Air Tight ATM-300R power amplifier

To audio designers in Japan and elsewhere, the single-ended, 300B-tubed amplifier is like a haiku: an art form defined by both its prescribed limitations and the potential such restraint offers for artistic expression. Here, the only hard-and-fast rule is a simple one: output devices are limited to one 300B directly heated triode tube per channel. From there, it’s a blank slate: Do you want AC or DC on the output-tube heaters? Tube or solid-state rectification? Low or high gain? Fixed or cathode bias? New parts, vintage parts, or a mix of both? Triode or pentode tubes as drivers? Capacitors or transformers—or nothing at all—between the plates of the driver tubes and the grids of the output tubes?

I suppose you could even drive your 300Bs with transistors, power the amp with an outboard switch-mode supply, add user-adjustable negative feedback that you can control from your telephone, or build it into a carbon-fiber box topped with ostrich leather—all as unthinkable as making a martini with vodka instead of gin.

I’ve never heard a single-ended 300B amp that I disliked. That said, these days my favorite examples are the ones whose sound steers clear of the excessive sweetness and thickness that some people associate with the genre. Such preconceptions are not groundless: In the mid-1990s, when I tried my first single-ended 300B amp, a Cary 300SEI integrated, the trend among manufacturers and DIY hobbyists was toward passive parts and circuit designs known for compounding rather than offsetting the 300B tube’s inherent warmth (footnote 1). In the 1990s, at least in the US, it was difficult to find a single-ended 300B amplifier that sounded as clear and as musically precise—let alone as emotionally compelling—as the Air Tight ATM-300R. But I’m getting ahead of myself . . .

The ATM-300R ($16,995 with Electro-Harmonix 300B output tubes, $15,995 without) is the latest incarnation of a design that made its commercial debut in 1999, as the ATM-300—then as now, a single-ended amplifier that uses one 300B directly heated triode per channel to produce 8Wpc. The third product in this lineage—Air Tight’s second single-ended 300B amp, a limited-edition product called the ATM-300 Anniversary, was issued in 2016—the ATM-300R improves on its forebears in various ways. Perhaps the most significant of these is the change to a hand-wound, paper-wrapped choke coil for the power supply, in tandem with a hand-wound, paper-wrapped mains transformer—both designed and made in-house. According to Air Tight’s Yutaka “Jack” Miura, son of company founder Atsushi Miura, the new iron results not only in better sound but in slightly higher output: 9Wpc for the 100V version sold in Japan. (For the 120V US version Air Tight retains the 8Wpc rating, apparently motivated by prudence.) Jack Miura also points to the new model’s redesigned power supply, and its use of a brand-new output-transformer design from Japanese transformer specialists Tamura.

One thing the first ATM-300 had that the ATM-300R lacks: a switch for selecting between zero negative feedback and 3 or 6dB of same. That said, the omission sets the stage for something more interesting: The ATM-300R has a fixed (unspecified) amount of feedback, yet in each channel, that loop’s output tap is on the primary side of the output transformer, not the secondary. As Jack Miura explained in an e-mail, “Taking NFB from the primary side . . . was used on very old amplifiers such as Western Electric, but not on recent amplifiers.” He described the drawback of doing so as the need for a transformer capable of very stable performance—that and the fact that primary-side feedback doesn’t produce overall signal/noise specs as impressive as those associated with the traditional approach. But Miura said that Air Tight’s approach “opens-up” and enlarges the soundstage, and offers “crystal clear highs while keeping rich mids and tight bass, unlike the ‘narrow shoulder’ [sound] (this is how my father describes typical 300B tube amplifiers with wobbling bass sound and no sparkling highs).”

Other design distinctions of the ATM-300R: It’s a three-stage design, using 12AU7 and 12BH7 dual-triode tubes—one each per channel—as voltage-gain stages and drivers, respectively. The high-voltage rail is rectified by a single 5U4BG two-part diode tube, supplemented with a slow-start circuit to prolong tube life. And the ATM-300R is an auto-bias (aka cathode-bias) design; an illuminated bias meter on the amp’s front panel is intended not to aid in calibration but to provide confirmation that the 300Bs are operating within their intended range. The ATM-300R was designed by Y. Hayashiguchi and K. Hamada under the direction of Atsushi Miura.

Like all other Air Tight amps, the ATM-300R is built into an enclosure made mostly of bent and welded steel, to which a machined-aluminum front panel is attached with hidden fasteners. Inside, running nearly the amp’s full width, is a solid-copper plate to which the signal tubes are fastened, and that serves as a signal ground plane. Except for a small, square circuit board to which some power-supply capacitors are fastened, the ATM-300R is entirely hand-wired, point to point—impeccably done. Insofar as I could tell, all component parts are contemporary, and include Sprague capacitors, Dale wire-wound resistors, and Alps potentiometers. With its dark-gray enamel finish and that unabashedly cool-looking bias meter, the very solid Air Tight amp—at 54 lb, it’s far heavier than its 16.9″ by 10.8″ footprint had led me to expect—was one of the most attractive amplifiers I’ve ever had in my home.

Installation and setup
Apart from its pushbutton power switch, the ATM-300R has only three user controls: separate Attenuation knobs for the left and right channels, and an identical knob, labeled Bias Tests, with three positions: Operate, L ch, and R ch. Especially with a high-gain amplifier, I love having the ability to knock down that gain at the amp, as needed, and these pots let me adjust the channel balance in a day and age when few preamplifiers come with Balance knobs.

Although its Tamura output transformers are wound with three secondary taps—for 4, 8, and 16 ohms—the ATM-300R’s left- and right-channel outputs offer a choice of two loudspeaker loads, labeled Low and High; the buyer specifies ahead of time which two of the three secondaries should be made active. (Although this can be changed in the field, it’s not a job for the consumer, given how easily transformer windings can be ruined by poor soldering techniques.) On my review sample, the 4 ohm secondary was connected to the Low output, the 8 ohm secondary to High. I relied on the latter for all of my listening.

All of the tubes that came with my review sample were made by Electro-Harmonix. For $19,995, you can have an ATM-300R with Takatsuki 300B tubes, a combination I haven’t heard.

The Air Tight ATM-300R spent a lot of time in my system chained to a chore no sane man or woman would think fair: playing, four times in a row, the historic recording by Georg Solti, the Vienna Philharmonic, and a cast of soloists headed by Kirsten Flagstad and George London, of Wagner’s Das Rheingold (3 LPs, London OSA 1309). As it happens, a recent thread on Facebook had prodded me to re-read producer John Culshaw’s book Ring Resounding, about his production of that historic first complete recording of Rheingold and, ultimately, Wagner’s entire Der Ring des Nibelungen. The Air Tight amp reproduced it so gloriously well that, for the next couple of days, one or the other of the set’s three LPs was always on my turntable.

Footnote 1: And that Cary had as its standard output tubes Cetron 300Bs—a tube once plentiful and underappreciated, now rare and lamented.

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