March 2003

Atma-Sphere M-60 Mk.II.2 OTL Monoblock Amplifiers
Review By Wayne Donnelly
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  You know, I never get tired of listening to and writing about music, but listening critically to and passing judgment on audio equipment -- even the cool stuff I get -- can get stale now and then. When that happens, the best antidote for my reviewer blahs is encountering something that delivers a fresh, reanimating music-listening experience. The Atma-Sphere M-60 Mk.II.2, the first OTL (Output TransformerLess) amplifier I have had a chance to live with, has really gotten my reviewing juices flowing again.

Different Drummers...

Experienced audiophiles know that certain categories of equipment are controversial and stir passionate exchanges between adherents and detractors. In the amplifier category, single-ended triodes (SETs) have inspired cult-like devotion from listeners who extol their tonal warmth, lush harmonics and emotionally engaging musical presentation. Critics of SET designs -- who include virtually every maker of push-pull tube amplifiers -- point to typically poor measurements, rolled-off frequency extremes, restricted dynamics and excessive tonal coloration. (I guess it's warmth if you like it and coloration if you don't.)

Similar if not quite so extensive controversies surround OTL amplifiers. Why? At least partly because OTL amplifiers are even less well known and understood than SETs. To put it simply, all other tube amplifiers require output transformers that step down the power tubes' high voltages and produce the lower impedances needed to drive loudspeakers directly. Output transformers are critical to sonic performance, and in the best tube amplifiers the output transformers account for a considerable portion of the parts cost. Transformer designs comprise countless variations, such as custom proprietary windings and expensive materials -- e.g., silver. 

OTL designs eliminate the output transformer in favor of driving loudspeakers directly from the power tubes. This approach, it turns out, is a tricky business, and the chief raps against OTL amplifiers have been not just unreliability, but also their susceptibility to catastrophic failure -- such as starting fires or damaging loudspeakers when they fail. That reputation is mostly attributable to OTL amplifiers based on the 1970s designs of the late Julius Futterman, which more recently were championed by the late Harvey "Gizmo" Rosenberg at New York Audio Labs.

Atma-Sphere's OTL amplifiers (they make no other kind) are quite different from the Futterman scheme. They use the patented Circlotronic™ OTL circuit developed by Ralph Karsten, the company's founder and CEO. Atma-Sphere Music Systems turns 25 in 2003, so it's fair to say that Karsten's design has stood the test of time. Judging by conversations with several owners of Atma-Sphere equipment, the marque has built up a high degree of brand loyalty. To understand the depth of that loyalty, take a look at the Atma-Sphere Owner's Group (completely independent of the company) web site -- there is a direct link to it from the Atma-Sphere site.

Atma-Sphere's philosophy is illustrated in the "Twenty-year Rule" described on the company's web site. Briefly, Atma-Sphere expects its products to be used for at least 20 years before needing major service (e.g., new capacitors). With that in mind, Atma-Sphere chooses parts and tubes that can reasonably be expected to be available -- or to have appropriate equivalents -- 20 years down the road. Long tube life is expected -- Karsten estimates at least 10,000 hours from the power tubes and 10-50,000 hours from the input/driver tubes. Atma-Sphere's two-year general warranty also warranties the tubes for one full year, a welcome departure from the typical 90 days.

Atma-Sphere currently offers one stereo (S-30 Mk. II.2, 30 wpc) and three monoblock models, the M-60 Mk.II.2 (60 wpc), MA-1 Mk.II.2 (100 wpc) and MA-2 Mk.II.3 (220 wpc). The basic circuit and design characteristics are the same throughout the line. Karsten says that product improvements are ongoing. For instance, the following upgrades were made to the M-60 Mk.II to create the current Mk.II.2:

· Larger power transformer, boosting the power supply capacity in the output stage by 50%
· Increased filter capacitance in the driver supply by 50%, improving stability and lowering driver stage distortion by 40%
· Added resistors to the output section to help the tubes work better together, reducing distortion by 90%
· Implemented quick-bias feature, adding two front-panel switches for using the built-in bias meter

Along with those upgrades came a price increase from $4,200 to $4,650 per pair. In this writer's opinion, that's a helluva deal.

Design Highlights

The all-triode, pure Class A M-60 Mk.II.2 is a wide-bandwidth Balanced Differential Design® amplifier. (I was surprised to find that Atma-Sphere has registered that term. I -- and many other reviewers -- have used it generically for years, sort of like people say Kleenex for any brand of facial tissue.) The amplifier has only a single gain stage, and Atma-Sphere holds the patent on the direct-coupled OTL output stage. There are no circuit boards; everything is hand-wired. A "star" ground circuit contributes to the amplifier's impressive quietness. 

If the owner should desire more power, the M-60 Mk.II.2 can be "monostrapped" to use two amplifiers for each channel. This option increases the power into 8 ohms from 60 to approximately 180 watts.

Exterior Tour

No one is likely to accuse Atma-Sphere of squandering money on "audio jewelry." The open (no tube cage) chassis is finished in wrinkly textured black. The four 6SN7 input/driver tubes are grouped front and center, flanked by two sets of four 6AS7G power tubes. The rear panel holds only the IEC jack and a single pair of (poorly marked) loudspeaker binding posts. The front panel is well populated: power and standby toggle switches and lamps (not LEDs); two more toggles for checking bias and DC offset. A small plate contains instructions for the bias/DC offset procedure, which uses the built-in meter mounted behind the tubes on the power supply cover, and the two trim pots for adjusting those values (located next to the switches). The XLR and RCA input jacks are also on the front panel.

The M-60 Mk.II.2 looks like something designed in Minnesota... hey that's right, it was! The look can be summed up, I think, as "retro." Personally, I like the utilitarian appearance. And besides, I do a lot of listening at night with room lights off, and those glowing tubes are very companionable in the dark.

First Review Setup

I slipped the M-60 Mk.II.2s into my main system, replacing the previously reviewed WAVAC MD-300B SET amplifier, and driving the highly sensitive prototype Von Schweikert dB-99 loudspeakers. [NOTE: Albert Von Schweikert recently informed me that he would not be marketing the dB-99 in its present form. A smaller restyled version is planned for future introduction.] The rest of the review system included my Thor TA-1000 line and TA-3000 phono stages; Basis 2800/Graham 2.2/Transfiguration Temper analog source; Bartha-modified Pioneer DV 434 and stock Philips SACD 1000 combination player for digital playback; Bybee/Curl Pro power conditioner; wire by Nordost, Transparent, DH Labs, TG Audio and Bybee (custom).

The system with the M-60 Mk.II.2s sounded very good from the start, although I assumed -- correctly -- that the amplifiers would improve with break-in. That took three to four weeks. It took me a couple of days to adjust to the change in the sonic environment from the WAVAC's seductively warm, lush harmonics and broad, deep soundscape to the harmonically leaner, spatially less expansive, but faster and more dynamic presentation of the M-60 Mk.II.2. 

The most startling quality of the Atma-Sphere sound, which grabbed me at the very first listening session, is its immediacy. The music seems simply to be there, transmitted directly from the artists to the ear. I could analyze the elements of that immediacy -- principally a combination of excellent transient rise and settling speed and extraordinary tonal neutrality -- but it seems a prosaic response to the sheer sonic poetry of the experience. 

The amplifiers did have some help. I tried several aftermarket power cords, getting notably superior results with Bob Crump's TG Audio Silver, and then even better with a killer new prototype cord from Jack Bybee. (The Bybee cords should be available in a few months.) I also tried different footers, including cones by Black Diamond Racing, Polycrystal and Goldmund, air suspension (Arcici Air Heads) and the eventual winner, a mix of heavier- and lighter-duty Vibrapods. All of the cones produced a more engaging sound than the stock rubber feet, but they all seemed to change the intrinsic sonic signature of the amplifiers, albeit subtly. The air-suspension platforms brought out some of additional low-level inner detail, but also slightly softened transients. The Vibrapods made the system quieter, revealed even more inner detail, and stabilized the imaging within a somewhat deeper soundscape. I strongly recommend them with the M-60Mk.II.2s.

I could have written enthusiastically about the M-60 Mk.II.2s after the first day of listening to them, and an even more positive appraisal after a month. But I couldn't shake the feeling that I was not giving the amplifiers a chance to show their best. Ralph Karsten -- and all of the Atma-Sphere collateral material -- had emphasized that these truly balanced differential amplifiers needed to operate in a balanced system in order to realize their full potential. I didn't have a balanced pre-amplifier on hand, but after a quick phone call to St. Paul (the city, not the saint), a few days later my friendly UPS driver delivered an Atma-Sphere MP-3 pre-amplifier.

Second Review Setup

Switching pre-amplifiers essentially meant starting over, as I now had to burn in the factory-fresh MP-3. But like its amplifier cousins, the MP-3 sounded pretty good out of the box, so the break-in time was not a onerous. 

After the reconfigured system settled in, I put on my metaphorical critic's hat and started to morph into a serious audio reviewer. It was time to crank up the left brain and get analytical. But the Atma-Sphere sound tends to disarm that analytical mindset, so over and over I found myself slipping back into right-brain mode, just digging the performance or getting happy feet as the music boogied along. Alison Krauss + Union Station's New Favorite CD proved to be particularly infectious.

But did the M-60 Mk.II.2s sound better with the balanced MP-3 than with the unbalanced Thor? Damned if I could tell, especially after a month's interval. So I spent a few days alternating the two pre-amplifiers and making A-B comparisons with a wide variety of music. For consistency, I used only the Thor phono stage for LP playback. I'll comment on the MP-3's phono section when I review that unit.]

The primary musical selections for those comparisons: Berlioz Symphonie Fantastique, Munch/BSO on RCA Living Stereo LP and XRCD2 reissue; Respighi Pines of Rome, by Reiner/CSO on Classic Records single-sided 45 RPM LP and by Oue/Minnesota on Reference Recordings CD; Patricia Barber Modern Cool on LP, CD and SACD; Emmylou Harris Wrecking Ball CD; Miles Davis Kind of Blue Classic Records LP reissue; Dvorak & Glazunov Violin Concertos by Milstein/Steinberg/Pittsburgh on Cisco LP reissue; and Bruce Springsteen The Rising CD.

After exhaustive comparisons, I'll sum up the sonic differences between those two fine pre-amplifiers as follows: with the MP-3, the system is slightly quieter. At normal listening levels, neither unit is noisy enough to be heard over ambient room noise, but ultimately the Thor's noise level became audible at a slightly lower volume. The MP-3 also delivers somewhat better low-frequency extension and pitch definition, although I sometimes preferred the bigger-sounding bass of the Thor. In another close call, the MP-3 mirrored the amplifiers' mid and high frequency tonal neutrality a little more accurately. The Thor excelled in spatial resolution, throwing a broader and deeper soundscape. Both units had superb image placement and stability.

Bottom line, the MP3 acquitted itself very well against a great pre-amplifier that costs about twice as much. Were I assembling a system around the M-60 MK.II.2s, I would feel very good about matching the amplifiers to the MP-3. But on the other hand, the total differences were subtle -- so close that I would not expect to consistently identify which pre-amplifier was in the system if I just walked into the room with music playing. S I don't think it's a matter of "balanced or bust" -- I could happily listen long-term to a Thor/M-60 Mk.II.2 combination.

Final Review Setup

The prototype Von Schweikert dB-99 loudspeakers I had been using throughout the review have active bass systems with solid-state 600-watt amplifiers built in. But those internal amplifiers get their input signal from the output of the power amplifiers, so the sound reflects very closely the bass performance of the primary amplifiers. When I reviewed the similar but larger dB-100 loudspeakers, I found that the loudspeakers' bass changed significantly each time I substituted a different amplifier. So I felt confident that I was getting an accurate picture of the M-60 Mk.II.2s' bass performance, even though the amplifiers had not been seriously taxed by driving those 99 dB-sensitive loudspeakers. Still, I delayed this review for a month in order to try the Atma-Sphere amplifiers with a more challenging load: my Eggleston Andras, which were returning from massive surgical alterations that turned them into what I call "Super-Andra IIs." 

With 88 dB sensitivity and average impedance of 6 ohms, dipping to 4 ohms in the deep bass, the Andras were a far more difficult challenge for the 60-watt Atma-Sphere OTLs. I must admit to some surprise that the M-60 Mk.II.2s responded so well to the challenge. The sound was simply glorious -- open, relaxed and quite dynamic. I was able to play the system at surprisingly robust levels, although not as loudly as I demand in my more manic listening sessions. (My reference amplifiers are 750-watt VTL monoblocks.) Apart from that limitation, the only other compromised element was looser, less precise deep bass. I attribute that to the nasty low-frequency impedance dip of the loudspeakers.

But there is a solution for owners of loudspeakers with tricky impedance curves who are interested in the Atma-Sphere sound. A member of the Atma-Sphere Owner's Group, Paul Speltz, offers the Zero Autoformer, a toroidal transformer that is installed between the amplifier and the loudspeaker to step up loudspeaker impedance so that the amplifier sees a constant 16-ohm load, which is optimal for these OTLs. Speltz' autoformer sells for a grand per pair. I intended to request one to use in this review, but never got around to it. Ralph Karsten enthusiastically endorses the product. You can contact Mr. Speltz through the Atma-Sphere Owner's Group web site; look under Tweaks.

Summing Up

Since I already gave away the punchline in the opening paragraph of this review, I'll make this short and sweet. The Atma-Sphere M-60 Mk. II.2 is as fine sounding -- and emotionally engaging -- an amplifier as I have ever lived with. Its uncanny ability to communicate the essence, the very soul of the music played through it is something I never got used to in the months I listened to it -- I was enchanted anew at each encounter. Moreover, these magical qualities are supported by a well-established and stable company that is dedicated to ongoing improvement and conscientious customer service. At $4,650 for the pair, the M-60 MK.II.2 is an outstanding value. I felt a genuine pang of regret when I packed up the amplifiers for their return trip to St. Paul. Much as I love them, I need a more power. Perhaps there are bigger Atma-Sphere amplifiers in my future.

Features And Specifications

Exclusively triode-based, vacuum tube circuitry 
Pure class-A operation 
Patented Atma-Sphere direct-coupled (OTL) output stage 
Balanced Differential Design
Mono chassis construction 
Easily monostrapped for greatly increased power 
Standby switch for long output tube life 
Only one stage of gain throughout 
XLR and RCA input connectors 
Proprietary, precision grade components used throughout 
Low-noise "star" ground circuit 
Fully hand wired electronics; no circuit boards! 
Upgrade options: custom Teflon coupling caps $300 Caddock resistor package $900
Power supply boost $300 
Classic open chassis construction with Wrinkletex finish 
Two-year general coverage warranty, one year on output tubes

60 watts/channel into 8 ohm load/ 45 watts/ch. into 4 ohms/ 80 watts/ch. into 16 ohms 
Power Bandwidth: 2Hz to 75kHz (+- 0.5dB) 
Frequency Response (1 watt): 1 Hz - 200 kHz within 1/2 db 
Intermodulation Distortion: 0.05% @ full power 
THD: typically under 1% with proper setup. 
20Hz square wave tilt: unmeasurable 
Phase Shift: less than 1 degree @ 20 kHz 
Power Supply storage: 140 Joules/chassis 
Output Section Risetime: 600 V/micro-second 
Output Impedance: ~4.1 Ohms 
Feedback: 1dB 
Power Consumption: 300 watts/chassis 
Tube Complement: eight 6AS7/channel and four 6SN7s 
Weight: 30 lbs.
Dim: 17 x 13 x 8.5 (width x depth x height in inches)

Company Information

Atma-Sphere Music Systems, Inc.
160 South Wheeler
St. Paul, MN 55105

Voice: (651) 690-2246
Fax: (651) 699-1175
E-mail: ralph@atma-sphere.com
Website: www.atma-sphere.com