In the pre-COVID-19 days, when audio shows were still a thing, Atma-Sphere typically displayed their amps in a large room with large, high-efficiency speakers. Entering the room you always heard fun and interesting music at a good volume that filled the large room. Between the large speakers would be glass. Lots of glass. A wealth of tubes that glow fiercely on amps with a classic industrial design, while the speakers reproduce beautifully clear music with a wide and deep soundstage. In the back of the room you'll find Ralph Karsten, the founder of Atma-Sphere, with long hair blowing in a '60s vibe. Ralph is the industry's biggest proponent of the transformerless (OTL) amplifier. He has been experimenting and perfecting his design since the late 1970's 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 would be interested in selling his amplifier. At the time, alongside the venerable NAD 3020, we mainly stocked solid-state amps like Dave Belles' A-mods and the Amber 70. As expected, Ralph's amps had a lot of tubes. The big thing was the OTL design. In the past, OTL was a difficult design to get right. OTL models developed in the 50's and 60's were somewhat unreliable and had a reputation for sounding great when working but also blowing up speakers when they weren't quite right. Ralph convinced us to try his designs and we became an early Atma-Sphere dealer.
Ralph worked on an OTL circuit design for many years, looking for a solution to the reliability issues that previous versions had failed to overcome. I asked him how he came up with his stable design and he said he had thought about the issues that had been at the heart of other OTL design flaws. How could he have done something different from what he had tried before? In his words, he napped, woke up and started drawing a design that came to him. I certainly woke up with an idea I was looking for, but I never woke up with a business startup idea that set the stage for a lifelong career. True to its vision, the patented Atma-Sphere OTL design has proven to be extremely reliable, with many of its amps still in service after decades of use. An advantage of the design is that owners of older versions can 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 using the same circuit, no matter how old they are. A great feature for Atma Sphere amp owners.
I had the opportunity to meet 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 reviewing one of his amp designs. The room was outfitted with his Novacron amps with the 6C33C triode power tubes. A favorite amp of mine. He surprised me by asking if I would be interested in a pair of M-60 Mk 3.3 amps, the Atma-Sphere 60 watts per channel OTL mono blocks, the two quads of 6SA7G power tubes alongside four 6SN7 tubes per amp contain. The M-60 has a classic industrial design and a cute mechanical power meter in the center, made exactly as it has been since 1937. Atma-Sphere even uses a company that has a WWII era machine to stamp the model and nameplate 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 send them out for review.
When they arrived in two very manageable boxes, I set about putting them up. I say manageable because each box weighed less than 25kg for monoblock Class A amps. The joy of OTL. No huge output transformers to lug around. The amps were well packaged and included another box inside containing all the tubes wrapped in bubble wrap and the power cord. Fitting the tubes was straightforward and even less of a project than some other amps as no tube fitting is required on the M-60 amps. There is no bias adjustment required as with many tube amps. If you don't actually need the full 60 watts per channel into 8 ohms, you could run them with fewer tubes (a fuse change would be necessary) and the amps would still be absolutely stable. The amplifiers are a symmetrical differential design running in class A2 mode. The amplifiers come with a small copper clamp installed in a balanced channel connector that allows for a single ended connection if required. To connect a balanced XLR cable, simply remove the clamp and connect your cables. Removing the bottom screws on one unit revealed pristine point-to-point wiring. Atma-Sphere produced a short video where you can see how they 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 via the BHK signature have been made balanced using all AudioQuest MacKenzie and Water XLRs.
Back to the tubes in the M-60, I asked Ralph why he chose the 6AS7 tube as his power tube of choice. Low cost and availability was one reason, but the other was that 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 for all the tubes that would fit his circuit profile. What was striking about the 6AS7 was that it was designed as a voltage regulator and these tubes need to be very linear. The more linear it is, the more effective it is at regulating voltage. This type of tube must be a power triode. Unlike a pentode like the KT88, which requires careful tube matching and regular biasing, the 6AS7 has a mu of two. It's such a low value that its grid is relatively insensitive to minor fluctuations. If you can set its bias point from the factory, it will be in the right place in the long run. The 7241 and 6336 can fill the same role, but at a much higher cost, with no improvement in sound or performance. He bought as many of the 6AS7 tubes as he could from some suppliers he found and stocked them up for the long haul. His house was full of it for almost 20 years. Their reliability and low cost has allowed Atma-Sphere to offer a full one year warranty on the tubes of every M60! If you need more power than 60W in Class A, Atma-Sphere makes amplifiers that can scale up to 500W into a 3 ohm load with the same OTL circuitry. However, I can't imagine what size of concert hall you would need to require 500W Class A power.
The talk of guarantees raises a point about people's beliefs regarding OTL amplifiers. The longstanding thought has been that OTL amps blow up or are unreliable, largely because Juliusfutterman's 1950s designs had reliability issues. Seventy years later, Atma-Sphere's OTL circuit designs have solved this challenge. Ralph says the most Sisyphic (his word, of course I had to use it!) task he faced early on was simply convincing people that his designs were stable and reliable. Forty years and no blasts later, he has more than proven how reliable a well-designed OTL can be.
In my time with the M-60 amps they have performed admirably without any hiccups, completely calling into question the historical challenges of OTL amps.
First, I connected the M60 Mk 3.3 amps to a pair of Zu Audio Omen Dirty Weekend speakers using a pair of Silversmith Audio Fidelium cables. The Omens are a light load at 12 ohms (running off the 8 ohm taps) and are 97 dB efficient. The M-60 pair conveyed a wonderful sense of realism through the omens. Listen to Pink Floyd'sThe dark side of the moonon SACD [EMI] via my PS Audio DirectStream DSD DAC and SACD transport The feel for dials was wonderful and the metallic tone of the clocks from the song Time rang out in all their bell-shaped glory. I can't just listen to one track of this classic, so a full listen was required. Played through the M-60 mono amps, the delivery was as wonderful as I heard it. The really stand out track was "The Great Gig In The Sky" with Clare Torrey's incredible vocal pyrotechnics. The clarity and body were exceptional. Her emotions radiated through and made this a special listening event. It had that visceral presentation I enjoy from a high end tube amp and the M-60 Mk 3.3 played so well.
Coming to audio in general is the life of an audio show party so it was time to see how dynamic the M-60 amps can be with a pair of speakers that really pack a punch for volume and clarity. Tower of Power recorded a 1981 direct-to-disc album by Sheffield Labs. The single What is Hip has a special energy and realism that comes from this live performance. Horns, guitars and drums drive this sonic powerhouse song. The M-60 delivered more than enough power with the necessary authority to bring this tune to life. The Zu Omen speakers brought the live performance into the room with floor-shaking fun.
After trading in the Zu Omen for a pair of DeVore Fidelity O/93s, it was time to fingerpick with Tommy Emmanuel. The title track fromendless road[Favored Nation] offers a masterful performance with his Maton guitar. Air and space between notes are hallmarks of a great amp while providing great tone. The M-60 amps delivered this in abundance over the O/93. The 10 ohm, 93 dB efficient O/93 proved to be another excellent match for the M-60. The realism of the fingers on the strings was a delight and reminded me of going to a Tommy Emmanuel concert with fourth row tickets. The M-60 amps made listening an experience rather than a simple listening session. Very adorable.
Finish listening to an all-time favorite, Toto's "Africa" [Total IV, Columbia], a song that was playing a lot on the radio when I first encountered Atma-Sphere as a company in 1982. The M-60 made the most of this SACD recording, with drums, xylophone and marimba anchoring syncopated rhythms that propel the song along. Soaring vocals moved across a wide and clear soundstage. The rain was indeed blessed.
The Atma-Sphere M-60 Mk 3.3 power amps proved to be sonic marvels. Body, space and dimensionality joined the classic “Pace, Rhythm and Timing”. There was also that hint of tube magic without the overpowering coloration. Things sounded the way they should, with just enough color to offer a tonal character that would keep me coming back for more. Truly one for the music lover.
Type: Tube Power Amplifier
- Input: 3V sensitivity (for full power),
True balanced RCA and XLR input
- Tube configuration: 8×6AS7G output tubes and 4×6SN7 driver tubes per channel
- Output power: 60 watts per channel at 8 Ω load, 45 watts per channel at 4 Ω load, 80 watts per channel at 16 Ω load
- Input Impedance: 100 kΩ unbalanced, 200 kΩ balanced
- Output Impedance: ~4.1Ω
- Gain: 20 dB (for 8 Ω)
- Power Bandwidth: 2Hz-75KHz
within ½ dB
- Frequency Response: 1Hz-200KHz
- Total Harmonic Distortion (THD):
typically 1% at full power. Intermodulation Distortion (IM): typically less than 0.04% at full power. Phase shift: less than 1° @20KHz
- Rise time output part: 600 V/µs
- Feedback: 1dB
- Dimensions (W×H×D): 43×20×33cm per case
- Weight: 13.6 kg per chassis (without tubes)
- Price: 7,700 USD/pro Pair USD
Manufacturer: Atma Sphere