sadly, was less impressive or interesting.

Not knowing that I was the circuit’s author, he went on and on about how I didn’t understand the circuit, nor could I understand his fabulous modifications (you know the kind, specific brands of coupling capacitors and resistors, which I found odd, as I had never recommended a specific brand of any part). I love when this happens. I flatter myself by believing that it could be said of me what is said about the economist Milton Friedman: that he loses all the debates, except for those in which he actually participates.

I remember when I was in high school visiting an electronic parts store and asking for some advice.  The salesman asked which school I attended. After I replied, he said, “You’re in luck then. There’s this student there named John Broskie, who knows everything there is to know about loudspeaker design.” I told him that I knew John Broskie, quite well in fact, and that I thought his knowledge was vastly overrated, that many fat volumes could be filled with what he did not know about loudspeaker design. The salesman was adamant, however; which was great, as I loved arguing for the opposite against his steadfast high opinion of me, until I finally revealed my identity.

In the case of the Aikido expert, I never told him who I was, in the hope that I might run into him again one day and that I could relay to him what John Broskie had said after I had e-mailed John Broskie a list of this tube guru’s vast improvements to the Aikido circuit. Unfortunately, although I doubt it, this guru just might read this journal, so the jig could already be up. (But then, maybe not, as I have read where a novelist had modeled his worst character after one of his own friends, then feared running into that same friend; but after running into him, the novelist was relieved to find that the friend never recognized himself, imaging that a different friend had been the true inspiration. Self recognition is indeed difficult.)





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