Tube Power Amp Reviews

Closing his review of the AirTight ATM-1S amplifier in the November 2014 Stereophile, Art Dudley wrote, "The AirTight ATM-1S is right up there with the best I've heard: the artisanal, the small, the artistically sensitive, the colorful and characterful, the smart. The AirTight is as serious an EL34 amplifier as I've heard, offering texture, color, poise, musical drive, and rightness, all in good proportion to one another. It also looks nice and represents at least decent if not outrageously good value for the money."

Art's ATM-1S was a step in a long succession of tube amplifiers produced by AirTight since its 1986 founding. First came the original ATM-1; AirTight has also produced the ATM-2 (which used KT88 tubes), the ATM-4 (6L6GCs), the 1S that Art reviewed (EL34s; the S stood for "special"), the ATM-2 Plus, and now the ATM-1 2024 Edition ($14,975). Over 38 years, AirTight has also unleashed a minibarrage of phono cartridges, phono preamplifiers, preamps—and amplifiers both mono and stereo.

Known informally as the ATM-1E—the E stands for "Edition"—the ATM-1 2024 edition power amplifier debuted at High End Munich 2024. Weighing 48lb and employing four New Sensor EL34 tubes (branded "AirTight") in a push-pull configuration, it delivers 35Wpc of class-AB power into an 8 ohm load. How does the 1E differ from the 1S Art Dudley reviewed? Apparently it differs a lot.

Description
I once shook hands with Bill Clinton at Bloomingdale's. His slick white hair and bulbous red face recalled Santa Claus without a beard—but in retrospect, that encounter feels pedestrian as a puddle. Even supermodel Claudia Schiffer's smile at the Soho Grand, moments after I interviewed Massive Attack, paled in comparison to the thrill of unboxing the ATM-1E. Its EL34 tubes and trademark gray finish filled me with awe and desire.

I promise, though, to work hard to set aside this predisposition in the hope of achieving a modicum of objectivity in this review. Readers can decide for themselves whether I succeed.

"There are no common things between the ATM-1E and ATM-1S except that both models are EL34 push-pull amplifiers," wrote Yutaka "Jack" Miura, managing director of A&M Limited (AirTight's parent company), in an email. Jack is the son of company founder Atsushi Miura. "The biggest difference is how each amplifier was designed. The ATM-1S was designed in 2006 by our old design team. The ATM-1E was designed 2022–2024 by our new design team of Yoshihiro Hayashiguchi (product designer) and Kiyoshi Hamada (circuit designer), who have designed all our products since 2015. All our current models except the ATC-3 preamplifier were designed by the new team, and sound of the ATM-1E aligns with our recent product lineup."

Another similarity in the two amplifiers is the size—but that's about as far as it goes. The ATM-1S employed Hashimoto transormers; the ATM-1E uses Tamura transformers, including the Tamura choke coil located ahead of the Tamura power transformer. While both amps use EL34 power tubes, the ATM-1S used one 12AX7 and two 12AU7s; the ATM-1E works one 12AT7 (for voltage gain) and two 6CG7 tubes (for phase inversion). Like the ATM-1S, the ATM-1E has a meter that allows the user to confirm that the power tubes are running in the correct range.

Also of note, the AirTight ATM-1E frame follows "monocoque construction that combines strength and acoustic benefits," Miura wrote. A lot of people assume that "monocoque" means something like "single shell," which is pretty close to a literal translation. ("Coque" means "hull" or "shell.") Actually, though, it means more than that. "Monocoque construction is a structural design technique ... that uses the external skin of an object to support structural loads, similar to how an eggshell is arranged," is how my AI assistant put it, apparently drawing heavily on Wikipedia. Continuing with the latter: "First used for boats, a true monocoque carries both tensile and compressive forces within the skin and can be recognized by the absence of a load-carrying internal frame."

I asked whether AirTight intended the strict definition or the loose one. The answer from Steve Huntley of US AirTight rep Axiss Audio: the strict one. "There is not an inner frame or even internal bracing to hold the weight of components," he replied. "All of the components, from PCBs to transformers, mount to the steel outer shell rather than to any other frame or bracing. That steel outer shell"—the monocoque—"is the structure that bears all of the load of the amp's components and also the footers on which all of the weight rests."

That monocoque is constructed of 1.6mm-thick carbon steel and painted with an automobile-quality finish. "It's that fully welded construction that we think characterizes the sound of AirTight," Miura explained. "While many amplifier manufacturers try to stop vibration on the chassis, we utilize microresonances generated by tubes, transformers, coils, and circuits to create our sound." Like some loudspeakers. "We call it 'resonance control.'"

"Utilization of the microresonances requires a painstaking process of trial and error," Miura continued. "How you mount the copper ground plane to the chassis will change the sound. If you change the material of the chassis, such as to stainless steel or brass, the sound will change. If you change the material of the bottom plate to copper or aluminum, or change its thickness, or add bracing, again, the sound will change. We've been researching this for the past 38 years, and we think we've found a rule of thumb that balances mechanical strength and a component's sound. We incorporated our know-how into ATM-1E and all other current AirTight models."

As with all previous AirTight amps, the ATM-1E is completely hand-wired point to point, using Chicago-made Belden and Japanese wire. The capacitors and resistors soldered to the copper circuit board are from exclusive brands such as AuriCap, Mundorf, TE Connectivity, Cornel, United Chemi-Con, Unicon, TDK, and KOA. Careful construction and meticulous parts selection are another bedrock of AirTight's technical approach.

Chosen for its grounding and antistatic properties, an extensive copper grounding plane in the ATM-1E also aligns with the "resonance control" methodology. Its extra surface area doesn't just optimize signal integrity; it also minimizes unwanted vibration.

"We feel the copper base board offers a clearer sound character compared to a polymer-based circuit board," Miura said. "We never mount tube sockets on a polymer-based circuit board. Differences in thermal expansion and contractions of polymer and metal will eventually damage the circuit board and cause cracks, and the whole printed circuit board will need to be replaced."

"For our tube sockets, we use a very basic plastic base with a steel-type socket," Miura noted. They're supplied by Japan's Sankyu Technos Co. You might expect to see pricier ceramic versions in an amplifier at this level. "We avoid ceramic bases for tubes, as tolerance to the pin holes are not as accurate as we want. We think this happens due to shrinkage of ceramic during the calcination process. We've experienced ceramic bases not matching the pins of the tubes. We do use a ceramic base socket for the 211 tube in the AirTight ATM-2211 mono, which requires high-voltage insulation."

For those who care, the ATM-1E employs negative feedback. "We cannot give you details about amount of negative feedback we are applying," wrote Miura. "It's not too small but not too big."

The ATM-1E's ½"-thick front panel is made of machined aluminum treated with Alumite, which hardens the aluminum and improves abrasion resistance. The faceplate holds a handful of user controls. Around back, two sets of gold-plated CMC Audio brass binding posts offer 4 ohm and 8 ohm connections; you can order a 16 ohm tap for those old-school high-impedance speakers. A single pair of RCA inputs and an IEC receptacle complete the back panel. The custom-made feet—four of them—are made of standard rubber.