Air Tight ATM-211 tube monoblock power amplifier Page 2
I normally place amplifiers on a PolyCrystal stand that can support a good-sized stereo amp or a pair of smallish monoblocks. However, the pair of ATM-211s was too big to be accommodated, so I got hold of a second PolyCrystal stand so that each amplifier had its own. I first listened to the amps plugged directly into AC, but found that using a PS Audio High-Current Ultimate Outlet resulted in a slightly lower noise level, with no downside such as restriction of dynamics.
The ATM-211 has what seems like an exceptionally rigid, well-built chassis; this, combined with the vibration-absorbing properties of the PolyCrystal stand, made me doubt that additional supports like Aurios or Rollerballs would provide any sonic benefit. I was about halfway though my auditioning when I got around to placing three Aurios Pro MIB supports under each amplifier, and, lo and behold, there was some "cleaning up" of the sound.
Among the goodies I took home from the 2002 Consumer Electronics Show was a set of PTFE/Titanium Duende Criature tube damper rings from Divergent Technologies. These are available in sizes to fit almost any tube ($17.50-$35 each), including the 211, and a couple of 211-sized rings were part of the set that Divergent's Tash Goka gave me. I tried these on the 211s, but can't say that I noticed any change in the sound. However, placing the damper rings on the small tubes did produce some improvement in transient clarity, so I left these on for the rest of my auditioning.
The ATM-211's volume-control potentiometer provides for some flexibility in setup but also leaves the user with another decision to make: at what level should this volume pot be set? I normally assume that, all else being equal, a potentiometer sounds best in its maximum position, where it's nearly out of the circuit—but this means that the preamp's volume control would be in the lower part of its range, which may not be optimal for the preamp. My Convergent Audio Technology SL-1 Ultimate preamp has a discrete resistor-ladder volume control, which is sonically superior to a potentiometer, but the steps at the low end are pretty big; being able to set it closer to the middle of its range would be an advantage. I eventually settled on setting the ATM-211's volume control at the 11 o'clock position, the maximum being at 12 o'clock.
One disadvantage of speakers as sensitive as the Avantgarde Unos (+100dB) is that electrical noise—originating in the source component, preamplifier, and amplifier, as well as in ground loops and RFI/EMI picked up by cables—can be a significant problem. The last amplifier I'd had in the system was the Quicksilver Horn Mono (reviewed in the May 2002 Stereophile), which is designed to have low gain specifically to minimize noise—and, indeed, it had the lowest noise level I've encountered yet in my system. The ATM-211's noise level was much higher: with the system on but not playing, the hum/buzz was noticeable when I walked into the listening room. Turning down the amplifier's volume control (and turning up the preamp volume to compensate) had no effect on the noise, nor did various alternative grounding arrangements, including floating all the grounds in the system.
I ran the test that consultant Neil Muncy recommends for checking a possible "Pin 1" problem (see "Noise Susceptibility in Analog and Digital Signal Processing Systems," AES Preprint No.3930), and it indicated no problem with grounding. I even tried plugging a CD player directly into the ATM-211 via short interconnects, but the noise persisted. The noise was roughly comparable in level to what I'd heard with amplifiers like the Wavelength Gemini, and was not loud enough to interfere with my enjoyment when there was music playing, but I wish it had been lower.
Audiophiles in the solid-state camp typically characterize tube equipment as sounding rolled-off in the frequency extremes, with soft highs and mushy lows, and they argue that tubes add colorations that may be euphonic but that represent a departure from true accuracy.
For anyone who holds this view, listening to the ATM-211 should be an ear-opening experience. I heard nothing that would make me think that the ATM-211 is even slightly restricted in the frequency range. Highs had a sparkle, a sense of air and openness, particularly apparent in the upper range of the piano keyboard and with percussion instruments like cymbals. If anything, the highs were on the borderline of being a bit too bright—I got a better tonal balance with the somewhat mellow Nirvana S-L interconnects than with the squeaky-clean Nordost Valhallas. (However, I preferred the clarity of the Valhalla speaker cables over the Nirvana S-Ls.)