ATMA-SPHERE M-60 MK. 3.3 OTL MONOBLOCK AMPLIFIERS

Back in pre-COVID-19 days when audio shows were a thing, Atma-Sphere usually showed their amps in a big room with large highly-efficient speakers. Entering the room, you always heard fun and interesting music at a good volume filling said large space. In between the large speakers would be glass. Lots of glass. A plethora of tubes glowing fiercely on amps featuring a classic industrial design while the speakers put out wonderfully clear music with a wide and deep soundstage. In the back of the room, you would find Ralph Karsten, founder of Atma-Sphere, long hair flowing in a 60s vibe. Ralph is the industry’s chief proponent of the Output Transformerless (OTL) amplifier. He has been experimenting and perfecting his design since the late 1970s when Atma-Sphere was first formed.

I first met Ralph around 1982 when he walked into our newly relocated audio store in St. Paul, Minnesota and asked if we might be interested in selling his amplifier. At that time, we were stocking primarily solid-state amps like Dave Belles’ A-mods and the Amber 70 along with the venerable NAD 3020. Ralph’s amps had – as you would expect – lots of tubes. The big thing was the OTL design. Historically, OTL was hard design to get right. OTL models designed in the 50s and 60s were somewhat unreliable and had a reputation of sounding great when working but also for blowing up speakers when they were not quite right. Ralph convinced us to give his designs a try and we became an early Atma-Sphere dealer.

Ralph had been working on an OTL circuit design for many years and was seeking a solution to the reliability challenges that previous versions had not overcome. I asked him how he arrived at his stable design and he said he had been thinking about the issues that had been at the core of other OTL design failures. How could he take a different approach than had been tried previously? In his words, he took a nap, woke up and started drawing a design that had come to him. I have certainly woken up to an idea I had been seeking but I never woke to a company founding idea that set the course for a lifelong career. True to his vision, the patented Atma-Sphere OTL design has proven to be highly reliable with many of his amps still in service after decades of use. One benefit of the design is that owners of older versions are able to upgrade to any new improvements. The current Mark 3.3 circuit design used in several Atma-Sphere amps can be added to any of the amps utilising that same circuit regardless of how old they are. A great feature for owners of Atma-Sphere amplifiers.

I had a chance to catch up with Ralph at the last US pre-COVID-19 audio show in Tampa in February 2020 and I asked him if he would be interested in a review of one of his amp designs. The room was featuring his Novacron amplifiers featuring the 6C33C Triode power tubes. A favourite amp of mine. He surprised me by asking if I would be interested in a pair of M-60 Mk 3.3 amps, Atma-Sphere’s 60 watt per channel OTL monoblocks featuring two quads of 6SA7G power tubes alongside four 6SN7’s per amplifier. The M-60 has a classic industrial design and a sweet mechanical power meter in the centre made exactly as they have been since 1937. Atma-Sphere even uses a company that has a WWII-era machine to stamp out the model and name plate on the front of each amp. They have a timeless look that never goes out of style. Little did we know it would take several months to ship them out for review.

Once they arrived in two very manageable boxes, I got to setting them up. I say manageable because each box weighed in at under 25kg for monoblock Class A amps. The joy of OTL. No hulking output transformers to cart around. The amps were well packed and included another box inside that contained all of the bubble wrapped tubes and power cord. Populating the tubes was straightforward and even less of a project than some other amps as no tube matching is required with the M-60 amps. There is no Bias adjustment required as with many tube amps. In fact, should you not need the full 60 watts per channel in 8 Ohms, you could run them with fewer tubes (a fuse replacement would be necessary) and the amps would still be completely stable. The amps are balanced differential design running in Class A2 mode. The amps arrive with a small copper staple installed in one balanced channel connector allowing for single-ended connection, if needed. To connect a balanced XLR cable simply remove the staple and connect your cables. Removing the bottom screws on one unit revealed immaculate point to point wiring. Atma-Sphere has produced a short video where you can watch them build the M-60: https://www.youtube.com/watch?v=yodVDiEIpqI&feature=emb_logo

I connected the M-60 amps to my PS Audio BHK Signature Preamp which offers five balanced and five single ended connections. All connections used through the BHK Signature were balanced using all AudioQuest MacKenzie and Water XLR’s.

Getting back to the tubes in the M-60, I asked Ralph why he selected the 6AS7 tube as the power tube of choice. Low cost and availability were one reason, but the other was he was looking for a tube with the lowest possible plate resistance. He considered the 7241 and the 6336 but they were expensive. The 6AS7 was inexpensive and available. Every few years he would do a price performance calculation on all the tubes that would fit his circuit profile. What stood out about the 6AS7 was that it was designed as a voltage regulator and those tubes have to be very linear. The more linear it is the more effective it is at voltage regulation. That type of tube needs to be a power triode. Unlike a pentode such as the KT88 which requires careful tube matching and regular biasing the 6AS7 has a mu of two. It is such a low value that its grid is relatively insensitive to minor variations. If you can set its bias point at the factory it is going to be in the right spot for the long term. The 7241 and 6336 can fill the same role but at much higher costs without any improvement in sound and performance. He bought as many of the 6AS7 tubes as he could from a few supplier’s he found and stocked up for the long haul. His house was packed with them for almost 20 years. Their reliability and low cost have allowed Atma-Sphere to put a full one-year warranty on the tubes of each M60! If you need more power than 60W in Class A Atma-Sphere makes amps that can scale up to as much as 500W into a three-ohm load using the same OTL circuit. However, I cannot imagine what size concert hall you would need, in order to need 500W of Class A power.

Talk of warranties brings up a point about people’s beliefs around OTL amplifiers. The long-term thought has been that OTL amps blow up or are unreliable, largely because Julius Futterman’s designs from the 1950s did reliability issues. Seventy years later, the circuit designs of Atma-Sphere’s OTL have solved that challenge. Ralph says the most Sisyphean (his word, naturally I had to use it!) task he faced early on was simply convincing people his designs were stable and reliable. Forty years and no blow ups later, he has more than proven his point about how reliable a well-designed OTL can be.

In my time with the M-60 amps, they behaved admirably with no hiccups of any kind completely rendering the historical challenges of OTL amplifiers moot.

Initially I connected the M60 Mk 3.3 amps to a pair of Zu Audio Omen Dirty Weekend speakers via a pair of Silversmith Audio Fidelium cables. The Omens are an easy load at 12 Ohms (Running off the 8 Ohm taps) and 97dB efficient. The M-60 pair delivered a sense of realism through the Omen’s that was wonderful. Listening to Pink Floyd’s Dark Side of the Moon on SACD [EMI] via my PS Audio DirectStream DSD DAC and SACD Transport The sense of scale was wonderful and the metallic tone of the clocks from the song Time rang out in all of their chime filled glory. I am unable to listen to just one track from this classic, so a full listen was required. Played through the M-60 mono amps, the delivery was as wonderful as I had heard it. The real stand-out track was ‘The Great Gig In The Sky’ featuring Clare Torrey’s incredible vocal pyrotechnics. The clarity and body were exceptional. Her emotion shone through making this a special listening event. It was this more visceral presentation that I enjoy from a high-end tube amp and the M-60 Mk 3.3 played so well.

Zu Audio is generally the life of an audio show party so it was time to see how dynamic the M-60 amps could be with a pair of speakers that can really bring it for volume and clarity. Tower of Power cut a direct to disc Sheffield Labs album in 1981. The single What is Hip has a particular energy and realism brought by this live performance. Horns, guitars, drums drive this sonic powerhouse song. The M-60’s delivered more than enough power with the authority necessary to bring this tune to full life. The Zu Omen speakers brought the live performance to the room with floor shaking fun.

Swapping out the Zu Omen for a pair of DeVore Fidelity O/93, it was time for some finger picking with Tommy Emmanuel. The title track of Endless Road [Favored Nation] offers up expert playing with his Maton guitar. Air and space between notes are a hall mark of a great amp, while also offering up great tone. The M-60 amps delivered this in abundance via the O/93’s. The 10 Ohm 93dB efficient O/93 proved another excellent match for the M-60’s. The realism of fingers on strings was a delight and reminiscent of when I attended a Tommy Emmanuel concert with fourth row centre tickets. The M-60 amps made the listening an experience rather than a simple listening session. Most delightful.

Wrapping up the listening with an all-time favourite tune, Toto’s ‘Africa’ [Toto IV, Columbia], a song that was on the radio a lot when I first encountered Atma-Sphere as a company in 1982. Africa offers up a compelling mix of percussion and vocal harmonies that have made it a classic rock staple. The M-60’s brought out the best in this SACD recording with drums, xylophone and marimba’s anchoring syncopated rhythms that drive the song forward. Soaring vocals moved along across a wide and clear soundstage. The rains were indeed blessed.

The Atma-Sphere M-60 Mk 3.3 power amps proved to be sonic marvels. The classic ‘Pace, Rhythm and Timing’ was joined by body, space and dimensionality. There was also that hint of tube magic without any overbearing coloration. Things sounded as they should with just enough colour to provide a sonic character that would keep me coming back for more. Truly one for the music lover.

TECHNICAL SPECIFICATIONS

Type: valve power amplifier

• Input: 3 V sensitivity (for full output),
  RCA and XLR true balanced input
• Tube complement: 8 × 6AS7G output tubes and 4 × 6SN7 driver tubes per channel
• Output power: 60 watts per channel into 8 Ω load, 45 watts per channel into 4 Ω load, 80 watts per channel into 16 Ω load
• Input Impedance: 100 KΩ single-ended, 200 KΩ balanced
• Output Impedance: ~4.1 Ω
• Gain: 20dB (for 8 Ω)
• Power Bandwidth: 2Hz–75KHz
  within ½ dB
• Frequency response: 1Hz–200KHz
  within 3 dB
• Total Harmonic Distortion (THD):
  typically 1% at full power. Intermodulation (IM) distortion: typically under .04% at full power.
Phase shift: less than 1° @ 20KHz
• Output section risetime: 600 V/µs
• Feedback: 1 dB
• Dimennsions (W×H×D): 43 × 20 × 33cm per chassis
• Weight: 13.6kg per chassis (without tubes)
• Price: $7,700/per pair USD

Manufacturer: Atma-sphere

URL:atma-sphere.com

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