July 2003

Follow-Up: Atma-Sphere M-60 Mk.II.2 OTL Monoblocks Combined With
Speltz ZERO Autoformers
Review by Wayne Donnelly
Click here to e-mail reviewer

  My highly enthusiastic March 2003 review of the Atma-Sphere M-60 Mk. II.2 monoblock amplifiers concluded with these words:

"At $4,650 for the pair, the M-60 MK.II.2 is an outstanding value. I felt a genuine pang of regret when I packed up the amplifiers for their return trip to St. Paul. Much as I love them, I need more power. Perhaps there are bigger Atma-Sphere amplifiers in my future."

What prompted that conclusion was my experience driving my reference Eggleston Andra II loudspeakers with those excellent 60- watt OTL amplifiers. At 86dB sensitivity and nominal 6-ohm impedance that drops to 4 and as low as 2 ohms in the very deep bass, the Andra II is a challenging load for a moderately powered tube amplifier-especially an OTL design. The M-60s are happiest -- even putting out 20 watts more per side-with 16-ohm impedance. And obviously, more efficient loudspeakers-say 90dB and up sensitivity-would give the amplifiers additional headroom for handling recordings with large-scale dynamics. 

The M-60s had surprised me by driving the Egglestons with a fair degree of authority, although they did gas out at lower playback levels than I favor in my more exuberant moods. There was also discernibly less deep bass than I had been getting with the Von Schweikert dB-99 loudspeakers. That accounts for my assumption that the 100- and 220-watt Atma-Sphere monoblocks would be more suitably matched with the 86dB sensitive Egglestons. 

When the review appeared, I began receiving e-mail us from a passionate M-60 owner. (I would like to acknowledge the gentleman by name, but unfortunately I lost much of my e-mail archive recently while switching from a Windows PC back to a Macintosh.) He insisted that the M-60s would easily have driven my speakers to high volume if I had taken the time to run them through the ZERO autoformers, which provide for impedance-multiplying connections that could be set to show 16-ohm impedance to the amplifiers. I had mentioned the ZEROs in the review, but had decided that I didn't have time to audition them properly. Apparently I had erred in skipping the ZEROs. 

Suitably chastened, I requested and received a loan extension from Atm-Sphere's Ralph Karsten. Next I contacted ZERO designer Paul Speltz, who promptly sent a pair of ZEROs housed in attractive hardwood enclosures. So I unpacked the amplifiers and put them back into the system with the ZEROs connected between amplifiers and loudspeakers. 

Senior Editor Dick Olsher reviewed the ZEROs here just last month, and I strongly recommend going to that article for Dick's insights and for technical information that I am not going to repeat here.

Setup and Listening

The only thing that required any thought with this configuration was deciding which multiplication factor to use. With a 4-ohm speaker it's pretty much a no-brainer: the 2x connection shows the amplifier 8 ohms, 3x = 12 ohms, and 4x = 16 ohms. But the Egglestons are nominally 6 ohms, for which this formula would yield 12, 18 and 24 ohms respectively. Just for the hell of it, I tried all three configurations, finally chose the 3x taps for a nominal 18-ohm load.

Paul Speltz had confirmed my assumption that the ZEROs need extensive break-in before they begin to sound their best, and the progress of the sound during the first few weeks of listening was painfully gradual. I did notice that the system could now play more loudly, and there was a modest improvement in the lower bass. On the other hand, there was a new slight harshness in the mid and treble octaves that tended to impede my usual progression from the analytical left to the hedonistic right half of the brain. 

Help arrived in the form of a brand-new audiodharma Cable Cooker 2.5 (review in progress). After several break-in sessions on the Cooker with listening tests after each session, I finally hooked up the ZEROs and left them there.

Being able to play the system at much higher levels without audible strain was now even more of a kick. Yes, at the upper end of the new volume envelope I could still evoke the traces of harshness and glare that signal imminent amplifier clipping. But the loudspeaker-driving capabilities of the M-60/ZERO combination were robust enough to satisfy me at least 90 percent of the time. I was also pleased to note that the deepest bass was restored to a degree that would be worth 3-4 points on the I've numerical rating chart. 

The other principal improvement with the ZEROs, which I had not really thought much about, was in spatial resolution. The soundscape did not seem appreciably wider, but there was a slight improvement in depth and a more significant gain in image scale and placement. Altogether, the greater sense of power and control, cleaner and deeper bass, and more precise spatial presentation were impressive. 

But believing that there is no free lunch (except at trade shows) -- or fidelity -- I kept listening to see if the prescribed extensive break-in had reduced or eliminated the subtle but nonetheless distracting flaws I had noticed in earlier listening. Surely I would perceive some slight loss of inner detail, some tonal coloration-something nasty! But I came up with zilch, nada. On the basis of listening to this one system, I'm not prepared to assume that the ZEROs will be as completely beneficial with other amplifier/speaker configurations. But I found no reason to hand out any demerits to the sound I heard. 

Paul Speltz sells the ZEROs direct (www.zeroimpedance.com) with 30-day money-back guarantee. They are wonderfully effective with the Atma-Sphere M-60 MK.II.2 so much so that I would consider their purchase as simply part of the price of the amplifiers. I think anyone who favors the sound of low-power tube amplification -- be it OTL, SET or push-pull -- is well advised to look into adding the benefits of the ZERO autoformers. There's no financial or technical risk-unless you can not count to 30 and a potentially terrific upside. 

NOTE: Specifications and Company Information for both products can be found in the original reviews.

Click here for original M-60 review.

 

Click here for Dick Olsher's ZERO review.