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Single-Ended Amplifier Survey

by Dick Olsher. Reprinted from Fi - The Magazine of Music and Sound.

Warning: this review is X rated. It contains materials not suited for young children or for audiophiles faint of heart. Explicit language and mind-altering content may cause cardiovascular distress. Do not let any audio engineer read this without first either medicating or properly restraining him/her.

The calendar on my wall says January 1996, but I’m knee-deep in single-ended (SE) power amplifiers whose roots go back to the 1920s and ‘30s. Seven decades ago, triodes ruled the day, and a single-triode output stage was standard. Nothing could be simpler: the entire audio signal (positive and negative portions of the waveform) was amplified by a single device. In audio, simpler is usually better. And although the notion of combining a pair of tubes in push-pull to amplify separate portions of the waveform was already known in the mid ‘20s, there was little incentive to complicate matters. Typical applications at the time required a fraction of a watt, and the simplicity, reliability, and sonic distinction of SE triode amps got the job done.

However, the world moved inexorably toward greater and greater power demands. First, it was the radio boom of the ‘30s and then the need to satisfy the appetite of the theater sound systems. Western Electric did its best to meet the demand for an honest 8 watts with a couple of 330A-based SE designs (the WE 86A and the WE91A). A Voice of the Theater horn-based system would just gorge itself on a single watt. Having 8 watts in its tank gave it the muscle to blast an entire movie theater with clean sound. Eventually, in the ‘40s and ‘50s, the consumer market became enamored with small box speakers. The growing popularity of relatively inefficient direct-radiator loudspeakers made an inexpensive 20-to-30-watt amplifier a commercial necessity. And that’s when power triodes and SE power amps really fell off a cliff.

The advent of the tetrode, pentode, and finally the beam power tube in 1936 made it possible to squeeze power more efficiently out of an amp relative to what was possible with triodes. A power triode is typically limited to a 25% efficiency; you have to dissipate 40 watts in the plate of a 300B just to produce 10 watts of audio power. In contrast, a 6L6 operated single-ended is 40% efficient, and when used push-pull in the now ubiquitous Class AB bias, it can even exceed 50% efficiency.

Another technical factor was the expense of a SE transformer. Unlike push-pull, DC bias current flows through the primary of a SE output transformer. The core must, therefore, be large and air-gapped to avoid core saturation. That means a large increase in iron, weight, and expense as the power level and bias current are pushed higher. Maintaining a decent power bandwidth also gets tougher and more expensive as SE transformers are pushed beyond several watts.

And so the audio world, driven by considerations of power, cost, and efficiency, plunged into the "dark ages" of push-pull. Designs became more compact per watt. Output transformers became cheaper to build and lighter on a per-watt basis. And with enough negative feedback push-pull designs looked just fine on the scope. If the general flow of events so far sounds familiar, it should: it’s the typical story of "technical progress," giving the consumer more for less money.

Rising From the Ashes

The trap of believing that if it measures better, it must sound better, discouraged most audiophiles from questioning such "progress." But not everyone was duped by the mass market conversion to push-pull. A few souls actually listened and heard the siren-call of Western Electric’s SE amps, by now regarded by the mainstream as technical dinosaurs. In an era when the Williamson amp was being proclaimed as the first true hi-fi amplifier, home constructors in Europe and Japan kept the SE flame alive. Slowly, the legend grew, until in the late ‘80s and early ‘90s, the technology finally re-established a commercial foothold. It was Audio Note’s Kondo who got the ball rolling in Japan with the Ongaku in 1988.

Why Bother?

Does it make sense to embrace the extra expense and reduced efficiency mandated by SE power amps? Is the hassle of finding a suitable load for low-powered triode amps worth the trouble? Is the additional dollar-per-watt expenditure really worth it? In my experience, the answers to all of the above questions are a resounding yes!

The payoff lies in terms of a heightened musical experience. If we set ourselves the goal of communing with the music’s deep structure (to borrow a linguistic phrase from Noam Chomsky), then it’s inevitable that we explore the SE design pathway.

I maintain that SE amplification, by virtue of the simplicity of its operating class, is inherently more capable of retrieving the music’s micromodulations and microdynamics. Of course, not all SE amplifiers are born equal. Considerations of power supply, passive parts quality, driver circuitry, and choice of active gain devices do make a major difference in the final sound. But when all else is equal, I’m convinced that SE will almost always exceed the musical intensity of push-pull .

The Audio Note Kit One Stereo Amplifier

The Kit One is a product of Audio Note UK and is imported and distributed by Angela Instruments. It is advertised as a kit for the experienced DIY tube audio enthusiast with the promise of delivering "eight glorious watts of pure Class A single-ended triode power."

My sample was assembled by an Angela Instruments technician and delivered fully assembled. Therefore, I can’t comment on the degree of difficulty involved in assembling the kit. But I’m told that if you can mod a Dyna, you can build the Kit One.

The design is simple yet quite elegant. One half of a 6SN7 dual triode is used in the input stage. The driver stage consists of a 5687 twin triode connected as a constant current cascade. A Russian 5U4G rectifier is used in the B+ power supply.

This "ugly duckling" sure took an instant liking to the Poly Natalias, sounding its best off the 8-ohm taps. The soundstage was admirably transparent with a superb feel for space, and always believable 3-D image outlines. There was plenty of detail, but above all else, the Kit One connected me with the heart of the music. It put me directly in touch with the music’s emotional punch.

Objectively speaking, reproduction wasn’t perfect, especially at the frequency extremes, but in the context of the Kit One’s overall achievement its flaws don’t seem to matter as much. For the record, the treble lacked sparkle, being shut-in and slightly soft sounding. Harmonic colors were on the gray side of reality; at times I kept wishing for a bit more vividness. The mids, however, were portrayed with the directness and honesty characteristic of a 300B. I really liked its rhythmic attitude, constantly propelling the music forward.

With the stock Golden Dragon 300Bs, bass definition was decent, but a bit weak in terms of impact. Rolling in a pair of Cetron 300Bs turned out to be a really good thing. Not only did the sound become smoother and sweeter, but bass punch improved to the point of being VERY respectable. I don’t know why, but this puppy can kick more ass than its infinitely more expensive cousins in the Audio Note family. The Vaic Valve VV300B also works well in this context, coaxing yet more purity out of them mids . . . but there’s a substantial price differential to contend with.

Ok, I’m impressed. The Kit One is fully competitive with amps costing several times as much. At its asking price it qualifies as a genuine bargain. Hell, let me be even more emphatic: the Kit One has got to be the greatest single-ended amp bargain on this planet. Build this ugly duckling and watch it turn into a sonic swan.

The Audio Note Meishu Phono Integrated Stereo Amp

First, a little math: Meishu Phono = M2 Phone + P3. The P3 is the power amp section of the Meishu (Japanese for master craftsman), with the M2 Phono is its MM phono preamp section. Since I did not use the phono section, I’m basically reporting on the sonic capability of the P3 amplifier ($3,995).

The Meishu is representative of Audio Note UK’s Level 3, or highest level of audio products. It is rated at precisely 8.5 watts per channel, which brings a smile to my face. I guess that for single-ended triode, every half watt is important. With its stock complement of Cetron 300Bs, the Meishu was a model of midrange refinement. As suave as James Bond in a tuxedo, its reproduction of harmonic textures was always supremely effortless. What a relaxing amplifier! Vintage 300B sound.

The soundstage expanded to fill the front of the listening room as in a super nova. Depth layering was particularly convincing. Low-level detail retrieval was superior to that of the Kit One, as was treble extension and harmonic color through the upper registers. The tonal balance was slightly on the warm side, almost tailor-made for the Poly Natalia.

On the debit side, the volume pot was noisy. Also, mid bass impact as in kick drum slam was a bit shy. Dynamic shadings weren’t fleshed out as well as with the Ongaku, but than that’s almost an oxymoron: you could say that of just about all other amps out there. Finally, there was a persistent and slight dulling of soprano upper registers.

In the end, the Meishu won me over with its ability to communicate feelings. Lorin Rowan’s "My Father’s Son" (My Father’s Sone, Black Dahlia Music BDM-78002) tells a personal story. Staying on pitch is secondary to sharing a range of feelings with the audience. The Meishu grabbed my attention, and didn’t let got ‘til the last bar rolled on by. What more can you ask of any power amplifier?

The Audio Note Ongaku Integrated Stereo Amp

A Kondo-san product. It was in 1988 that Hiroyasu Kondo rolled out the 211-based Ongaku, thereby setting at least a couple of world records. The one that comes immediately to mind is the dollar-per-watt ratio, which has escalated over the years to its present 3.3 kilobuks per watt. For most of us working guys and gals, the price tag is what gets the most attention. Price is like obesity--it stands out--but unfairly in this case. You see, the Ongaku is really a misunderstood product. It should be viewed as an artisanal product, all the way down to the component level. Where else would you find hand-crafted silver-foil caps and hand wound silver-wire transformers? Here’s a passionate attempt to reveal the 211’s full sonic glory.

Kondo understood a basic audio truth: everything is in the signal path. The power supply, wire, resistors, caps, and of course, the output transformer, all contribute to the sonic signature of a tube amp. Normally, it is par for designers to make a variety of engineering compromises during the birth of any product. Not so with the Ongaku. Off the shelf parts were rejected in favor of hand-built parts. Kondo’s experience with silver wire led him toward a radical solution: avoid copper in the signal pth. That explains the approximately 21 pounds of silver residing in the Ongaku; a veritable silver mine!

Another most wise design decision was to opt for a full-wave vacuum tube bridge rectifier. Sure it’s more expensive to implement than a solid-state bridge rectifier, but in terms of sheer musicality there’s nothing like it. Tube rectifiers in my experience are much less noisy and more microdynamic than solid-state diodes. The rest of the tube complement should be studied closely by the industry. The 6072 (12AY7) and 5687 are great sounding twin triodes, but out of favor relative to the popular triumvirate of preamp tubes: the 12AX7, the 12AU7, and the 12AT7.

Let me set the scene for you. Audio Note’s Mike Trei and Herb Reichert are at the doorway lugging the Ongaku into the listening room. Lesley’s on hand, quite curious to see and hear what all the fuss is about. As the Ongaku is lowered into place between the Poly Natalias, Lesley gives it a quizzical once over. She turns to me while pointing at the amp: "Is this it?" she asks. "Sure doesn’t look like a $****** amplifier." I say, "OK, let’s check it out." The lights are dimmed, and we all settle down for a quick listen. A couple of hours later, and Lesley is flushed with excitement: "I’ve never sounded any better," she declares. Lesley on Lesley, as definitive as it gets. And I’ve got to agree, the Ongaku fleshes out feelings inherent in the human voice like nothing else. The morning after, Lesley says something she’s never ever said before about any audio product I’ve had in the house: "Do you think we could buy an Ongaku?" Exsqueeze me!? Lesley’s in love with an amplifier!? I couldn’t believe my ears. It just didn’t seem like a rational thought at first, but the idea of having to give up the Ongaku was too much to bear. No, in the end we didn’t buy it. Of course, if we didn’t have a son in college, and could afford to liquidate a small house, we would have. Oh well, maybe if we win the lottery, dear.

The first watt sets the stage . . . and it belongs to the Ongaku. Absolutely the best midrange I’ve ever heard. Incredible spatial and low-level detail resolution. The sound-stage unfolds with breathtaking delineation of the depth perspective. Harmonic colors are close to perfection, with just an occasional slight dulling of the brilliance region. The Ongaku’s most endearing attribute though is its reproduction of the human voice micromodultion, which leads the auditory system to embrace the Ongaku without any reservations. Recordings that I didn’t much care for or enjoy previously were all of a sudden getting me involved in the music. I remember a humbling experience in my French class in High School (many years ago). I thought that I was pretty good at it, until one day we donned headphones and listened to tapes of native speakers. The speed at which the speech patterns scrolled by was such that I could comprehend nothing. With the Ongaku in the chain, it felt like musical textures were "slowing down" for me so that I could readily deciper the music’s deep structure.

About half way through my listening tests, I started complaining about the bass quality and dynamic constrictions in the range from loud to very loud. It was at that point that I decided to change out all of the rectifier tubes and rolled in the RCA 211s (that Herb had left with me). The Ongaku instantly gained punch and rhythmic drive through the lower registers to the point where it was able to rock ‘n’ roll. According to Herb Reichert, the original set of tubes had seen considerable duty. I would certainly plan to retube at least the rectifier tubes every 1,000 hours.

For an amp that’s probably not a champion on the test bench, the Ongaku blows away the competition in crucial areas that touch the soul. It is able to communicate the musical message like nothing else I’ve heard to date. The Ongaku is truly a sonic talisman that will bestow upon its bearer a lifetime of musical joy.


This page was updated on: 06 Aug 1998
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