Atma-Sphere S-30 MK 3.3 OTL Power Amplifier

Quality and Quantity

Equipment report
Tubed power amplifiers
Atma-Sphere Music Systems S-30 MK 3.3 OTL
Atma-Sphere S-30 MK 3.3 OTL Power Amplifier

Aside from the need to tune the bass output, the other first impression was of a slightly recessed treble range. It was the sort of tonal balance that highlighted the midrange. And there is no need to mince words here: The mids were drop-dead gorgeous—pure, detailed, and sensuous. Various front end substitutions only served to confirm that the S-30 was lacking a bit of presence region—that is the range from 3kHz to 8 kHz was a bit subdued, though I can’t state with certainty whether this is an intrinsic characteristic or (more likely) caused by a speaker load interaction. The S-30 took about 30 minutes to sound its best, and when it did, it became clear that it was capable of delivering an outstanding first watt. The majesty of the cello was fully fleshed out with finely detailed and luxurious textures. Vocals were nuanced dynamically and delivered with plenty of emotional drama. Image outlines were palpably etched within the confines of a spacious soundstage with an impressive depth perspective. Could the midrange get any better? I reached into my collection of vintage 6SN7s and rolled in a sextet of 6SN7GTB types, mostly RCAs. The answer was a resounding yes! Think sweeter, smoother harmonic textures. If you would rather invest in just a couple of vintage 6SN7s, replace the front pair. In trying to voice the amp for a slightly livelier tone, the 6F8G, the mother of all 6SN7s, worked well for me. You’ll need a pin-out adapter, but this tube is well worth the extra effort. It brought a shade of romanticism to the upper mids that compensated nicely for the S-30’s slightly dark presentation.

Although the S-30 is pitched as an entry-level product, I found it capable of competing favorably with far more expensive amps. A case in point is the First Watt SIT-1, an exceptional single-ended solid-state design from the fertile mind of Nelson Pass. While the S-30 lacked the full measure of the SIT-1’s bass impact and midrange transparency, it made up for it by being oh so much more seductive. Coupled with a wonderful reproduction of soundstage depth perspective, the S-30 loudly proclaimed its tube heritage. There was no mistaking its tube virtues.

It was also instructive to contrast the sound of the S-30 with that of a pair of refurbished Leak TL12+ monoblocks; an EL84-based ultralinear design from the late 1950s. This rather conventional transformer-coupled British classic has gained quite a following in recent years due to its absolutely lovely sonic character: it sings with sweet textures, tight bass lines, and accurate tonal colors. Matched up against the S-30 it sounded a bit polite dynamically and lacked the OTL’s engaging intimacy. There are good engineering reasons for eliminating the output transformer, and from a perceptual standpoint the payoff is increased transient speed, coherency, and immediacy. The S-30 delivered on the promise of a good OTL, eliminating a layer of complexity and nudging me closer to the musical experience.

It shouldn’t surprise you to learn that I own a Futterman H-3 amplifier, a fairly recent purchase that has been lovingly refurbished. There’s no doubt in my mind that the H-3 is still a contender today, a full 50 years after its introduction. But it’s a tweaky product that the average consumer would have a difficult time maintaining. Naturally, I was curious about a confrontation, Futterman vs. Circlotron, a mano-a-mano duel for OTL supremacy. Let me start by saying that both amps acquitted themselves very well, perhaps on balance I would award the S-30 a narrow decision, though there were noticeable sonic differences. The H-3 was the more tonally neutral of the two, which I think is due to its much lower source impedance and thus minimal load interaction. In particular, the presence region was reproduced more realistically. In this regard, the S-30 benefitted from being mated with a brighter front end. The S-30’s midrange textures were purer and sweeter sounding, and I came to realize that I wasn’t totally happy with H-3’s capacitor coupling of the output stage. Listening through a large and mostly electrolytic cap bank seemed to add a slight layer of grit and veiling to the sound.

Although the S-30 is guilty of slight timbre alterations, and presents a somewhat dark tonal balance, these acts of commission, most likely due to load interactions, did not generally interfere with the music, but point out the need to mate the amp with a compatible loudspeaker and front-end electronics. The S-30 lights a fire under the soundstage, illuminating it with commendable transparency. By eliminating the output transformer, the S-30 is able to connect the listener more intimately with the music than a host of more expensive transformer-coupled designs. The Atma-Sphere S-30 strikes an intelligent balance between quality and quantity, focusing as it does on delivering a strong first watt. Its asking price is a small price to pay for such a sizeable chunk of musical joy. Highly recommended!


Power output: 30Wpc into 8 ohms, 45Wpc into 16 ohms
Input impedance: 100k ohms single-ended, 200k ohms balanced
Input sensitivity: 3V
Gain: 23dB at 8 ohms
Power bandwidth: 2Hz–75kHz +/- 0.5dB
Frequency response: 1Hz–200kHz, +/–3dB
Dimensions: 17" x 8" x 13"
Weight: 24 lbs.
Price: $4300

1742 Selby Avenue
St. Paul, MN 55104
(651) 690-2246

Associated Equipment
Basszilla Platinum Edition DIY loudspeaker; Audible Illusions L3A line preamp; Brown Audio Labs SP-1B preamp; Experience Music autoformer volume control; Kuzma Stabi Reference, Revox B795, and Sony PS-X600 turntables; MacBook Pro laptop running Amarra V3.03 software, April Music Eximus DP1 DAC; ModWright modified Sony XA-5400ES SACD player: FMS Nexus-2, Wire World, and Kimber KCAG  interconnects;  Kimber KCAG speaker cable; Monarchy Audio AC-Regenerator; Sound Application power line conditioners

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