Atma-Sphere M-60 Mk. 3.3 OTL Monoblock Amplifiers

Tubed power amplifiers
Atma-Sphere Music Systems M-60 Mk. 3.3
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:

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