s o l i d s t at e

By Wim de Haan
The Pragmatic Class D Amplifier
With the "best-kept secret" in audiodom, you can construct your own Class D amp.
lass D amplif iers are gain- amplifier using B&Os ICEPower teching in popularity these days. nology based on the 200ASC board as Hypex and Tripath offer great used by Cary Audio and Jeff Rowland. Searching eBay is often part of my products for DIY enthusiasts. daily routine, and one day I came across Class D has some advantages over class A, AB, B, or tubesone of which is a tiny amplifier that was completely units high efficiency. At normal output known to me. Even the companys name power there is no need for heatsinks, Pragmatic didnt ring a bell, but it piqued simply because heat dissipation is al- my interest nonetheless. The ad listed most nonexistent. At higher output the following features: class D, low disonly a rather small heatsink is required. tortion of typical 0.01% to 0.05%, no Another advantage is the amps physical ripple in the passband, and audiophile sound. size and weight. One of the very early class D designs Searching for pragmatic amplifier was the Sinclair X-10 amplifier, which in the Google search engine, I was diswas sold in the early 60s as both a kit appointed by the number of related and as a complete product.This power hits. Either this amp was a forgotten amplifier yielded 10W of output power treasure or just not a good item, but I but, in reality, produced only a couple of decided to take a risk anyway. I purwatts.Those who successfully built the chased three of the ten pairs of amplikit reported that the amplifier radiated fiers the eBay auction offered. The final like a radio transmitter. price was $115 USa good dealand In the 80s I repaired high-end audio I later regretted that I did not buy all for a specialized audio shop. Many rare ten pairs. amps passed through my hands, including the heavily-built Sony TA-N88 The Pragmatic audio power amplifier. This model was Class D Module one of the first commercial amplifiers The modules (Photo 1), measuring only to use class D pulse width modulation 8.7 5.1 2.0cm (L W H), use (PWM) technology, along with the use of a switching power supply. At that time I did not really pay attention to this. My interests in class D started in 2000 after reading an article on Tripaths class D technology. I even ordered some modules. However, for some reason, I never got around to using them in a project. Last year I listened to Tripath and Hypex UcD based PHOTO 1: Pragmatic mono class D module. class D amps and built a class D 26
SMD components throughout. Because the modules use an EMI/RFI PCB enclosure, you cannot determine component locations or any device numbers. So the amplifier technique and circuit design remain a mystery. Pragmatic Communications Systems has been selling these class D products since 1996. Prasanna Shah, who designed this class D module, says the following: One reason for our anonymity is that we have not been able to spend money on magazine advertisements. We have invested money in technology and performance rather than advertisements. We have sold our amplifiers to lovers of high fidelity all around the globe (Sweden, Norway, Denmark, Switzerland, UK, Germany, the Netherlands, Belgium, Italy, France, Hong Kong, Singapore, Australia, USA), and most of the sales have been word-of-mouth referrals. Shah continues: The Pragmatic amplifiers have been tested against very expensive Mark Levinson amplifiers at Madrigal Audio Labs listening station. at listening sound levels they could not tell the difference between the Pragmatic amplifier and Mark Levinson amplifiers, except that the Mark Levinson amplifiers were an order of magnitude more expensive, weighed more than 100 times more, and dissipated tremendous amount of heat energy into the room.
The Practical Realization
These tiny amps are very easy to use. Each amp comes as a complete working module, so

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there is no need for component soldering or stuffing a circuit board using SMD devices and no need for adjustments. Each amp comes with three pairs of wires: power supply (red and black), line input (orange and black), and a speaker pair (positive output marked on PCB). All you need is to connect the 12V DC PSU (at least 5A per channel) and a fine line input signal (plus, of course, speakers) to have a full power amp.
I first set up a laboratory power supply and two Pragmatic modules, with no enclosurejust some wires laying around on the floor. All worked without hum or hiss. I built three amplifiers. Amp One ( Photos 2-4 ) uses an ABS enclosure and a Dell SX280 miniPC 12V DC switching power supply, both of which I bought for not too much at a radio flea market. The standby function of the PSU is used for switching on and
PHOTO 2: Prototype Amp One.
PHOTO 3: Overview inside Amp One.

PHOTO 4: Amp One.

off, with the speakers not suffering from any dangerous pops or clicks. I built this amp for a total cost of less than 70 euros (about $65 US). Because the first amplifier performed so well, I built another, better-looking, amp. Amp Two (Photos 5-7) uses a switching power supply by XP Power (Farnell 1337158), which was a leftover from a former project. A PSU by Mean Well, such as the SP-150-12 or SP-100-12, would be a better choice, pricewise, but the XP PSU fits the used enclosure perfectly, and the specs are great. Amp Two uses a Sansui VU meter, which I also bought at a radio flea market. The VU meters use a dedicated DIY circuit board by JLM Audio Australia, and these are directly mounted on the VU meters. This PHOTO 5: Front Amp Two. circuitry is powered by the 12V DC from the switching power supply. The 12V scale lighting uses a flexible white LED strip. Because I still had one amplifier pair left, I built a third amplifier ( Photos 8-10). This amp uses a conventional power supply with a toroidal mains transformer and a voltage regulator; I used one single 120VA transformer and a LM338 voltage regulator for each channel. The LM338 comes in TO-3 housing and is good for 5A. PHOTO 6: Amp Two overview. I also purchased all parts for this amp at a radio flea market. The LM338 goes for 1.50 euros and the 120VA mains transformer for 20 euros. I bought the ABS enclosure for just 15 euros. Use of a switching power supply is interesting, not only because of its price and compact size, but also because this kind of power supply works on a similar principle as that of a class D (or pulse PHOTO 7: Amp Two inside overview. width modulation) ampli-

audioXpress June 2009

fier except that the reference voltage of a PWM amplifier is the varying audio signal instead of a fixed voltage as in a switching power supply.

TECHNICAL ASPECTS

Its monoblock design provides extremely clean state-of-the-art power. The SIWA-248 unit accepts any volumecontrolled line level input and uses a 12V-15V DC power supply. It has continuous power of 24W and a peak power of 60W. Advanced protection circuitry guards against system damage from overload, overheating, or short circuits.

TABLE 1

COMPANY SPECS Power: 12-15V DC Continuous power: 24W per channel Peak Power: 60W Signal-to-noise ratio: >100dB Dynamic range: >100dB Frequency response: 20Hz to 20kHz Applications: Features and Benefits Compact monoblock amplifier Residential or commercial audio amplification compact design High efficiency High clock frequency Very linear gain and phase character-
istics Flat frequency response from 10Hz to 50kHz No ripple in the passband Very high signal-to-noise ratio Very high dynamic range 24W (peak 60W) Only 12V DC power required Module works with 4 and 8 loudspeakers Very low and uniform distortion on all frequencies and very high signalto-noise ratio and dynamic range.

SOUND IMPRESSIONS

How to connect the Pragmatic module power supply.. +12V DC = red... ground = black. line input.. signal in = orange.. ground = black. speakers... - output not marked on PCB.. + output is marked on PCB. Please note: the amplifier circuitry is designed in such a way that you can not connect the negative loudspeaker output to ground.
To evaluate the sound performance, I used a large Tannoy D700 and a small-size Dynaudio. The Dynaudio is based on a T330D Esotar tweeter and 17WXL04 low mid unit, both by Dynaudio. At first you notice the ease with which music is reproduced. Sound
PHOTO 8: Front Amp Three. PHOTO 10: Amp Three inside.
PHOTO 9: Amp Three overview.
PHOTO 11: The Pragmatic SAA-224 stereo standalone amplifier.
is very clean, dynamic, and precise with a beautiful soundstage. The Dynaudios 4 load and 84dB sensitivity are no problem for these low power modules; however, the Tannoys 94dB sensitivity allows headbanging levels. Big Band sound, such as in Gerald Wilsons In My Time CD, drives both speakers to a very high level. Music is reproduced with great authority and ease, rhythm and pace are excellent, and it is also free of any distortion artifacts. Also, at low and moderate levels all music can be felt and heard; its dynamic behavior is excellent. Chet Bakers Great Last Concert (Enja) sounds impressive, with very good tonal balance and great atmosphere/ambience. Piano solos such as classical pianist Arcadi Volodos Live in Carnegie Hall (Sony) are wonderfully reproduced; the Rachmaninoff pieces are extraordinary and captured by this amp in the best way. Audience, the bench, recording/hall noises/ambiencethis amp offers high-resolution sound with impact and drive. Another great piano solo recording is Marc-Andr Hamelins Nikolaij Medtner four-CD box (Hyperion). This wonderful recording by Tony Faulkner is real, and, with these Pragmatic amplifiers, the Dynaudios fade away. The low and high units melt together as the left and right speaker and results in a threedimensional sound. The music of Russian composer Nikolaij Medtner results in a wonderful journey. Natalie Coles voice on Take a Look is full-bodied with lots of air around it; the differences in recording of the different tracks can easily be detected. Fusion jazz such as Lee Ritenours Wes Bound (GRP) will make you smile. Such big sound from such a tiny amplifier. Also noticeable is that the higher frequencies dont show any rolloff. The Keith Jarrett Trio Live at the Blue Note six-CD set is a real treasure; I guess these Pragmatic modules already knew this. Yes, I am aware that I am overly enthusiastic. These modules are really fun and capable of reproducing music in all its pros and cons.

setup. Musicality is enormous; they can rock and shake. Intimacy, imaging, soundstaging, and high resolution are its strong points. I guess these modules are probably the best-kept secret in audiophile circles. And remember: this class D design dates from 1996, which is remarkable because class D back then was not in the picture nor as developed as today. Just before publishing deadline, I found out that the Pragmatic Company never sold these modules separately, so
the modules I bought on eBay were likely taken from a Pragmatic SIWA in-wall configuration. However, the Pragmatic Company sells modules as standard products. Check out their website for the standalone amplifiers such as the mono MBA-124 and the stereo SAA-224 amplifier (Photo 11), as well as in-wall configurations such as the SIWA-224; the first two are beautifully made standalone units with external power supplies. A 12-channel version is also available. aX cial applications. The success of these products has led to a strong and dynamic technology base, which has been applied to a new generation of products such as CATSTM and others such as the DMSTM, TrueMusicTM wireless hi-fi system and TrueViewTM wireless video system. The companys philosophy is to use its technologies to solve problems that have gone unresolved for decades. Further products include highperformance, low-profile line-array speakers, high-efficiency audio amplifiers (monoblock, stereo, multichannel, in-wall), high-performance TV/CATV/FM tuners, category 5 (CAT5) audio/video transmission and cross-point matrix switchers, hard disk music servers, ambient audio levelling systems, microphones, and IR detectors. PADSTMMulti-room audio distribution system on a single category 5 wire. DMSTMMulti-source, multi-zone distributed music system on a single category 5 wire. DAVESTM 4.43Distributed Audio Video Entertainment System. CATSTMMulti-source, multi-zone DVD quality audio/video signal distribution and IR control on a single category 5 wire. CATS TM HDHigh definition audio/video on category 5 wire. The Class D tiny power amplifiers are part of the so-called Hide-AwayTM Series (SIWA). All are trademarks of the Pragmatic Company.

About the Designer

Prasanna Shah is the Chief Pragmatist and Thinker at Pragmatic Communications Systems, Inc. He earned his M.S. degree in Electrical Engineering from Stanford University and B.S. degree in Electrical Engineering from Santa Clara University, Santa Clara, Calif. He is a member of Tau Beta Pi and Eta Kappa Nu Electrical Engineering Honor Societies. He has almost three decades of experience working on semiconductors for analog and digital audio, video, cellular telephones, wireless communications, fiber optic communications, data communications, data acquisition, and industrial controls in the Silicon Valley. He holds one patent on power line communications, several patents on audio, video, and wireless multimedia systems, and many more patents pending in the area of industrial automation, health care patient interaction, and power generation/energy storage technologies. An avid audiophile and music lover, in his free time he likes to apply his pragmatic thinking to solve complex problems.

About the Company

EPILOGUE
These interesting Pragmatic modules are very quiet, even in my simple test
Pragmatic Communications Systems, Inc. (www.wireless-experts.com and www.pragmatic1.com) is a Silicon Valley company established in 1994 to design, develop, and produce a variety of innovative products for wireless audio, video, and data communications. The majority of the companys products until 1997 were primarily designed for industrial and commer-

We Interview Nelson Pass

From LP cutter-head electronics to car amps, from the 1kW/ ch X1000.5 to the 10W/ch FirstWatt series, from open-baffles to giant pipe speakers, Nelson Pass has designed them all. and successfully so. Lets meet this man who chooses his own direction, and gives generously to the audio DIY community.

By Jan Didden

Jan Didden (JD): Mr. Pass, most readers will be familiar with the audio product line that bears your name, from Pass Labs. Some might also know that before Pass Labs you founded Threshold Corporation, known for among others, the Stasis amplifier series. Less people know that before starting your own company you designed amplifiers for others. Can you talk a bit of that history? Nelson Pass (NP): Threshold was successful for about eight years before other companies showed interest. For the record, my ownerships have been Threshold, Pass Labs, and General Amplifier (First Watt). I have collaborated with 13 other brand names that I can remember. A couple of these I
should not name, several do not deserve to be named, and the following are ordered chronologically: I worked at ESS, where Peter Werback designed the amplifiers. When I gave up my little side business building car amplifiers (back in the days of Craig and Kraco), I offered Peter my small customer base, and the resulting company, Linear Power, was quite successful until he died in a plane crash. So I filled in part time for a couple years, helping out with amplifier design work, an accounting system, and a subwoofer with motional feedback. After this I was introduced to Rob Ain and Alec Chenin of Adcom, where I designed the GFA555 amplifier and its relatives. These products were very
successful commercially, teaching me to charge more money for my work. These amplifiers used bipolar transistors in a simple four-stage circuit. The input and voltage gain stages were biased by constant current sources, which drove Darlington emitter-follower output devices. It had an interesting clip detector which sensed feedback overload in the voltage gain stage and lit an LED. It also had a very effective bias circuit which fixed the bias voltage at the output of the driver transistors through local feedback, giving the amplifier improved thermal stability. JD: The Adcom amplifiers were quite successful, werent they? They had and still have a very good reputation,
Multi Media Manufacturer | January February 2010
and still command a good price on the used market. Your reference to feedback bias circuit is intriguing. Can you give some more details about that? NP: Its pretty simple. For production convenience, you want to make the bias current of the amplifier output stage easy to adjust and you want it to stay at that value forever, so I made up a simple self-adjusting bias circuit. Here are two diagrams, the first showing a simplified version of the GFA555 (Fig. 1) and the second another version which I later patented (Fig. 2). In the GFA555 circuit, the bases Q7 and Q8 see the voltage across R1 and begin conducting when that voltage is
enough to bias up their PN junctions (about 1.3V). This controls the voltage across the bases of drivers Q3 and Q4 with a local feedback loop and biases output devices Q5 and Q6. The loop eliminates the thermal drift of Q3 and Q4, which improves the output stage bias current stability. This still leaves the thermal drift of output devices Q5 and Q6, but the thermal characteristic of the PN junctions of Q7 and Q8 tend to compensate for this if they are in thermal contact with the output stage. The second circuit is similar, but uses an optical isolator, which eliminates any electrical connection between the
current sensing and bias control parts. Of all the collaborations, Adcom was the most personally enjoyable. I enjoyed working with Rob and Alec, and we had a good relationship up to the sale of the company. I still talk to Rob occasionally. The relationship with Nakamichi was more profit oriented. Around 1985 we were approached for a patent license and design work, which resulted in the Nakamichi versions of the Stasis amplifiers. Actually, we did not have to do a lot of workthey took our existing schematics and rendered them into their own version of the product. At the end of the process, they

FIGURE 1: Simplified version of the GFA555 bias generator.
FIGURE 2: Opto-isolated feedback bias generator as patented by Mr. Pass.
brought one up to Foresthill to have it tweaked, and there we were. Nakamichi also led to design work for several car audio amplifiers by Soundstream. This project was a lot more work, but ultimately resulted in a product line that included the D200, D100, A100, A50, and so on. There wasnt anything groundbreaking in the
circuits, but it was my first commercial experience with switching power supplies. JD: In the 80s switching power supplies were relatively new and didnt have a very good reputation. The Sony TA-N86 and 88 come to mind. When they failed it was difficult to find

PHOTO 1

a repair service that actually understood enough about these circuits to be able to repair them. What was your experience at the time? Now that the technology for switched supplies has matured, would you use them for power amplifiers? NP: This is not really my area of expertise. As a concept, I would consider using them, but it would take a considerable investment in time and energy that I dont have available at the moment. I would be far more likely to use them in low power equipment, where they can be made to have lower capacitive coupling to the AC line and where noise issues are a little easier to deal with. But for the time being my attention is elsewhere. So, shortly after founding Pass Labs in 1991 I did some work for Mobile Fidelity, replacing the Ortophon electronics driving their cutter-head. This was a particularly challenging project, as the original electronics were very complex, with tons of ancient op amps in the circuit. The goal was to create a better-sounding system by simplifying the active circuitry as much as possible. The functions included balanced inputs, reverse RIAA equalization (full and half speed), additional equalization, feedback, feedback equalization, and balanced outputs. The outputs went to balanced Class A power amplifiers which drove the cutter-head, and feedback from the cutter-head came back to the control unit.
PHOTO 1: Nelsons Kleinhorn experiment, not an exercise in modesty. PHOTO 2: Nelson (right) and the author PHOTO 2 (left) at the first Burning Amp Festival in 2007.
Because cutter-heads are very delicate and expensive to repair, there were numerous delicately adjusted protection circuits. The cutter-head drive coils were cooled by helium and monitored for temperature by running a very small amount of DC current through them. A circuit sensed the DC voltage across the coils and interpreted this as temperature. The protection needed to respond quickly, so the filters were quite sharp to avoid being triggered by the AC drive signal. To accomplish all this, I first measured all the characteristics of the electronics and the cutter-head as precisely as possible and created a model of the system that I gave to a simulator (an early version of MicroCap). Then I began simplifying the active electron-

ics by consolidating as many functions as possible into a few active stages. In the end I was able to get all the gain, equalization, and feedback folded into one active stage which still delivered the same objective performance. The power amplifiers were cousins of the A75 design and predecessors of the later Adcom MOSFET designs. The project was successful, paid well, and broadened my experience in vinyl production and circuit simulation, but it was a tremendous amount of work over a couple years. JD: You mentioned the A75. I believe that was a successor to the A40 which you published in Audio Amateur. Still mostly bipolar, if I remember correctly. At what point did you go to mostly or all-MOSFET designs? And what was
the main reason for it? NP: I cut my teeth on bipolars in the late 60s, and with the exception of JFET inputs, followed occasionally by small MOSFETs, the Threshold products were bipolar. In 1980 I took a small leap with the power MOSFET version of the HK Citation 12 for an audioXpress DIY project, and after that my DIY pieces, including the A75, were MOSFET. Threshold continued to make bipolar, but when I left in 1991 to form Pass Labs I had by then decided to concentrate on Class A FET design. There were two reasons for it. First, I found the FETs easier to work with in terms of obtaining equivalent or better performance with simple Class A circuits. Second, I liked the sound of the FETs better than bipolars, and
still do. Now that some decent power nominally successful. I think the same JFETs are available, I am getting more is true of home theater. The customer interesting results, and I look forward is more focused on dynamics and arto working with them more in the ticulation, and some of the traditional future. high-end two channel qualities fall by Speaking of MOSFETs, in 1992 Adthe wayside. com came back to the table for a new series to replace the GFA555 family, JD: I dont want to open a whole can which had grown long in the tooth. of worms, but you seem not to be This became the GFA5800, 5500, overly concerned about getting very and so on. The circuits are familiar by low distortion. For your First Watt now: three stages of MOSFETs, using amps you have made the point that an input differential pair biased by a at the low levels where most listening constant current source, this driving a takes place, the distortion is very low. P channel VAS, also current sourced, Yet, at high-level peaks (especially with and followed by complementary MOS- low- to medium-output power amps), FET followers. I also delivered a simple distortion does rise considerably. But line stage preamp design, the GFP750, which was vaguely similar to the Aleph P preamp, but also incorporated a switch for active versus passive operation. I designed the Adcom car audio products, the GFI 4702, 4600, and other amplifiers. These were circuits based on the MOSFET designs for the home audio equipment, and they were pretty unusual for the car audio market. They FIGURE 3: First Watt F1, how simple can it get? seem to still have a good reputation today. people like your amps. Does it have to JD: Is there a principle difference in do with the way the distortion rises, designing car audio amplifiers or home or with the spectral make-up of the stereo amps, apart from the obvious distortion? 12V nominal supply in cars? NP: The characteristic I get is the NP: Its a very different aestheticthat result of simple FET circuits running is to say that the customer has very in Class A. This means minimal or no different expectations. When I did the feedback, low-order harmonic characAdcom car series, I applied highteristic, and monotonic but pedestrian end hi-fi sensibilities to the design. distortion numbers. It sounded great in the living room, That approach usually (not always) but did not particularly impress the delivers the sound that I like, and customers, and the product was only apparently my taste matches up with

a portion of the buying public. The only magic involved takes place in the neural networks between our ears. What happens there is still poorly understood and varies between individuals. Whatever the deal is, ultra low distortion doesnt seem to have much to do with it. JD: I know you have been involved with speaker design as well; can you tell us a bit more about that? NP: I started out building speakers, and found myself at ESS about two weeks before Oskar Heil arrived. I spent the next couple years working up all manner of enclosures and crossover networks, sharpening my electronics skills, and soaking up all there was to know about a start-up manufacturing operation. Subsequently I spent a year running the service department at the local Sun Stereomore valuable experience poking around inside the broken amplifiers of McIntosh, Phase Linear, Sony, Pioneer, and JVC, while I worked on my own designs at night until it was time to launch Threshold in 1975. But the speaker bug never leaves you, and so I have always maintained a tablesaw and a router. We have always kept a few examples of commercial speakers so as not to lose touch with the marketplace, but most of the time my listening space has been graced with unfinished plywood boxes. I met my wife Jill at ESSshe was the Heil diaphragm testerand she doesnt have any problem with that. When I met her I had the Claw, a straight bass horn with a 50ft2 mouth and 9 depth.
Since then she has lived with the Kleinhorns (nearly as large, but stereo) and El Pipe-Os at 12 high, so a nice set of open baffles doesnt cross the line. Of course, the day finally came when Pass Labs decided to offer a loudspeaker, the Rushmore, which dipped our toes in the water, and several years later I surprised her with SR1s which have grille cloths, followed now by SR2s (SR = Son of Rushmore). I spend a lot of time with those funny little full-range speakersLowther, Feastrex, Fostex, and so on in open baffles. They have their own appeal and mate nicely with my little First Watt amplifiers at 10 to 25W. JD: You have been an early contributor to the audio DIY scene; I still remember an early article of yours in what was at the time The Audio Amateur magazine on cascodes, how they work, and their advantages. You have a knack of explaining engineering things very clearly in a few words, and you obviously enjoy doing it. You are also a very active contributor at www.diyaudio.com, being very generous with advice, tips, and complete amplifier designs that people can build. What do you get out of that interaction? NP: I got my start when the boss at ESS sent an in-house paper I wrote to Audio Amateur. The first I heard about it was when he gave me the check for it. Over the last 36 years Ive tried to do something fairly regularly for Ed Dell; in fact, I have a couple of new pieces in progress. As to style, I like to speak to the teenager (me) who wanted to know this stuffthats my audience. There are always people who appreciate a decent explanation that gets to the meat and potatoes. I see it all as light entertainment with a little education

thrown in. The academic paper approach has its place, but it seems intended for people who mostly understand the stuff already. If you want to communicate with DIYers, you depend more on colorful analogies, a little hand waving, and very little differential calculus. I get lots of personal satisfaction out of the whole enterprise. It gives me an outlet for some cool ideas and things that otherwise would stay bottled up, and I have an excuse to explore offbeat approaches purely for their entertainment value. Also, the process of communicating DIY stuff is a two way streetI would say I get about as much as I give. (Authors note: Nelsons latest super-simple DIY design is the DeLite amp, which he presented at Burning Amplifier Festival 2009. Compared to DeLite, FirstWatt is overly complex. The DeLite design and construction is discussed at http:// www.diyaudio.com/forums/passlabs/153832-pass-delite-amp-baf. html).
JD: Nelson, thank you very much for your time and your frankness during these discussions. As you said earlier, diyaudio has a special place in your heart. Do you have any advice for those DIYers out there who aspire to become successful designers? NP: Ah yes, the concluding bit of sage advice: First of all, design for yourself, not for other people, and dont be afraid to try something. Then, when others want your designs, take great care of your reputation. You will not have much control over something that goes out into the world, but your name will still be on it. And, finally, be prepared for a lot M3 of work.
The A40 power amp article was published in The Audio Amateur, 4/78, and the A75 power amplifier appears in The Audio Amateur 4/92 and 1/93. His other works are posted at www.passdiy.com.