AOM Logo November 1998


You Are The Music
The Atma-Sphere M-60 Mk.II Reference Monoblock Power Amplifiers


Anthony Kershaw

Ralph Karsten, owner and head designer of Atma-Sphere Music Systems, built the prototype of his output transformerless (OTL) M-60 amplifier way back in 1980. By all accounts, the amp is still running some twenty thousand hours later! And so goes the myth of tube amplifier unreliability. If the design is strong and stable and uses high-quality parts, most products will last. Eighteen years after the first incarnation (which I have not heard) Karsten has produced a Mark II version.

The Reference Model
Like all designers, Karsten has striven to improve his gear. To that end, he has gone one step further and created a "Reference" model of the M-60 Mk.II, offered for an additional US$1500.00 over the base Mk. II, using higher-quality parts but built on the same chassis and utilizing the same transformerless design. It is this "ultimate" Mk. II that was submitted for review.

Myth
I had heard much positive talk about, but little music through, the M-60 Mk. IIs, and thus was intrigued by the hubbub. In the past, I have reviewed equipment where, to my ears, the hype surrounding it was misplaced, so I was a little apprehensive when this particular product was offered for review. Right off, I'll commend Mr. Karsten for suggesting that the local distributor bring the amplifiers over to the house and spend some time setting them up. And, let us be thankful for small mercies, the amps were broken in.

After spending some time with the References, my mind began to direct itself to the body of the review. These same thoughts, I have to admit, caused a little trepidation. Simply, these amplifiers sounded divine, yet the trick was to transfer their unique sound quality into written words that would be meaningful to the reader, and do justice to the equipment and designer. After a few fits and starts at the keyboard, I thought it better to retreat to the comfort of the listening chair and see what future listening sessions would hold.

Atma-Sphere M-60 Mk.II Reference Power Amplifier

Reality
Structurally, the M-60 Mk. II Reference is built like a tank. On first look, it is a superbly crafted piece of industrial art. But while not particularly attractive to the eye, the amplifier is a serious attempt at quality production, and will give the purchaser the feeling that his M-60 will also be around in twenty thousand hours. What you get for a relatively small sum is a state-of-the-retro-art design crammed full of what tube lovers crave: a triode-based, pure class-A, dual-mono chassis amplifier using quality parts such as Caddock non-inductive resistors and HEXFRED rectifiers, one gain stage, a standby switch, both XLR and RCA input connectors, proprietary, precision grade components and fully hand-wired electronics. Mr. Karsten provides a two-year general warranty and one year on the output tubes. All auditioning of the Mk. II Reference was done with the supplied tubes. And while not feeling the need to experiment further, local owners have noted great success with tubes of vintage stock.

OTL
Technically, the M-60 Mk.II Reference is a tour-de-force. To an absolute non-tech, removing the cover revealed a thing of beauty. The wiring is meticulous, and, unlike other point-to-points I've seen, does not look like a rat's nest. Power wise, you get 60 Watts per channel into an 8 Ohm load, with 4 Ohms bringing home 45 Watts. Karsten calls his design "…the first reliable OTL". Although the output transformerless circuit was developed initially by New York's Julius Futterman, Karsten has refined the concept and patented his design, calling it Balanced DifferentialTM. This, says Karsten, "…results in our amplifier having fewer parts for the amount of gain available than is possible in conventional triode circuits, while at the same time having lower distortion and noise". Karsten goes on to say "…in each audio channel, there are two single-ended triode amps running 180 degrees out of phase with each other (differentially). So distortion is canceled at every point throughout the amp (without feedback), leaving only the music. Thus, Atma-Sphere combines the power of push-pull with the musicality of single-ended, but with better sound than either, because the output is direct coupled (no output transformer). Thus, our gear is wide in bandwidth, very dynamic, extremely neutral, and virtually distortion-free". Quite a testimonial.

Tubes
The M-60 Mk.II Reference's tube complement of sixteen 6AS7s and eight 6SN7s (divided between two chassis) make for a logistical quandary. The heat coming off of these amplifiers is almost Dantesque! Along with careful electronic and component matching, you must have a well ventilated, good-sized room that is air-conditioned. For best results, place the amplifiers near your speakers, as far away from the listening chair as possible. This unpleasant by-product of multiple power-tubes is the only caveat I can muster about the M-60s. The good news: Under normal conditions, the tubes will last in excess of five years. Cost to retube? About US$350.00.

Bias
Biasing the tubes on the M-60 is a simple operation. First move the positive leg of each speaker cable from the amplifier's output terminal to the biasing posts, leaving the negative leg attached. The large meter located on the amplifier's front panel will read DC offset - the offset is adjusted via the right screw pot on the front of each amplifier to a reading of zero. Once this is done, one depresses the switches next to the trim pots on the amplifiers' faceplates. This will send the amps into biasing mode. Via the left screw pot on each amplifier, the bias is adjusted until it reads 0.52 mV on the meter. This process should be carried out when the amps are new (after the first hour) and then after a week, a month and then annually.

Sound
I compose my reviews whenever the chance arises, be it in the car, in between students at the studio, conducting Schubert's Ninth or when the flute part is resting for long-stretches in the orchestra. So it was a happy time during a recent teaching session - considering my earlier reservations about the text - that the idea for the thrust of this review came to me. I was trying to explain to a shy student that her flute was but a vessel, the amplifier if you would, from which her music flowed. She was the music. Later, this got me thinking about how Ralph Karsten has succeeded in making his amplifier a vessel from which music (his, possibly?) flows. There is no coloration, no harshness, and no lack of detail or balance in all octaves, just pure and very beautiful expression allowing instruments or voices their full measure. And what pleasures those measures brought.

The sound produced was utterly transparent, yet left all instruments and voices full-bodied with their unique timbres intact. No bloated bassoons or shrill violins here. Slow piano attacks, a dead giveaway for a weak amplifier, were never evident. Piano tone on good recordings was as original as Steinway and Bösendorfer engineers might have imagined. And how quick was the string decay on an original shaded dog of Rubenstein's Chopin Ballades (RCA LSC-2370)? Magnificently fast! Rubenstein's filigree touch never sounded so clean.

Time and space do not permit a flow of consciousness about the hundreds of hours of music enjoyed, but some moments were just exquisite. One of my greatest pleasures is preparing the Classic Records reissues reviews that appear in Audiophilia [Parts 1 and 2 of this ongoing series can be found in our archives - Ed.]. Upcoming are reviews of two of Fritz Reiner's greatest: Iberia (LSC-2222) and The Reiner Sound (LSC-2183). On both records, the Chicago Symphony sounded vibrant and real, producing head-shaking thrills and a few spills, all clearly defined by the M-60 References. Score one for the music and nil for the hardware. Surely, a match made in audio heaven.

Voices were reproduced in similar fashion. The loveliness of Diana Krall's performances on Love Scenes (Impulse IMPSD 234) was a gift from the M-60s that my Gallo Nucleus Solos adored. This is such a musical CD. The great songs drew me into their spell causing many a late night. The same must be said of Jessye Norman's very powerful soprano voice on her latest CD, Mahler's monumental Das Lied von der Erde (DGG 439 948-2). The colors Norman projects in this song-symphony are nothing short of miraculous, varying her tone from girlish impudence to the gravity of near-death. All was unencumbered beauty with the amplifiers allowing the intense emotions to the fore. The cleanliness of the sound during this CD was spectacular, allowing this listener to eavesdrop deep into the super-fine live performance from Berlin's Philharmonie.

During listening sessions with other Audiophilia staffers, one of my sneaking pleasures was observing the mesmerized looks on their faces after they had sat quietly for a while. The music took on such an intimate quality, they seemed at one with the performers. If I were an audio designer, I could not ask for a higher compliment.

Lately, I have had the opportunity to audition tube amps from Sonic Frontiers and Jadis, and solid-state amplifiers from Canada's SimAudio. And while all the amps were very fine, they all had a sonic signature, albeit a beautiful one, that shaded the music. Only when hearing an amp like the M-60 Mk.II Reference does one become aware that many other amplifiers cause an ever-so-slight blurring of inner detail. With the M-60s, you are in the driver's seat and you may never want to give it up.

Conclusions
The aforementioned Julius Futterman instigated the transformerless design and parlayed it into some fine, but ultimately, unreliable amplifiers. Ralph Karsten has taken time, ingenuity and great care in bringing his version of this unique design to market. There are many tube amplifiers available for purchase, most of which, for good reason, stress reliability. Understandably, audiophiles abhor spending hard-earned dough on clunkers! Reputations of many have not been good. Solid-state guys merely laugh! Well folks, laugh no more. With the M-60 Mk. II Reference power amplifiers, you can get the power you crave and the tonality of which you dream, both combined with excellent reliability. Except for the side effect of an unwanted tan, these relatively inexpensive amplifiers are one of few that I would consider a lifetime purchase.

M-60 Mk.II Reference Monoblock Power Amplifiers
Manufactured by Atma-Sphere Music Systems
160 South Wheeler, St. Paul, MN 55105, USA
phone: (651) 690-2246, fax: (651) 699-1175
web: http:/www.atma-sphere.com
Price: US$5300/pr.
Source of review sample: Canadian Distributor Loan
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