select_150w_tWhen you’ve been immersed for several years in high-end audio, chances are you take for granted certain things that may surprise non-audiophiles. One of these is that a lot of today’s audio gear uses the venerable vacuum tube. In fact, many audio “civilians” are surprised to learn that vacuum tubes still exist in the 21st century, and that companies such as Atma-Sphere have been using and refining tubed circuits in their amplifiers and preamplifiers for over 30 years with no plans to change. The Music Amplifier M-60 Mk.3.1 is an example of how Ralph Karsten, founder and president of Atma-Sphere, has refined the sound of a venerable circuit.

201101_atmasphereAtma-Sphere is one of the few companies whose tube amps don’t use output transformers -- the output tubes drive the speakers directly. Transformers tend to smear the sound and limit the frequency response, and while many manufacturers wind great transformers, there’s no transformer like no transformer at all -- something that becomes clear when you hear an Atma-Sphere amp. You hear bass as deep as that from a solid-state design, but with levels of detail and tunefulness that most solid-state amps miss. The difference is not subtle. And unlike some output-transformerless (OTL) amplifiers, Atma-Sphere’s have been, in my experience, virtually bulletproof. You can even remove or insert output tubes while an Atma-Sphere amp is playing (an oven mitt is recommended). Most tube amps will be damaged if you turn them on with no speakers connected to them, but not the Atma-Spheres; being OTL, they can withstand that normally fatal condition. I don’t recommend that you actually try either of these, but if a connector slips off an Atma-Sphere’s speaker terminal, it’s nice to know it won’t trash the amp.

Recently, Atma-Sphere revised the input circuits of all its amplifier models. Still using 6SN7 tubes, the new circuit has 90% less distortion than the previous model, according to the company. The new models bear a “3.1” designation, and Atma-Sphere offers a low-cost upgrade to owners of their predecessors. The output circuit still uses the 6AS7G dual-triode tube in a self-biasing circuit, so the user doesn’t have to reset the bias when a new tube is inserted. All Atma-Sphere amps use fully balanced differential circuits, though unbalanced jacks are provided for use with unbalanced preamps.

Description

Unlike solid-state amplifiers but like other tubed OTL amps, the M-60 Mk.3.1 monoblock ($6950 USD per pair) produces more power into higher speaker impedances. Claimed to output 60W into 8 ohms, the M-60 Mk.3.1 produces 45W into 4 ohms or 80W into 16 ohms. Its input impedance is 100k ohms into its unbalanced (RCA) jacks, and 200k ohms into its balanced (XLR) jacks. I can’t conceive of an active preamp that couldn’t drive the M-60, but I’m not so sure about passive preamps. The M-60’s input sensitivity is 2.83V for full output. If you have very sensitive speakers (over 100dB), a tube in the M-60’s cascode voltage-amplifier section can be replaced with an optional jumper plug ($60/pair).

Another advantage of the M-60’s OTL output section is a sensationally fast risetime of 600V/µs. Its class-A circuit uses only 1dB of feedback. The amp’s output impedance is approximately 4.1 ohms, which is rather high, and could interact with an irregular speaker impedance to produce frequency-response variations. It also provides a low damping factor for normal speaker impedances.

The M-60 Mk.3.1 looks like a prop for a 1950s science-fiction movie. Unabashedly retro, it’s painted in black Wrinkletex, and an industrial-looking circular meter is mounted at the center of the transformer housing, which runs across the rear of the amplifier. But wait -- if this amp is transformerless, why is there a transformer cover? That’s because it still has power transformers -- two toroidal ones in each chassis. Remember, it’s an output-transformerless design. The meter is used to set the M-60’s DC offset to zero, and also indicates the power output. A trim panel is available for the meter panel in anodized brushed gold, black, or silver. The review samples had the silver panel, which looks a lot more attractive in person than it does on Atma-Sphere’s website. To my weird aesthetic, the M-60’s looks were quite appealing.

The M-60 Mk.3.1 measures 17”W by 8”H by 13”D and weighs 30 pounds; if that seems rather lightweight for a 60W tube amp, remember that there are no heavy output transformers. And while those dimensions should easily fit most rack shelves, a dozen tubes per amp produce a lot of heat -- the M-60s should be placed where they’ll be well ventilated. If there’s a shelf above the amplifier, expect the shelf and whatever equipment that rests on it to become noticeably warm. I didn’t try any accessory feet, but my Atma-Sphere S-30 Mk.3 stereo amp has responded well to several such feet I’ve tried, especially if they absorbed vibrations from the shelf.

The M-60’s balanced and unbalanced input jacks are on the rear panel. If you use the RCA jacks, you’ll need to insert a jumper (provided) across two pins of the XLR jacks; to use the XLR jacks, remove the jumper. Also on the rear are hefty copper speaker connectors by Cardas. An IEC inlet is provided for the power cord.

201101_atmasphere_wide

Setup and use

The tubes are shipped in their boxes -- the user plugs them into their sockets. I noticed that there were no 6AS7G tubes provided. Was that a big mistake? No, Atma-Sphere uses Chinese and Russian tubes, which have Russian nomenclatures equivalent to the 6AS7G. These tubes are specially selected for the Atma-Sphere circuits, and are the only ones recommended for the best sound. If that sounds scary expensive, it’s reassuring to learn that replacement tubes are relatively cheap: $26 per 6AS7G-equivalent output tube, $28 per 6SN7 input tube.

Balanced Audience Au24 e cables connected the M-60 Mk.3.1s to my preamp, and Clarity speaker cables connected the amps to my speakers. As I usually do, I used an aftermarket power cable for the amps, in this case Purist Audio Designs’ Venustas, which has improved the sound of just about everything I’ve used it with.

Turning the M-60s on is not just a matter of throwing a switch and waiting to hear sound; you first have to turn on the tube heaters by flipping a toggle switch at the far left of the front panel. The amber light next to the switch comes on, and a few seconds later the tubes start to glow. Let the tubes warm up for at least a minute, then flip up the Standby toggle at the far right to apply high voltage to the tubes. The amplifier is now ready to play.

But according to the manual, the very first time you turn on an M-60, you need to adjust its DC offset to zero. I checked the DC offset again after about an hour’s play, and occasionally thereafter. There may be some drift as the tubes age or after one or more has been replaced, but generally, the DC offset is quite stable, so frequent adjustments aren’t needed.

Fortunately for me, Atma-Sphere had already played the review samples for about 100 hours. I let them burn in for another hundred hours before beginning to listen critically. If you’re not convinced that burn-in is necessary, that’s your call, but Atma-Sphere recommends 150-200 hours of it; as a reviewer, I follow manufacturer’s recommendations.

My Audio Research LS26 is a fully balanced line stage that has paired remarkably well with my Atma-Sphere S-30 Mk.3, so I used it as well with the M-60 Mk.3.1s. I used the LS26’s balanced output connections, which, not surprisingly, sound somewhat better with a balanced amplifier. Although I usually augment my main speakers with JL Audio Fathom f110 subwoofers, when I review an amplifier, I disconnect the subs so I can assess the review unit’s bass performance.

At first, the M-60 Mk.3.1s sounded awfully congested and distorted, with no gain, and generally unpleasant. I was getting ready to send them back when it dawned on me: I’d plugged the Atma-Spheres into my power conditioner, which was limiting the current it fed to the M-60 Mk.3.1s. Plugging the amps directly into the wall transformed them into the excellent-sounding monoblocks I had expected.

Although I didn’t have the low-noise jumper plugs, I didn’t really need them, so quiet were the M-60s. If I placed my ear directly in front of the 103dB-sensitive driver, I could hear a faint buzz that was totally inaudible at my listening position.

Sound

Once I’d properly connected the Atma-Spheres to the wall socket, they sounded open and fast, with good frequency extension at both ends of the audioband. My S-30 Mk.3 amplifier had led me to expect excellent deep bass, with good detail and tunefulness, and the M-60 Mk.3.1s didn’t disappoint. With Folia Rodrigo Martinez, performed by Jordi Savall and his band and ripped from their La Folia 1490-1701 (CD, Alia Vox AFA 9805), a large bass drum goes down to the middle of the 20-30Hz range. The M-60s reproduced the drum with plenty of impact and weight, but with more extension and detail than I sometimes hear even from solid-state amps.

I often evaluate a component’s high-frequency performance by playing Eiji Oue and the Minnesota Orchestra’s performance of Argento’s For the Angel, Israfel, ripped from Reference Recordings’ 30th Anniversary Sampler (CD, Reference RR-908). Through the M-60 Mk.3.1s, the high opening chimes sounded just about right quantitatively, neither truncated nor exaggerated. Qualitatively was another matter: highs sounded sweeter than I’m used to, and the flutes right after the initial chimes sounded unusually tonally complete and expressive. The cascabels (sleigh bells) were audible throughout this piece, as were the castanets. Sometimes those percussion instruments blend into a background blur, but not through the M-60s.

In heavy rotation chez moi has been a recent download: the chamber orchestra I Musici’s Serenata Italiana (24/96 FLAC, Foné/HDtracks). This collection of romantic and modern Italian music includes Respighi’s Ancient Airs and Dances, a suite for modern orchestra based on Renaissance works for lute. The M-60 Mk.3.1s trod a very narrow path between solid-state and tube sound, presenting the complete harmonic envelope of the lovely string sound as beautifully as you’d expect from a tube amp, but without the second-harmonic distortion that tubes sometimes produce. The M-60 Mk.3.1s realistically depicted the sound of this medium-size chamber orchestra, with not only accurate and complete tonal structures of the various stringed instruments, but also accurate dynamics for each individual instrument. Like many high-resolution recordings, Serenata Italiana provided a realistic soundstage; though not unnaturally pinpointed in space, the instruments sounded as if located within a small orchestra -- which they were.

Atma-Sphere amps usually throw stunningly wide, deep soundstages, and the M-60 Mk.3.1s followed in that tradition. The soundstage of the title work of the Tallis Scholars’ Allegri Miserere (24/96, FLAC Gimell/Gimell) was just gigantic, expanding both laterally and in depth to vividly depict a small ensemble singing some distance behind the main chorus in a large church. Voices were tonally whole and precisely located in the soundfield. If you need to illustrate to your friends how breathtaking a hi-rez digital soundstage can be, this is a good recording for that purpose -- and it doesn’t hurt to have amplifiers like the M-60 Mk.3.1s.

Comparison

I first compared the M-60 Mk.3.1s to Atma-Sphere’s own S-30 Mk.3. After I’d reviewed and then purchased it in March 2007, my S-30 Mk.3 (current price $3950) became my reference amplifier. It doesn’t sound as sweet as some single-ended triode (SET) amps, but seldom does it run out of power. I’ve compared the S-30 Mk.3 to several other amplifiers in the 25-35Wpc range, and it has proved strong competition for several much more expensive models. In short, it kicked their butts. I’d planned to have my Mk.3’s input circuit updated to Mk.3.1 status, but had decided to keep the earlier configuration until this review was completed, so I could compare the two circuits. The S-30 Mk.3 is built on the same chassis as the M-60 Mk.3.1, but is a stereo amplifier with 16 tubes. Unlike the M-60 Mk.3.1, the S-30 Mk.3’s input jacks are in front, which is inconvenient, as it requires longer interconnects, which must be routed from the rear of the preamp to the S-30’s front panel. Not only is that arrangement tricky, it’s ugly.

The S-30 Mk.3 sounded very similar to the M-60 Mk.3.1s -- no big surprise -- but the newer amp sounded somewhat sweeter, more relaxed, and less mechanical. The difference wasn’t huge, but it was there. I attribute it to the new input circuit’s lower level of distortion, and I’m sending my S-30 Mk.3 back for an upgrade posthaste!

I then compared the M-60 Mk.3.1s to the Audio Research VS115 ($6495). The ARC is rated at 120Wpc into 8 ohms, but less into my speakers’ 16-ohm impedance -- probably in the same ballpark as the M-60 Mk.3.1. The VS115 has a traditional tubed output stage with output transformers, and uses four 6550 tubes per channel. Unlike the M-60 Mk.3.1, the VS115’s tubes must be manually biased -- a pain. At 62 pounds, the stereo VS115 weighs about as much as two M-60 Mk.3.1s.

Still, the ARC VS115 is not without its own charms. Its portrayal of the tonal characteristics of instruments and voices is extremely attractive, and its high frequencies could be the poster child for tube amps: open and extended, with no added tube distortion. Playing Folia Rodrigo Martinez, its bass is well defined and extended, if a smidgen less incisive than the M-60 Mk.3.1’s, which seemed a tad more dynamic. The ARC is eerily quiet -- aside from some hum during warmup as the capacitors charge, it’s hard to tell the amplifier is turned on at all. (Good thing I can see the glowing tubes and pilot light.) And which amplifier produced a better soundstage was a toss-up -- sometimes, the VS115’s extra power can be very handy into normal 4- and 8-ohm speaker loads.

Sonically, it’s hard to choose between the Audio Research and the Atma-Sphere; both have strong attractions. But if forced to choose, I’d probably pick the Atma-Sphere for its superior bass and speed.

Summary

Displayed on Atma-Sphere’s website is an impressive collection of awards. When he reviewed the M-60 Mk.3.1’s predecessor, the M-60 Mk.3, our own Tim Aucremann bestowed on it a coveted Reviewers’ Choice award. Since then, Atma-Sphere has improved the input circuit to reduce distortion, which has resulted in a sweeter, less mechanical sound. I like it a lot. If I didn’t already own Atma-Sphere’s S-30 Mk.3, I’d be sorely tempted to send in a check for the M-60 Mk.3.1s. For most speakers, the extra power will be well worth the cost. So since Atma-Sphere took a Reviewers’ Choice amp and made it better, I see no reason not to bestow Ultra Audio's Select Component award on the latest version, for the same reason.

A quote on Atma-Sphere’s home page says: “These OTL amps are arguably the most beautiful sounding amps in the world regardless of price.” You won’t hear any argument from me.

. . . Vade Forrester
vadef@soundstagenetwork.com

Associated Equipment 

  • Speakers -- Affirm Audio Lumination speakers, JL Audio Fathom f110 subwoofers (not used for amplifier evaluations)
  • Amplifier -- Audio Research VS115 stereo amplifier, Atma-Sphere S-30 Mk.3 stereo amplifier, Art Audio PX-25 stereo amplifier
  • Preamplifiers -- Audio Research PH5 phono preamp, Audio Research LS26 line stage
  • Analog sources -- Linn LP12 turntable on custom isolation base, Graham 2.2 tonearm, van den Hul Platinum Frog cartridge
  • Digital sources -- Meridian 508.24 CD player, Sony SCD-XA5400ES SACD/CD player; Hewlett-Packard dv7-3188cl laptop computer running Windows 7 and Foobar2000 music server software; Auraliti PK100 music player; Benchmark DAC1 Pre DAC; Audio Research DAC8 DAC (review unit)
  • Interconnects -- Crystal Cable Piccolo, Purist Audio Design Venustas, Audience Au24 e, Clarity Cables Organic, TG Audio High Purity Revised
  • Speaker cables -- Crystal Cable CrystalSpeak Micro, Purist Audio Design Venustas, Audience Au24 e, Clarity Cables Organic, Blue Marble Audio
  • Power cords -- Purist Audio Design Venustas, Audience powerChord e, Clarity Cables Vortex, Blue Marble Audio Blue Lightning
  • Digital -- Wireworld Starlight 52 USB cable and Gold Starlight 6 S/PDIF cable
  • Power conditioner -- Audience aR6-T

Atma-Sphere Music Amplifier M-60 Mk.3.1 Mono Amplifiers
Price: $6950 USD per pair.
Warranty: Three years parts and labor; one year on tubes.

Atma-Sphere Music Systems, Inc.
1742 Selby Ave.
St. Paul, MN 55104
Phone: (651) 690-2246
Fax: (651) 699-1175

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

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