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Equipment Review
June 2006

Atma-Sphere M-60 Mk III Mono Amplifiers

by Tim Aucremann


"As close to a straight wire with gain as my system has known."

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Review Summary
Sound "'Big air. Like a ski jumper. Sound floats in space, waves of music washing over me.' 'There is no doubt.' say my notes, 'these amps are superb.'" "The entire performance amplified through the M-60 Mk IIIs was a testament to extraordinary clarity." "I heard all the small noises -- the little ticks, bumps, and coughs made by a roomful of musicians working. Detail yes, but it wasn't not spot-lit or clinical." "The bonus prize at the bottom of the Atma-Sphere box was discovering how stunningly fall-over-gorgeous the M-60 Mk IIIs render the human voice -- especially the work of female vocalists."
Features "The M-60 Mk IIIs employ "a single gain stage, operate fully in class A, and incorporate Karsten’s patented Balanced Differential Design. In essence, one monoblock chassis comprises dual single-ended-triode amplifiers, each handling one half of the audio waveform. The whole shebang is balanced from input to output." "Earlier amps can be upgraded to current spec, which also includes renewal of their warranty."
Use Speltz Autoformers (see sidebar) make the OTL M-60 Mk IIIs suitable for use with a much wider range of speakers. "Cardas XLR and RCA input jacks reside on the front panel -- keep that in mind when measuring interconnect length." "In case an output tube goes south, a bridge circuit protects the speakers and ensures the only part affected is the tube that failed."
Value "If you have higher-sensitivity 8-ohm speakers, you could find the M-60 Mk IIIs to be a true bargain."

In a world where Internet buzz is fleeting, Atma-Sphere is no overnight sensation. For almost 30 years, Ralph Karsten and company have been building reliable output-transformerless tube amplifiers based on his patented Circlotronic circuit. One doesn’t need golden cochleae to appreciate the accolades festooned across the company’s home page. Karsten’s amps and preamps have garnered enough audio trophies, including multiple Reviewers’ Choice nods from SoundStage!, that you may need your toes to add ‘em all up.

And so it comes to pass after a nine-year run in their prior vintage, the Atma-Sphere M-60 60-watt monoblock evolves to Mk III status. But this is not a case of new wine in old bottles. No, the M-60’s design remains grounded in same basic output circuit Karsten has been refining since the dawn of modern high-end audio.

So how has he done it? How has Karsten taken a cool idea -- coupling speakers directly to output tubes -- and brought it to life with an innovative implementation and managed to delight lovers of music and finicky audio reviewers across many years? How has he pulled it off? Turns out, he is a good pupil.

Ralph put it to me like this:

"Having done this a while, all I can tell you is this: At just about any given time after making an improvement, people have asked me 'What will you do next to improve the circuit?' The answer has consistently been 'I don't know.' Some people take this to say that I am out of ideas, but it does not work that way. Instead, I regard myself as a student of the circuit, and it is teaching me all the time. And, over time, it becomes apparent from this teaching what is required next. So, it has been an ongoing evolution for nearly 30 years. The Mk III series thus is an evolution. Although it is safe for me to say that it is at a fairly refined state, given history I suspect that we will find ways to improve the amplifier as time goes by."

To understand what makes Atma-Sphere amplifiers different and appreciate the recombinant nature of the M-60’s evolution to Mk III, we turn to its genetics. Here is a tube amplifier whose OTL moniker describes it in terms of what it doesn’t have, namely a transformer between its output circuit and the speakers it drives. So what’s the big deal about not having an output transformer?

Consider a typical push-pull amplifier. Its output transformer doesn’t amplify the audio signal; it sits between the amp’s output circuit and the speakers, and its primary job is to step down higher tube voltages and support the impedance needed to drive the loudspeakers. However, when the audio waveform expressed as an electrical signal switches from positive to negative and back again, it takes energy from the signal itself (albeit a small amount) to switch the polarity of the magnetic field in the transformer. This energy requirement is effectively a reduction of signal and ultimately results in loss of sonic detail.

Other transformer factors may negatively affect the audio signal. Among these are loss of power as heat (some of us know from unfortunate experience just how hot a transformer can get) and the capacitance of the wire in the transformer’s windings. For OTL disciples, the bottom line is this: Without needing to listen through or around a transformer, the designer is well positioned to hear what his circuit has to teach. Karsten puts it this way:

"Since our amplifier topology is very simple and there is no output transformer to mess things up, you might say that the transparency of the circuit allows one to hear things that are obscured in other designs. So it is a big deal when we are able to find a better coupling cap! Similarly, we have been able to improve our wire and our resistors due to this same transparency. That's why I say I am a student of the circuit."

Coupling output circuitry directly to the loudspeakers is but one way the M-60 follows a simpler-is-better approach. From the 30-watt S-30 to the 500-watt MA-3 behemoth, all Atma-Sphere amplifiers use a single gain stage, operate fully in class A, and incorporate Karsten’s patented Balanced Differential Design. In essence, one monoblock chassis comprises dual single-ended-triode amplifiers, each handling one phase of the audio waveform. The whole shebang is balanced from input to output. Differential and cascode amplifier circuits combine to reject noise, reduce distortion, and increase bandwidth. In case an output tube goes south, a bridge output circuit protects the speakers and ensures the only part affected is the tube that failed. To demonstrate the reliability of his circuit, Karsten has been known to yank tubes from the amps while playing. The amps simply keep going.

Specs from Atma-Sphere say an M-60 Mk III delivers 60 watts into 8 ohms and 45 watts into 4 ohms. Output impedance is about 4.1 ohms, and the amp employs a teeny amount (1dB) of negative feedback.

Incorporating an improved signal path and layout, the M-60 Mk III rolls up incremental refinements made across prior iterations of the long-lived Mk II series. Priced at $5350 USD per pair, the latest M-60 replaces the mirror-image layout of the Mk II with identical twin chassis sporting rounded shoulders and large VU/power meters. A single soldering artisan hand builds a complete M-60 in about a week. There are no printed circuit boards inside; point-to-point wiring means the amps are easier to update.

Each monoblock measures 17"W x 8"H x 13"D and weighs in at a mere 30 pounds. The amps are swathed in a black wrinkletex finish and sport that très retro look loved by Sputnik-era couturiers A Caddock resistor package and Teflon coupling caps are among the available upgrades. The only option on the review pair was polished stainless front panels. Earlier amps can be upgraded to current spec, which also includes renewal of their warranty.

Each M-60 Mk III monoblock uses eight relatively inexpensive 6AS7 power tubes and four 6SN7s. The output tubes bias automatically to a range suitable across their lifespan. Cardas XLR and RCA input jacks reside on the front panel -- keep that in mind when measuring interconnect length. To use the RCA inputs with my single-ended preamp, I shorted the XLR connectors with supplied jumpers. Also up front are toggle switches and lights for main power and standby. Hefty Cardas binding posts for speaker cables are around back, along with fuses and an IEC power-cord receptacle.

After letting the tubes warm up for a few minutes you will want to set proper DC offset using the meter, a screwdriver and a small trim pot on the front of the amp. The amps are easy to operate, and the review pair ran flawlessly. Each monoblock consumes 300 watts, and the amount of heat thrown from 16 power tubes encourages placing the amps out in the open. The M-60 Mk IIIs work best when their tubes are hot. I always gave the amps at least an hour of warm-up before critical listening. Toasty and quiet, they will attract cats in winter.

Review context

My vinyl rig consists of a Shelter 901 cartridge mounted on an SME V tonearm attached to a Teres 255 turntable. A Pass Labs Xono phono stage connects to a Conrad-Johnson ACT2 -- this line-stage preamp continues to prove itself a superb instrument for reviewing amps. My reference amplifier is the Conrad-Johnson Premier 140. My CD player is a CEC-transport-driven Parasound 2000 that rides on Symposium Rollerblocks. All other electronics mount on Walker or Mapleshade cones, and everything plugs into a Shunyata Hydra Model-8. Speakers are Audio Physic Avanti Centuries rated at 4 ohms and 89dB sensitivity.

The phono cable is an FMS Blue II. Otherwise, all wires come from Shunyata Research. Interconnects are Altair and Antares Helix, speaker cables are the revelatory Andromeda Helix, and power cords include the Python Helix Alpha, and Taipan Helix Alpha and Vx. For this review, I plugged both the C-J Premier 140 and Atma-Sphere amps into Hubbell 8300HW wall outlets.

Encountering a hummer of a ground loop during my last review, I was pleased to learn that all grounded components in an M-60 Mk III get wired centrally in a star-ground topology that helps inhibit noise and hum. After a steady diet of transformer coupled push-pull amps, I truly enjoyed the idle quiet of the Atma-Spheres.

This ain’t no disco. This ain’t no foolin’ around.

Bill Layer of Atma-Sphere makes the point: "It can be difficult to convey the sound of an OTL to someone who has never heard one." I’ll begin coyly by saying the M-60 Mk III sounds different from any amp I’ve ever heard in my setup. From the specifications of metal and glass, to the ineffable orbit of late-night listening, these Atma-Sphere amps proved a joyous trip. While I’ll describe what I heard, truth be told, Bill’s right -- words are difficult to come by.

"Big air. Like a ski jumper. Sound floats in space, waves of music washing over me." "There is no doubt." say my notes, "these amps are superb."

Speltz Zero-Autoformers

Being a fan of symphonic performances and having a decent-sized room, I enjoy playing my music at a "proper" level. Truth told, per specs alone, my Audio Physic Avanti Century speakers are not ideally suited to the M-60 Mk IIIs with their 45-watt delivery into a 4-ohm load. A 4-ohm speaker needs twice the current to deliver the same relative sound pressure level as a 16-ohm speaker. The M-60 Mk IIIs’ high output impedance of 4.1 ohms is also a factor. Matching them with a 4-ohm speaker yields a low one-to-one speaker-cone damping factor -- a potential source of flabby-woofer syndrome.

Unfettered happiness in the OTL world means using a speaker rated at 8 ohms or higher -- preferably one that possesses a flatter impedance curve with respect to frequency. So there it is, a cautionary note about amplifier-speaker synergy. Here I am asking the M-60 Mk IIIs to drive a less-than-ideal load (and feeding these balanced amps via a single-ended line-stage preamp). Disadvantage for Atma-Sphere?

Not exactly. Along with the amps, Atma-Sphere sent a pair of Paul Speltz's Zero-Autoformers ($950 per pair; $450 per pair "naked"). These are speaker-impedance-multiplying devices that sit between an amp and speakers, enabling a felicitous mating. At first glance, these looked like VPI Bricks dressed up in birdseye maple (cherry is also an option), each sporting six binding posts. Input coming from the amp side enable multiplier combinations of 2X, 3X, and 4X against the speaker’s nominal impedance; the other connectors are outputs to the speakers. I evaluated the M-60 Mk IIIs with and without the Zeros, using them with Shunyata Research Andromeda Helix speaker cables and short generic Atma-Sphere-made jumpers. With the Zeros in place, each amp saw my speakers with a load of 8, 12, or 16 ohms depending on hook-up configuration.

The Zeros did not change the fundamental character of the M-60 Mk IIIs, and that’s what is described in my review. However, the Speltz Zeros do improve the coupling of amps and speakers. Some specifics: Toward the end of Shostakovich’s "Festive Overture" as heard on Moscow Sessions [Sheffield Labs TLP-27], there are three massive bass-drum whacks. The M-60 Mk IIIs do a fine job conveying their substance -- and how the drummer quickly damps his instrument so the boom does not bloom. However, without the Zeros in place, the heft of initial impact and authority in the fundamental is less than I’ve heard on amps with more power. Using the Zero-Autoformers to make the M-60 Mk IIIs think they’re seeing 8-ohm speakers brought more body and brawn to lower registers -- and more satisfaction to my ears. Harps and violins were beautifully extended and airy in the upper frequencies; horns and trumpets were as clear as sunlight with the right brassy edge on an attack that quickly burnishes into mid-tone glow.

During double-forte passages, without the Zeros, lower treble on brass sometimes sounded a bit strained. I assumed this was distortion, as the issue passed when the volume was lowered or when speaker impedance was increased. Reconfiguring the Zeros as quadruple multipliers brought even more power into 16 ohms, but led to a hint of dryness in the midbass. Extended highs also lost that last bit of sparkle and finesse. At lower volume, without the Zeros, the M-60 Mk IIIs had a touch more air and extension on top.

Reflecting on their sonic value, I found the Zeros on the plus side of the equation and found myself using their 2X taps (8 ohms) almost exclusively. As they say, your mileage may vary. There's no harm done in trying the Zeros at 3X or 4X to determine what sounds best with your speakers.

...Tim Aucremann

Those words were the first I wrote while listening to Jacques Ibert’s "Escales" (Ravel: Rapsodie Espagnole [Mercury/Classic Records SR90313]. First impressions may change and often they’re incomplete, but time only confirmed my initial enthusiasm for the M-60 Mk IIIs. These amps are the real deal.

"Escales" opens with a breathy flute backed by gossamer violins and quickly moves into a lush Mediterranean tutti of strings. In the Tunis-Nefta portion, I heard the bodies of cellos resonating as their bows’ sharp-but-delicate attack backed a melancholic Moorish oboe. An amp needs to be quick to convey these bowing transients, and the M-60s delivered the performers’ artistry. During the concluding Venice Anime, the timbral character of different woodwinds was clear, as the distinct entrance of one overlaps another in mere seconds. My ear was drawn to the frisky tambourine. I did not hear rattley metal but the thinness of its light clappers snick-snacking into one another. Instruments had weight and body. Notes from bass strings and timpani were distinct in pitch, their start and end well defined, though not ultimate in firmness.

The entire performance amplified through the M-60 Mk IIIs was a testament to extraordinary clarity. My internal focus jumped easily between individuated sections to soloists to the Detroit Symphony as a whole. Interior orchestral phrasing was made obvious; I heard what each instrumental section was doing, dynamically and temporally, front to back, side to side. The work is big, busy, and dynamic, but notes did not congest or smear. Clarity and rhythmic aptitude coupled with detail and a real sense of musicians grouped in space. These were the hallmarks I heard the first time I listened through the M-60 Mk IIIs, and they pervaded every recording I threw at them.

By clarity further virtues are revealed. Consider the Phil Spector-like wall of sound that is Mussorgsky’s "Night on Bald Mountain" as found on Witches Brew [RCA/Classic Records LSC-2225]. The vertical spatial array of musicians and notes was striking. This piece certainly delivers an orchestra in the space of a symphony hall. The M-60 Mk IIIs don’t just plop you down in the same seat and leave you there. Depending on the recording, I had the sense of hearing down from the balcony, hearing up from rows close to the stage, or being on the same plane as the musicians.

On the Susskind/LSO performance of "Appalachian Spring Suite" [Everest LPZ-2034], even though I was sitting below the tweeter line, I had the sense of looking down obliquely at the London Symphony. Perhaps this illusion of perspective can be cashed out as a product of microphone positioning and recording-hall reflections. Whatever the cause, the M-60 Mk IIIs relayed its effect. They are highly resolving; I heard all the small noises -- the little ticks, bumps, and coughs made by a roomful of musicians working. Detail yes, but it was not spot-lit or clinical. Rather, it was detail as subtle sonic cues to the acoustic envelope, the physical reality of the recording locale. I’ve yet to hear another amp relay these differences in aural perspective. If you pardon the pun, the M-60 Mk IIIs deliver atmo-sphere in spades.

Musicians and instruments were not simply positioned to the left and right or front and back like two-dimensional playing cards stood up in an orchestral arrangement.  In my mind's ear, I sensed the performers in space and time, as their different instruments projected and radiated into the surrounding air according to the type and directionality of the instruments themselves. Players and sections took on a dimensionality that was as visceral as I’ve heard from my system.

On Ravel’s "La Valse," which precedes "Escales" on the Mercury record, the M-60 Mark IIIs easily let me hear large swells as they rose from trough to peak, rippling across different instrument sections. The dynamic shimmer from massed strings vied for the best I’ve heard in any system.  Transients were natural, not incisive and even in busy passages I heard decay from individual instruments.

The bonus prize at the bottom of the Atma-Sphere box was discovering how stunningly fall-over-gorgeous the M-60 Mk IIIs render the human voice -- especially the work of female vocalists. Sit back and close your eyes to Elizabeth Schwarzkopf performing Strauss’s "Four Last Songs" backed by the Berlin Radio Orchestra with Szell at the helm [EMI ASD288]. Oh my goodness! Hear her power and the command she has of her instrument. Hear how her song comes from her chest and throat. Her voice projects with directness and authority. The amps did a top-flight job of integrating the singer with the orchestra; she neither receded nor came too far forward. Small tone-bending inflections and the barest whisper of a trilled German "r" came through beautifully. I had the sense of her head tilted upward as if she were singing into a microphone above her. The performance was truly sublime, and the Mk IIIs carried me away.

From Renée Fleming to Linda Ronstadt, from Shirley Bassey to Lucia Popp, I must have gone through every recording I own of a female vocalist at least twice. If portraying this music the way they do was the only thing the M-60 Mk IIIs could offer, I’ll still buy a ticket.

Dare to compare

My current reference amp is the Conrad-Johnson Premier 140 ($7500). It’s a classic big-watt push-pull stereo design with 4-ohm taps, and it drives my Audio Physic speakers with what I previously described as "grace and fluency … power and control." The recent addition of a Shunyata Python Helix Alpha power cord tightened up the 140's deep bass, improving its already nimble transients and lending a dash of vigor across the board.

So here’s the home team’s clean-up hitter versus the visiting Atma-Sphere 60-watters, which have taken the field under less-than-ideal conditions (see sidebar). The Premier 140 is not entirely divested of earlier C-J amps’ signature warmth; tonally it bears a touch of caramel coloration that is most evident by comparison. The M-60 Mk IIIs? Here I recall the master’s words: "Hard to hear the dark side is." The Mk IIIs yield Goldilocks' tonal neutrality -- not too cool, not too warm. For me, it’s just right. And here, neutral does not mean thin, lean or shallow; it means accurate. In fact, the Atma-Sphere amps let me hear deeply into music’s complex blend of fundamentals and overtones. They’ve got what I call "tonal depth."

When it entered the title cut on Procol Harum’s Broken Barricades [A&M Records SP-4294], the orchestra sounded utterly clear through the Atma-Sphere amps, and the horn behind Gary Booker was harmonically richer. The Mk IIIs' sound has no dusting of the faint glaze I heard, by comparison, pervading music through the C-J amp. The Premier 140 is assured in its power reserves and delivers authoritative, deep bass. Lower frequencies through the Atma-Sphere amps are better articulated and make differences in timbres more obvious. I was surprised at their low-end control.

Hear the piano rouse and push the bass during its solo on "Simple Sister." Both amps rock the Procol Harum tunes, though the M-60 Mk IIIs had such irrepressible energy and quickness that the 140 did not seem quite as crisp. Listen to Booker’s voice -- the C-J recessed it back into the soundstage. Via the M-60s, his voice had its own pocket of air and was not smudged in with the piano. There was coherence to Booker himself -- there he sat at the piano, his head bobbing side to side with a dimensionality that just ain’t happening with the Premier 140.

All things being equal (of course they never are), the Premier 140 is a better match on paper with my 4-ohm Audio Physic speakers. However, at $2000 less, the Atma-Sphere M-60 Mk IIIs bring unique attributes to the musical feast and make a very compelling case for themselves in my system. Would I like a little more power? Sure I would, but if you have higher-sensitivity 8-ohm speakers, you could find the M-60 Mk IIIs to be a true bargain.

The crux of the biscuit

Like ‘em? You betcha. A lot. What can I say? I’m smitten.

The Atma-Sphere M-60 Mk IIIs are different from any amp I’ve heard to date. Clarity of voice, purity of tone, rhythmically adept, spacious, dimensional -- the audiophile phrases run down the page and chase me back to my listening chair, where they disappear when these little amps turn signal into magic. Pay attention to the speakers you ask them to drive and they will reward. They are not perfect, but they are as close to a straight wire with gain as my system has known. Considering the adversity I asked the amps to play through, there’s not much higher compliment I can give. If you listen to the M-60s Mk IIIs, be sure to invite Ms. Fleming or Ms. Ronstadt (or whoever your favorite female vocalist is) to go with you.

Ralph, you’re a good student. Keep listening to your circuit.

...Tim Aucremann

Atma-Sphere M-60 Mk III Mono Amplifiers
$5350 USD per pair.
Warranty: Three years parts and labor, one year for tubes.

Atma-Sphere Music Systems
1742 Selby Avenue
St. Paul, MN 55104
Phone: (651) 690-2246
Fax: (651) 699-1175

E-mail: ralph@atma-sphere.com
Website: www.atma-sphere.com

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