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Equipment Review
October 1999

Atma-Sphere M-60 MK II Mono Amplifiers

by Todd Warnke

Reviewers' Choice Logo
"…at the $4200 price point, I know
of no real competition for the
M-60 Mk II."
 

 

Review Summary
Sound "A rich and vibrant continuousness" and tremendous clarity; "very extended but also extremely smooth and non-fatiguing top end," but the mids "really are where it shines"; the bass has no solid-state whomp, but Todd has "never heard bass so real, so tangible and so organic."
Features Stable and reliable OTL circuit; uses tubes in current production; very long estimated tube life.
Use Inputs and outputs are on opposite sides, making connection tricky if you have stiff cables; runs very warm.
Value Very high; OTL monoblocks with glorious sound at a little over $4000.

Spare parts?

Almost by definition, to follow the path of the audiophile is to abandon Siddhartha’s Middle Way. Yes, in modern terms we are extremists, and that’s why the inexperienced audiohead will look at the Atma-Sphere M-60 Mk II mono amps and choose one of two paths. The first is to look at the M-60 Mk IIs, see only OTL circuitry, panic at the non-standard approach, and then listen to it out of that perspective alone. The second is to look and just see an amp, drop it into a system willy-nilly and then listen. But we the Illuminati, we who have been initiated into Inner Mysteries, we will control our base audiophile tendencies and follow the Middle Way and in doing so avoid being blinded by the OTLness of the M-60 Mk II. Nor will we be blind to that same OTLness. We will combine these partial truths in our search for full truth and be the richer for it.

The first step on this middle path is to deal with the shades of OTLs past. For those who weren’t around back in the day, Julius Futterman was the first to market widely an OTL, or Output TransformerLess, amp. Like many firsts, audio and otherwise, the idea was better than the execution. The NYAL Futtermans had a tendency to sound clean, pure and direct -- and then to blow up. They also had poor speaker protection, and so what followed was often more than just the loss of a tube; often it meant the loss of a speaker too, which is not the best way to build a reputation for either Futterman or OTL topology. Futterman and his company eventually paid for the reliability issue, but the OTL approach was unfortunately smeared by association.

But OTL had a value that went beyond Futterman, and so the basic idea hung around. By 1978 when he founded Atma-Sphere, Ralph Karsten had worked out many of the issues associated with the OTL approach, not the least of which were the reliability and safety problems. The result of Karsten’s work is that the basic, long-lived Atma-Sphere OTL circuit is stable with prolonged tube life, and when the tubes do go down, as happens in all tube designs, they go down safely. These are not just words either; Karsten’s company has a 20+-year track record to back them up. Since then other companies have joined, contributing their own variation on the OTL, with GRAAF being the big name in Europe while Transcendent and Joule-Electra are the other major US players. Each of these companies has helped rebuild the reputation of the OTL, and each offers reliable and safe products. Still, not one of them has the experience or track record of Atma-Sphere.

So why OTL anyway? Significantly, one non-amplifying and (as OTL proponents claim) non-essential part is removed from the circuit. Second, as any tube designer worth listening to will say, one of the final arbitrators to quality sound is the output transformer. Yes, circuit design is important, but the last thing the circuit sees is the output transformer, and no circuit can compensate for a poor transformer. Remove the transformer and you remove that issue. Third, the transformer exists largely to step voltage down and to match impedance loads. If that can be accomplished or compensated for elsewhere, the need for the transformer goes away. And lastly, with that blasted transformer out of the way, the output signal flows directly from tubes to drivers, and in a hobby where the "less is more" credo is often given no more than lip service, this is one time when it is real.

Atma-Sphere takes this less-is-more attitude even further. The M-60 Mk II uses but a single stage of gain. Further, simple triode tubes are used exclusively throughout the line. Continuing the theme, the M-60 Mk II eschews the use of negative feedback, although it can be set at the factory to a very low 2dB of negative feedback. Lastly, the amplifying circuit is pure class A, and while this is not necessarily a case of less is more, when you’ve gone this far, it pays to finish the job properly.

All this simplicity does not mean the M-60 Mk II is a basic, no-brainer design job. The direct-coupled OTL design, while built on simple parts, is extremely sophisticated. Using a combination of differential, cascode and bridge circuits, the patented Atma-Sphere Balanced Differential Design, which is used in both Atma-Sphere amps and preamps, is fully balanced from input to output, although the amp accepts RCA or XLR inputs. The differential or balanced portion of the circuit results in high common-mode noise elimination, or, in English, noise common to both phases of the signal is canceled out. The bridge circuit contributes to the reliability of the design and allows tubes to fail without taking speakers or any other parts with them. The complete circuit is stable into any load, offers wide bandwidth, is quite dynamic and is very quiet. The circuit is also easy to use as bias is set individually for each tube bank (there are two banks of four tubes per channel), and the tubes do not have to be individually matched.

Speaking of tubes, the M-60 Mk II is a monoblock design and uses eight 6AS7 output tubes and four 6SN7 input tubes per chassis/channel, for a total of 16 output tubes and eight small signal tubes. The stock output tubes are Sovteks, while the 6SN7s are JAN Philips. Ralph Karsten predicts that in standard usage, the output tubes should have about a 10,000 hour life and that the inputs tubes will last from 10,000 to 50,000 hours!

Atma-Sphere uses an internal 20-year policy when making parts and tube selection decisions since they estimate that their products will have at least a 20-year life before needing a significant re-build. With that in mind, all parts are selected with the idea that they or proper equivalents will be available in 20 years. Certainly appropriate value caps and resistors will be available in 2019, but predicting tube availability 20 years down the road is risky. To that end, Karsten figures that the best way of assuring that tube replacements will be around is to use only current-production tubes. The 6SN7 is currently made by Sovtek, and the 6AS7 is manufactured by both Sovtek and Svetlana. The Russian 6AS7 tubes are quite good -- and fairly cheap too. On the other hand, the Sovtek 6SN7 is passable at best. Fortunately there is a large NOS supply of the 6SN7, and while the prices vary greatly, there are many great-sounding tubes out there for reasonable bucks (see the Spring 1999 issue of Vacuum Tube Valley for an excellent history and survey of 6SN7 tube types). With the extended life of the 6SN7 in the M-60 Mk II, if you own a pair of amps, it makes sense to buy a couple of your favorite sets now and put ‘em away. With those 10k to 50k hours of life, chances are you could be covered for 20 years too.

As for the specs on the M-60 Mk II, each monoblock is rated at 60 watts into 8 ohms, and 45 watts into 4 ohms. At 17"W by 12"D, each chassis is a bit smaller than a standard amp, but the combination will take more space than a single-chassis stereo amp. In addition, all those tubes and that class-A output stage make for a hot partner. Whereas a class A/B solid-state stereo amp may fit and run cool enough for the bottom shelf of your rack, the M-60 Mk II wants and needs to be placed on the top, where it can breathe. Once the amps are there, you can’t help but notice the looks. With a gray wrinkle-tone finish, a flat space in front to hold the tubes and raised rear ledge with a bias and DC offset meter in the middle, the look is decidedly late ‘40s/early ‘50s military/tech. Whether you like it or not is certainly a personal issue; I know that I like it best at night and with the lights out.

There are a couple of other oddities about the M-60 Mk II. The Cardas five-way speaker posts are in back, but both the RCA and XLR input jacks are up front. Atma-Sphere points out that this is optimal from a design viewpoint as the distance from jack to input tube is about an inch and the low-level signal isn’t routed by, around or through anything else. But it can be awkward in use, especially if you have stiff or bulky interconnects. Lastly, the 6AS7 output tube often emits a purplish-blue glow as it works. For anyone coming from the EL34, 6550, or KT88 worlds, this can be a bit startling. It shouldn’t be as this really is a sign of a great 6AS7, and when you get one that dances along with your music, you have a special tube indeed.

The warranty on the M-60 Mk II is three years, parts and labor (one year on tubes), but if you have an older model M-60, or any Atma-Sphere product for that matter, and send it to the factory to bring it up to current specs, you get a new three-year warranty. If that doesn’t alleviate the Futterman legacy, I don’t know what will.

The paths

Now, about those two paths. If you see only OTL when you look at the M-60 Mk II, chances are you’re still thinking Futterman. Don’t. The M-60 Mk II by all accounts (check out the Atma-Sphere Owners Group at www.condor-connection.org/asog/ for other’s experiences) is one of the most stable amp designs on the market. On the other hand, it is a tube amp, and a relatively unique one at that. Without an output transformer, the amp causes the speaker interface issue to take on added importance.

Since it is stable into any load, you could just drop it into any system. Don’t do that either. To hear what it can do, and in reality this applies to every single piece of audio gear ever made, it needs to be placed into an appropriate system. What it likes best is to see a flat-impedance load. Efficiency matters also, but no more than with any other 60-watt amp. High impedance helps, but again, no more than with any other tube amp. And in many ways, a flat impedance matters about the same for the M-60 Mk II as it does for other tube amps, but without it, most definitely you will miss what you pay for with the M-60 Mk II.

I’m lucky because my main speakers, the Merlin VSM-SEs, offer both a relatively high and extremely flat impedance curve. Bobby Palkovic of Merlin often shows with OTLs and did much of his final voicing of the speaker with OTLs. (Atma-Sphere must feel the same about the Merlin as they have used the VSM-SE at shows.) The other speakers I used in this review, the Dunlavy SC-III and the Silverline SR 12, are also OTL friendly, especially the Silverlines. So having wisely chosen the Middle Way -- recognizing the needs of the M-60 Mk II but not fearing them -- we can now listen to the amp knowing that what we hear is what it can do.

Play time

I am one of those all-too-common humans who must learn from his own mistakes. Paradise means nothing without a sojourn through the inner circles of the Inferno, so perhaps it is best that I come to the Atma-Sphere amps now, for I fear that their splendor would have been lost on my once-neophyte ears. Why? Because they sound like music, and without knowing how remarkably rare that is, I would have taken the amps for granted. It’s only now, after spending time in the Circle of Transistor Sizzle and Artificial Hype (a place filled with those who suffer the hubris of thinking that music, a natural magic, can be entirely quantified and understood with numbers), as well as time in the Circle of The Opiate of Tube Lushness (a place filled with those who suffer the sin of sloth, being satisfied with the fatness of tone but without the will to correct other deficiencies), as well as untold hours in combinations of these two hells, I can hear the natural magic and accuracy of the M-60 Mk IIs. And yes, from the very first, I heard both -- magic and accuracy.

Associated Equipment

Loudspeakers – Dunlavy SC-III, Merlin VSM-SE, Silverline SR 12.

Amplifiers – Assemblage ST-40, Blue Circle BC6, Warner Imaging VTE-201S.

Preamplifier – BAT VK-3i.

Digital – JVC 1050 CD player used as transport, Dodson DA-217 Mk II DAC.

Analog – Rega Planar 25 turntable, Rega RB600 tonearm, Dynavector Karat 17D2 Mk II cartridge.

Interconnects and speaker cables – Audio Magic Sorcerer, Cardas Golden Cross, Cardas Neutral Reference.

Accessories – Golden Sound DH Cones; VansEvers Reference 85 power conditioner; Audio Magic and VansEvers power cords; SoundRack Reference stand.

Beginning with accuracy, and at the top, the M-60 Mk II has a very extended but also extremely smooth and non-fatiguing top end. Cymbals had full and burnished tone, but when slammed, they cut and crashed but never hurt. Dynamic contrasts in the treble were exceptionally well handled. Live jazz recordings, such as the musically and sonically superb Keith Jarrett recording At the Blue Note [ECM 1575-80], where Jack DeJonnette’s brush work alternates between almost subliminal timekeeping and full-throated roar, were so excellently captured that the border between here and there, between my place and the Blue Note, was as thoroughly erased as I’ve ever experienced. If there weren’t so many things that the M-60 Mk II did well, I would be tempted to point to the way it handles the treble as its defining characteristic.

Except that the mids really are where it shines. And in this respect, I could write the entire review around Joni Mitchell alone. Misses [Reprise 9-46358-2], her 1996 flip side to the nominal best-of album Hits [Reprise 9-46326-2], offers re-mastered HDCD sound. Taken mostly from the later half of her career, Misses also contains a couple of songs from early on, when her voice was as clear and pure as Waterford crystal. I’ve commented on the contrast between the two Jonis, the innocent and the mature (I prefer the older, wiser one), but in many ways this was an uninformed opinion. The M-60 Mk II has such a pure midrange that the differences between Joni on "For The Roses" and "Sex Kills" is not just years and cigarettes; it’s exactly 22 years, 3 months and 10 days, and an average of 1.4 packs of Marlboros each and every one of those days. Even better, the mids are so accurate that the two Jonis are not two different singers, but the same one separated in time.

Piano, that traditional bugaboo of all amps, with its range both dynamically and tonally, is extremely well served by the M-60 Mk II. I’ve been reacquainting myself with Bill Evans of late, and the Atma-Sphere amps are a major reason why. Drop in Waltz For Debby [Riverside OJCCD-210-2] and listen to "My Romance" and then on Turn Out The Stars – The Final Village Vanguard Sessions June 1980 [Warner Bros. 9-45925-2] and listen to any of the three versions of "My Romance" on that set. While the club is the same, the accompanists, piano and recording technique have all changed. So too has Evans, and the M-60 Mk IIs let that through, not just the gross differences, but the subtle ones that allow for a near telepathic connection to Evans as a person, not just a player.

My Evans tour also showed the only lack I could find in the mids of the M-60 Mk II. There is a very smooth and shallow leaning out of the lower mids that a piano, especially in solo, is the perfect instrument to expose. Mind you, it took swapping preamps and wire to be sure that what I was hearing was the amp and not something else, which tells you how small the deviation is, but at least to these ears, it is there. On the other hand, I’ve heard that the Svetlana 6AS7 has a slightly richer upper bass and lower midrange, and so those tubes may remove this issue entirely.

As for the bass, while lacking the familiar solid-state thump and monster-amp extension, the M-60 Mk II couples with near-hydraulic efficiency to the room, so even though it may not wake the neighbors, it pulses with a startling realism. And in doing this it scores best in the timing, tonality and texture of the bass. For examples look to Gary Peacock’s playing on the aforementioned Keith Jarrett disc, or Charlie Haden’s work on the Ginger Baker Trio album Going Back Home [Atlantic 82652-2]. The feel on both these albums, and so many more, is that of an actual standup bass in actual space. I know how many times I’ve read that before, and so I don’t expect you to believe me, but I can seriously say that excluding oddball bass events like cannons, full pipe organs and the like, I’ve never heard bass so real, so tangible and so organic.

When it comes to retrieving detail, the M-60 Mk IIs are champs. But unlike with many, if not most, amps, the way the M-60 Mk IIs retrieve detail is so natural that it is never sonic onanism. The detail is always connected to a musical or physical event. Events like horn blats are attached to the horn, while hall echo is attached to the hall. Sounds never float in midair, waiting for you to connect them to something; they all belong to real events, and it is obvious what they belong to. This leads to a rich and vibrant continuousness that mimics reality.

And we could end there except that we need to talk about the clarity of the M-60 Mk II, which really is the defining characteristic of the amp. Some amps that have a reputation for clarity actually overemphasize various elements. Not so with the M-60 Mk II. For example, it doesn’t have a stage that is deeper or wider than with other amps. In fact, the stage seems just slightly squashed as compared to the best that I’ve heard. Rather, it gives every single instrument, player, singer and object in a recording session its own presence and place. This isn’t accomplished by klieg lights glaring across the stage, but rather by removing the veils that are such a part of the playback process that we have become inured to them. With these veils gone, the result is immediacy, directness and intimacy. With the M-60 Mk II in my system, all the sounds, not just bass, couple with the listening room. We are not in the recording booth, looking through a glass at the musicians; we are in the session room. With the veils lifted, notes strike with suddenness, which gives the amp stunning speed. But the clarity of the amp also gives those notes a full body and texture, not just a leading edge. This is remarkable clarity, and indeed it is much a part of the magic as of the accuracy of the M-60 Mk II.

The M-60 Mk II has the most organic, non-mechanical and yet lively presentation I’ve experienced. Musical happenings are just that, happening. Sucked in isn’t strong enough; the M-60 Mk II issues an imperial decree to join in the festivities. Some amps are rock amps, some jazz ones, and others are classical. With the M-60 Mk II, I found myself playing everything, all the time. And it all worked. I cannot even begin to describe the hours I’ve spent with the M-60 Mk IIs in my current reference rig, so full of indescribable but musical ecstasy have they been. Dancing (a horrible sight for others, but a personal joy to me), singing (terror for others, but Ella the Ratdog puts up with it, even joining in on occasion), conducting, and just blissing out are regular occurrences. As I’ve said elsewhere, the key really is a complete system that works together, and the other partners in these experiences -- primarily Dodson DAC or Rega/Dynavector source, BAT preamp, Merlin VSM-SE speakers, Cardas wire -- are significant. But the Atma-Sphere amps brought it all together, and this may just be the highest compliment I can give any piece of audio gear.

At this point, no doubt, you’re wondering if Robin’s maiden name is Karsten. It’s not, nor is the M-60 Mk II perfect. While the images it throws are as resolute as Job, as I just mentioned the stage is a bit flatter than I suspect reality dictates. It can place images outside the speakers, but in general, it also has a slightly narrower stage than other amps I’ve heard. And as I mentioned earlier, the deepest bass lacks whomp. And there is that very slight leanness in the lower mids. The M-60 Mk II can also exacerbate other system issues. That very detailed presentation, when coupled with other gear that is a bit sharp, can result in a too-forward presentation. Finally, other than its effect on your heating bill, the M-60 Mk II will not warm the room. It’s not dry, clinical, or etched, but then neither is it the place to go for anything close to traditional tube warming. Beyond that, and after three months of close and almost continuous listening, and even though saying this may cause me to lose my critic’s badge, I can’t find another flaw. If anything, all three months of listening did was draw me deeper into the amp.

Any takers?

It’s been a good couple of years at the Warnke Mountain Home for Broken-Down Audio Scribes. The amps that have played here have been, without exception, excellent. But even in excellent company, some stand out, and none more than the M-60 Mk II. But the competition really is good, and each of the other amps I’ve reviewed of late has its strengths.

The $3700 Blue Circle BC6 is $500 less than the M-60 Mk II and simply gorgeous. It also is one of the most overtly emotional high-quality amps I’ve heard. Compared to the Atma-Sphere, it seems to go for the gut with more quickness and surety. On the other hand, it is also darker, slightly less detailed and less dynamic. The $3200-more-expensive Ayre V-1 out-muscles the M-60 Mk II, can control anything, and heats the room nearly as well, an important virtue for cold Colorado nights. But it too sounds a bit less detailed. At the end of the day -- and with my tastes and in my room and listening to music I like with my gear -- I simply have yet to encounter any amp that can do so much so right and so little so wrong as the M-60 Mk II.

Back to the listening room

I don’t doubt that there are amps that can equal the M-60 Mk II and perhaps better them -- amps such as the larger Atma-Spheres or the Joule-Electra OTL amps that I have heard at shows but not at home, or perhaps the Lamms that Marc Mickelson loves so much. But most certainly at the $4200 price point, I know of no real competition for the M-60 Mk II. I’m not saying that the M-60 Mk II is the end of the line and that if you don’t have it or like it that you’re a tin ear, but rather that the particular combination of virtues and almost complete lack of faults offered by the M-60 Mk II is extremely rare and coincides almost perfectly with how I perceive music, both live and recorded. To get this level of performance you need to place the M-60 Mk II in appropriate context. My advice is to try Merlin speakers with them -- from the comments on the Owners Group list, this combo is common and uniformly loved. But then this kind of matching applies to every piece of audio equipment. Beyond this, the only thing I can add is to go listen for yourself, and that’s exactly where I’m headed right now.

...Todd Warnke
todd@soundstage.com

Atma-Sphere M-60 Mk II Mono Amplifiers
Price:
$4195 USD per pair.
Warranty: Three years parts and labor; one year for tubes.

Atma-Sphere Music Systems
160 South Wheeler
St. Paul, MN 55105 USA
Phone: (651) 690-2246
Fax: (651) 699-1175

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


Atma-Sphere responds:

Todd's comments regarding the ideal speaker for a particular amplifier are right on the money. Having had plenty of time to study this issue in the last 20 years, we have found some interesting facts that fly in the face of "conventional" wisdom. One of them is the issue of speaker sensitivity that has been part of the myth surrounding OTLs. The truth of the matter is that OTLs are the least-load-sensitive amps made, depending on size, and are frequently far less sensitive to load than transistor amps. A practical example is the transistor amp that doubles its power when the load impedance is cut in half -- say from 8 to 4 ohms. By contrast, the M-60 Mk II loses 15% to 20% of its power with the same change, so the load sensitivity (for the M-60 Mk II and tube amps in general) runs opposite to that of transistor amps. Attention needs to be paid to this detail when choosing amps and speakers.

The way to work it with such an OTL amplifier is to provide a speaker load that has only minor impedance variation, such as the Merlin used in the review, or have a speaker that has most of its impedance variations occurring above a certain minimum impedance (that impedance depending on the size of the amp). An example of this second type of speaker is the Quad. Despite the rather wide range of impedance variation that the Quad is known for, using it with the M-60 will produce virtually constant power at all frequencies. Constant power at all frequencies would seem to be an essential part of the goal of accurate reproduction.

Thanks for the opportunity to make this point, and for the attention!

Sincerely,

Ralph Karsten
Atma-Sphere Music Systems

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