"Dragging high-end audio (kicking and screaming) into the future for over 29 years."

That’s Atma-Sphere's motto, and I like it. Although the company’s products have received an impressive array of awards from other reviewers (including Reviewers’ Choice recognition five times), my encounter with the S-30 Mk III was my first with an Atma-Sphere product, and I was eager to hear how well this amplifier lived up to the company motto.

Transformers degrade an audio signal: That’s the premise on which Atma-Sphere bases its designs, and it’s hard to argue the point. Although transformer design has come a long way over the last half-century, it’s inarguable that any transformer, no matter how good, imposes some fingerprint on the audio signal. So for 29 years, Atma-Sphere head Ralph Karsten has been making tubed amplifiers with no output transformers, known as output-transformerless (OTL) amplifiers. Atma-Sphere owns the patent on the Circlotronic OTL circuit, which it uses not only in its amplifiers but also in its preamps. Another design feature the company strongly believes in is fully balanced differential circuits. Although Atma-Sphere amps have unbalanced RCA inputs in addition to balanced XLRs, the former are only there for audiophiles like me who don't own a balanced preamp.

In the bull-pen

The S-30 Mk III ($3300 USD) is the baby of the Atma-Sphere amplifier lineup. It’s the only stereo amp; all the others are monoblock designs, including the gargantuan MA-3, which, with its 480 watts, is the most powerful OTL amp in production. As you can tell from its name, the S-30 Mk III is in its third version, and it has been restyled from the Mk II and Mk I versions. The Mk III looks more like a conventional amplifier, with power transformers in the rear, left-channel tubes on the left side, right-channel tubes on the right side, and driver tubes in the center.

Styling is gloriously retro. The amp would make a great prop for a 1950s science-fiction movie. The chassis is painted a black crackle finish (Atma-Sphere calls it "Wrinkletex") and has no fancy faceplate. A full-width transformer cover (power transformers, you know) spans the rear of the amplifier. On the front panel, there are two pilot lights: a yellow one on the left that illuminates when the main power is on and the amp is in standby, and a red one on the right that illuminates when the amp is out of standby and ready to make music. I thought it was a tad quirky that you throw the switch marked Standby to make the amp play. A third switch, in the center of the front panel, controls the VU meter in the center of the transformer cover. You use this to adjust DC offset via screwdriver-operated potentiometers on the front panel. Metalwork is neat and serviceable, though hardly deserving work-of-art status. A $150 optional stainless-steel trim panel is available to adorn the front of the transformer cover. Whether that’s a worthwhile investment is a personal decision, but it does increase the glow from the 16 tubes. My wife observed that the S-30 Mk III’s tubes looked like candles.

The S-30 Mk III is rated at 30Wpc into 8 ohms and 45Wpc into 16 ohms. Driving a 4-ohm load with the S-30 Mk III is not recommended. If you must drive 4-ohm speakers, an external autoformer is suggested. Karsten emphasizes that this transformer is not just an external output transformer, but matches the amp’s 16-ohm output impedance to a 4-ohm speaker.

The amp uses 6AS7G triode tubes for the output section, five of them per channel. The output stage is run in class-A operation with a miniscule 2dB feedback. Biasing is automatic, relieving the user of the need to bias all those output tubes. Karsten explained that the tubes are run at only 140 B+ voltage, which should assure long life and probably explains why Atma-Sphere can offer a one-year warranty on the tubes, the longest tube warranty I’ve encountered. Driver tubes are all Chinese 6SN7s. The two channels can be strapped for increased mono power. Hand-wired in dual-mono configuration, the simple circuit (only one gain stage) uses proprietary, precision-grade components. A star ground circuit is used to assure low-noise operation.

Both the XLR and RCA input jacks are located on the front panel. In the rear, a pair of copper Cardas binding posts provide a firm grip on the speaker cables. I love Cardas connectors. I’ve replaced the stock connectors on two of my amps with them, and the result was better sound and much more reliable terminations. I’d like them a lot better on the S-30 Mk III if they were clearly marked to show which terminal was positive and which negative. If you squint closely at the plastic bezel around the binding posts, you’ll discover that the positive terminals are those toward the sides of the amplifier.

The S-30 Mk III’s specifications surely don’t look like those of the SET amps that I normally use. Power bandwidth is specified at 2Hz-75kHz +/- 0.5dB, frequency response at 1 watt is 1Hz-100kHz +/- 3dB. Total harmonic distortion is specified as 1% at full power, while intermodulation distortion is typically less than 0.04% at full power. Most SET amps are rated at a 10% distortion level. I don’t know at what level distortion becomes inaudible, but I do know it’s lower than 10%. Phase shift is less than 1 degree at 20kHz, possibly as a result of the output section’s rise time of 600 volts per microsecond.

Comparing these specifications to those of a normal tube amplifier shows us how much output transformers limit the audio signal. The S-30 Mk III’s specifications would look good for a solid-state amplifier. However, the amp isn’t easy to drive. While its input impedance is 100k ohms through the RCA jacks, and 200k ohms through the XLR jacks, input sensitivity through the RCA inputs is a very low 2.83 volts for full power output. That means you’ll need a preamp with lots of gain.

Some early OTL amplifiers tended to go up in smoke if not presented with just the right operating conditions, but Atma-Sphere amps are designed to be bullet-proof. The S-30 Mk III is stable with all loads and inputs. Karsten performed an interesting trick at the 2006 Rocky Mountain Audio Fest. He took a quarter and shorted out the speaker terminals while the amp was playing at a high level. Of course, the music stopped, but when he removed the short, it started playing again with no problems at all. He also performed an even more surprising trick by reaching down, while the amp was playing along merrily, and removing one of the output tubes. Nothing happened; the amp continued to play just fine. If you’d done either of those things to some of the early OTL amps, you could expect large repair bills. The S-30 Mk III will actually play at a reduced output level with only one tube per channel.

I had an unexpected opportunity to test the amplifier’s ruggedness when a 6AS7G tube failed with a bang (literally) and a rather scary light show. The amplifier was not damaged, nor was the speaker for that channel. So Atma-Sphere’s claims were substantiated by a real-world tube failure. That’s where it counts!

Batter up!

As I inserted the 16 tubes in their sockets, I was glad I received this amp to review during the fall and winter -- it will help cut down the heating bills! While the amp gets pretty warm, it produces nowhere near the searing heat of a single 845 tube. However, my cat clearly appreciated the amp, lying on the floor nearby to soak up the warmth.

Because all of my preamps have only unbalanced outputs, I wasn’t able to drive the S-30 Mk III though the preferred XLR connections. The terse instructions (one sheet of paper) directed users who want to use the RCA inputs to construct a jumper and run it between pins 2 and 3 of the XLR jacks for quieter performance. Some may wonder "What’s a jumper? What do I make it out of? Once I’ve got one, what do I do with it?" It would have been nice if there had been a description of a jumper (just a wire) and a picture showing how to connect it. I used Radio Shack hookup wire as jumpers and, at first, got a buzz and distorted sound from the left channel. I decided to be more elegant, so I obtained some Radio Shack XLR plugs and soldered a jumper wire between the appropriate pins. That produced a much better connection. I wondered why jumpers weren’t included with the amp.

Since the input jacks are on the front panel of the amplifier, connecting preamp to amp can pose a challenge. In my equipment rack, the amplifier is on the bottom shelf, with the preamp on the shelf above it. Running interconnects from the rear of the preamp to front of the amp required routing them under the preamp and down the front of the rack into the amp’s input jacks. That required some very flexible cables; several would not bend sharply enough to allow such connection. I first tried Crystal Cable Piccolo interconnects, whose small diameter makes them extremely flexible, but with the S-30 Mk III, their frequency extension seemed somewhat restrained. The Dual Connect DC-I100 interconnects were also very flexible, but their high-frequency extension seemed a bit restrained, although their lows were quite strong. So that left the only other flexible interconnect I had, the $145 DNM Reson cables. These had plenty of extension at both frequency extremes, and they portrayed a wide-open soundfield. The Piccolos were a bit smoother in the midrange, however.

Speaker cables were less problematical. I used Dual Connect DC-S200 and Crystal Cable Micro speaker cables for most of my listening, but I also tried Blue Marble Audio and Purist Audio Design cables with good results. I used Atma-Sphere’s ordinary-looking stock power cord because I believe components should be tested in as-delivered form. However, I also tried an aftermarket power cord -- a Purist Audio Venustas -- to see if it made an improvement. Wow! Everything snapped into sharper focus. I heard more detail, better instrumental separation, more spaciousness, better defined dynamics, and deeper (though no more powerful) bass. If you ever wonder if those high-priced power cords are worth their cost, here’s proof of their efficacy. It’s a good thing I made all my observations before trying the Purist Venustas -- I couldn’t bring myself to put the stock cord back afterwards.

Although my deHavilland Mercury 2 preamp displayed its customary elegant sound with the S-30 Mk III, its low gain (10dB) meant I had to crank up its volume control rather high for recordings that were made at a low level. So I substituted my vintage Audio Note M2 Signature preamp, which has lots of gain. It drove the S-30 Mk III to ear-splitting levels with the volume control around the 12 o’clock position. Its smooth sound was also a great match for the S-30 Mk III’s dynamic presentation.

Play ball!

The first time I sat down to listen to the S-30 Mk III, stone cold right out of the box, the result was thrilling. I was using Opera Consonance M12 speakers along with Dual Connect interconnects and speaker cables. The bass response was more extended than I was used to, and the overall sound had greater detail and impact, with surprising spaciousness. I briefly worried that I was getting too much bass from the speakers, but I soon got over it. The S-30 Mk III also sounded great through my Second ReTHM speakers. There were oodles of dynamic power, superb soundstaging, realistic instrumental timbres, and, of course, the legendary handling of voices from the crossover-free Lowther drivers. But most of my evaluation was done with the Opera M12s, whose more laid-back presentation made a better match for the S-30 Mk III’s sound.

The Atma-Sphere amp's soundstaging was the best I’ve ever heard in my home, the Opera speakers producing a well-defined dimension of depth for the first time in my experience. OK, what I heard wasn’t nearly as precise and holographic as with line-source speakers like the Dali Megalines, the best-soundstaging speakers I’ve encountered, but the S-30 Mk III took my two-way speakers to new levels, especially with the Dual Connect cables. In the title track of the Tallis Scholars’ CD Allegri Miserere [Gimell 454 939-2], the position of the solo choir that sings from behind the main choir was quite well defined spatially. I thought I’d heard the choir reproduced accurately before, but the S-30 Mk III and Dual Connect cables did it noticeably better.

The S-30 Mk III had lower audible distortion, greater impact, and just as much of the presence as any SET amp I’ve heard. I suspect that the M12s, with their 97dB sensitivity, really benefited from the additional power beyond that of my SET amp. Ron Meyer’s spectacular Percussion Demo CD [Encore Performance Recordings EPR-2524], the most realistic recording of percussion instruments I’ve heard, spans the dynamic range from barely audible to downright scarily loud. There’s a well-deserved warning about the dynamic range on track 8, so, of course, I had to listen to this track with the S-30 Mk III. When the drummer whacked the bass drum extremely hard, the amp wasn’t fazed at all. Similarly, on Jordi Savall and associates’ "Folia: Rodrigo Martinez" from La Folia [AliaVox AV9805], the S-30 Mk III tracked the continuously changing volume level in a demonstration of exemplary microdynamics. The ultra-detailed ReTHM speakers present microdynamics effortlessly, better than any speaker I’ve heard; but the Opera M12s are no microdynamic slouches.

The S-30 Mk III was very transparent and easily revealed changes in components in the system. For example, it made the differences between interconnect cables easy to distinguish. That made it a valuable tool for a reviewer. But, more important, the music lover would benefit from how the S-30 Mk III extracts detailed musical information from recordings.

High frequencies sparkled, but weren’t over-emphasized. At the other end of the spectrum, bass was firm and detailed, and seemed to descend lower than I’d heard before. The overall presentation was quite neutral, with no part of the frequency spectrum emphasized. But the midrange is where the music lives, and the S-30 Mk III’s midrange performance didn’t disappoint. The title track from Chris Jones’s Roadhouses and Automobiles [Stockfisch SFR357.6027.2] portrayed his slightly rough voice with first-rate detail, allowing every nuance of his expressive interpretations to come through clearly. With "Folia: Rodrigo Martinez," Jordi Savall apparently was having so much fun that he couldn’t resist singing along with the music toward the end of the piece. His impromptu vocal accompaniment was easy to discern and identify as someone singing. With some components it’s hard to tell if the sound is a voice or an instrument playing -- or if it’s even there at all. The S-30 Mk III had no such trouble conveying exactly what was what.

In my ceaseless quest for a memorable turn of phrase, I was tempted to say something like "This amplifier combines the strengths of solid state -- extended frequency response and low distortion -- with the strengths of tubes -- spaciousness, dynamics, and tonal fullness." Although that would have been true, I realized that wouldn’t tell the whole story. To be more precise, the S-30 Mk III doesn’t sound much like either solid state or tubes. It faithfully delivers whatever is on your discs -- it's a conduit to the music. Perhaps that's what OTLs are all about.

The home team

The primary amp in my reference system is the $7500 Art Audio PX 25. An SET amp rated at 6Wpc, it’s obviously at a disadvantage versus the S-30 Mk III if power output is an important factor. With my Second ReTHM speakers, however, it’s not; 6 watts are just fine for 102dB-sensitive speakers. With the 97dB sensitive Opera M12s, power output becomes more of a factor, and while the PX 25 does a surprisingly good job driving the M12s, it sometimes ran out of steam on musical climaxes that wouldn't trouble the S-30 in the least. Its bass goes surprisingly deep for that of an SET amp, but it wasn’t as well controlled as that of the S-30 Mk III.

Where the PX 25 excelled was in the midrange. Although not as lush as some SET amps' middle frequencies (which I think is due to its lower second harmonic distortion), the PX 25 still has a very palpable, reach-out-and-touch-it midrange. The S-30 Mk III sounded leaner, not as palpable. The PX 25 also has stunning dynamics, but the S-30 Mk III seemed fully its match in that respect.

Which topology do I think is better? I will always have a soft spot for SET amps, but OTLs are gaining fast because of the Atma-Sphere S-30 Mk III.

Home run!

Now I understand why Atma-Sphere has won all those awards. The S-30 Mk III is the best amplifier I’ve yet heard in my system. Dynamics, soundstaging, frequency extension, timbral accuracy, vocal nuance and expression -- it has it all. It’s not uncommon to review a component that is superior to my reference in one area or another, but it’s a rare and welcome event to get one that makes improvements in all areas, and at a much lower price. The Atma-Sphere S-30 Mk III did that for me. If your speakers have an impedance of 8 ohms or higher, if this amp's ultra-retro styling doesn't turn you off, and if 30Wpc will comfortably drive your speakers, I can’t think of an amplifier I’d recommend more highly. If you need more power, Atma-Sphere has an extensive lineup of amplifiers with higher power ratings. You know, the ones that won all those awards.

Keep in mind that of all my observations were made using the amp’s unbalanced inputs. I suspect the S-30 Mk III would sound even better driven by a balanced preamp. So even though I went nuts over this amplifier, it may be even better than what I heard! At $3300, its price is hardly chump change, but it's still a big-time bargain in today’s audio market.

I can’t assess whether the Atma-Sphere S-30 Mk III drags the whole high-end establishment into the future, but it certainly dragged my system’s performance up several notches, making the S-30 Mk III an easy selection as a Reviewers’ Choice. Ralph, make some more space in your trophy case!

...Vade Forrester
vade@soundstage.com

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