Tube Power Amp Reviews

It's no secret, especially to those who've been following Stereophile for more than a short time: In the first half of 2007, I took the plunge and bought a Shindo preamplifier and monoblock amplifiers—handmade products characterized by low output power, generous numbers of vintage parts, steel casework finished in a signature shade of green, and richly textured, impactful sound with lots of sheer musical drive. And while we tend not to alert the major newspapers whenever someone on staff buys new electronics, the change was notable for two reasons: The compatibility of Shindo's amplifiers is limited to loudspeakers of higher-than-average sensitivity and impedance; and, throughout the seven years that followed my switch to Shindo (footnote 1) both my system and my point of view regarding domestic audio in general have evolved in the direction of the artisanal and the vintage.

Mild concern—among owners of low-efficiency loudspeakers, who might despair of finding in my reviews any relevance to their own systems; and among competing electronics manufacturers, who might wonder if an amp must be painted green for me to like it—is understandable. There isn't much I can do about the former except, I hope, write compellingly about things that people think they don't want. As for the latter, I can only assure you that my mind remains open—and that the world of hi-fi still contains many other musical-, organic-sounding, beautifully made preamplifiers and power amplifiers, the likes of which perform best when playing through horns, single-driver systems, and other high-efficiency loudspeakers. Of the proof I've heard most recently, there may be none better than the Air Tight ATM-1S ($9500), which first caught my eye at the High End 2014 show, in Munich.

Description
A&M Limited, the Japanese company that manufactures Air Tight electronics and phono cartridges, was founded in 1985 by Atasushi Miura and Masami Ishiguro. Miura, who had worked for Luxman Corporation since the 1950s, was intent on retiring—but when he learned that Luxman intended to commemorate his departure by scaling back their line of tubed audio products, he decided to bring to market his own tubed designs. Thus, in 1987 or so, A&M introduced their first product: the Air Tight ATM-1 amplifier, a 36Wpc push-pull stereo power amp that used two EL34 power pentodes per channel.

Over the years, other designs were added to the Air Tight line, during which time the company's debut amplifier underwent various refinements. Eventually, in 2010, the ATM-1 was replaced by the ATM-1S—the S standing for Special. Among the more notable updates are the new model's user-adjustable biasing system (I'll come back to that in a moment) and its use of solid-state rectification in place of the original's pair of 5AR4 diode tubes.

As stereo tube amps go, the ATM-1S is rather compact: The consumer pays for no more than necessary of Air Tight's steel casework, finished in the company's trademark gray lacquer. A machined aluminum faceplate, anodized to match the rest of the case, is itself no thicker than it needs to be. Inside, the whole of the amplifier is hand-wired, with passive parts aligned, tagboard style, above a solid-copper ground plane. The latter appears to be coated with varnish or lacquer, presumably to discourage corrosion.

Interestingly, the ATM-1S has two pairs of input jacks (RCA): one on the rear panel, another on the front. Adjacent to the latter is a rotary selector switch, for choosing between the input pairs, plus left- and right-channel level controls, the latter fronting a nice pair of Alps potentiometers. In the original ATM-1, the front-mounted input jacks were provided for owners who wished to connect their CD players direct to the amp, without a preamplifier; I believe there may also have been a difference in circuitry between the front and rear input pairs. In the ATM-1S, all four input jacks are connected straight to the selector switch, all are affected by the level controls, and all address the same three stages: a single 12AX7 dual-triode tube for voltage gain, one 12AU7 dual-triode per channel for phase inversion, and a differential pair of EL34 or 6CA7 pentodes operating in class-AB. The rail voltage—a steady, unwavering 460V, which is on the high side for the EL34—appears on both the plates and the screen grids of the output tubes.

The quality of parts used in the ATM-1S is generally excellent, as is the internal build quality. My only criticism is that the center of the top panel of my well-traveled review sample appeared to be sagging a bit under the weight of the transformers. While that speaks well of the quality of the latter—the output transformers are by Hashimoto, while the mains transformer is built in-house—it does, I think, indicate the need for an internal brace of some sort.

Installation and setup
Installation of the Air Tight ATM-1S was as straightforward as it gets for a tubed amplifier, thanks in no small part to good-quality packing that allows the amp to be shipped with its tubes already in place. The making of loudspeaker connections was only slightly puzzling: Air Tight's specifications indicate output impedances of 4 and 8 ohms, and the spare but decent instruction manual advises the user to "select the red terminal between 4 ohms and 8 ohms in accordance with the impedance of your loudspeakers"—yet only a single pair of terminals is supplied for each channel, and I found no provisions, inside or out, for switching. In any event, I heard no indications of a mismatch with either my 16-ohm Altec Valencia or my 10-ohm DeVore Fidelity Orangutan O/96 speakers.

During the ATM-1S's time in my system, I relied solely on my usual combination of Auditorium 23 copper speaker cables and Shindo silver interconnects; the former are relatively affordable, the latter moderately less so. (Apart from a really nice pair of Audio Note interconnects bought used in the 1990s for pennies on the dollar, I own no four-figure cables.) While poking around inside the ATM-1S, I noticed that the ground terminal of its IEC socket was unconnected to either circuitry or chassis. That doesn't necessarily mean the owner won't hear an improvement when using an aftermarket AC cord—yet because the qualities of many such cables pertain to their provisions for grounding, the Air Tight may be a dubious candidate for experimentation. In any event, I maintained my usual practice of ignoring aftermarket AC cords.

Apart from being unpacked, carried to its resting place, and plugged in, the only thing the ATM-1S asks is that its user check and occasionally adjust the bias current for its four EL34 tubes. For this, the amp has a nicely styled ammeter and four individual potentiometers, plus a rotary switch to select among the latter—all mounted on the top panel. This system proved easy to use, the entire procedure taking less than two minutes.

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Footnote 1: From the similarly low-power Lamm Industries ML2.1, driven by an original Fi preamp.