⌈ When, in 1986, Messrs. ATSUSHI MIURA and MASAMI ISHIGURO founded A&M Limited, which owns the Air Tight brand, they couldn't even suspect that 38 years later the company would still be producing award-winning top tube gear. And so it is. We are testing the latest version of the ATM-2 Plus power amplifier, which can also work without a preamplifier, directly with the signal source. ⌋

IF IT'S JAPAN, IT MUST BE ABOUT TUBES, if it's tubes from Japan, it must be 300B, right? When we talk about this country in terms of audio, we see it maybe not exactly that way, but something like that. We think of it as a place where traditional methods of sound reproduction are cherished in a special way. That's a stereotype, because this is, after all, the country where digital audio recording was born (Denon, 1971), where one of the two inventors of the Compact Disc format was based, where - finally - both DVD and Blu-ray were developed. It's also where some of best-known audio companies came from, such as transistor powerhouses Denon, Technics, Accuphase, Luxman and - smaller but equally important - Esoteric.

But, we should add, it's a stereotype based on strong foundations. Because Japan is indeed home to great tube devices, horn speakers and turntables. And it was there that the tube renaissance took place, when the Kondo OnGaku amplifier prototype was developed in the late 1980s. It was a single-ended triode (SET) design, the production version of which changed the thinking about modern amplifiers.

▌ KT88

ONE COMPANY THAT’S BEEN A PART OF THIS trend is Air Tight, and its latest product is the Plus version of the ATM-2 amplifier. Although it has both 300B and 211 tube amplifiers in single-ended mode in its lineup today, its first product was a pentode-based, push-pull device. The story of the brand started in 1986 with the ATM-1.

This was a stereo power amplifier based on EL34 tubes operating in an ultra-linear push-pull mode; the 300B SET, model ATM-300, was not developed until thirteen years later. In 1988, a more powerful, almost identical-looking version of the ATM-1, based on more powerful tubes, KT-88 beam tetrodes, model ATM-2, was ready. This amplifier was produced until last year, and it is the model that has been offered by this Japanese manufacturer for the longest time in its history.

Testing the ATM-2 in its original version in 2013 (!), I noted that Air Tight is extremely conservative in its tube choices. Like Audio Research, Jadis, or - earlier - McIntosh, it tried to "get the most" out of them. Already at that time, a "boom" was brewing for more powerful versions of KT-88 tubes, allowing up to 120 watts in class AB (Ayon Audio Triton Evo Mono) and as much as 50 watts in class A (Jadis I70), yet Air Tight remained faithful to the double 8s.

LET ME ALSO REMIND YOU that one of the most famous amplifiers using KT88s is the McIntosh MC275 (more → HERE ˻ PL ˺, → HERE ˻ PL ˺, → HERE ˻ PL ˺, → HERE ˻ PL ˺ and → HERE). The Binghamton-based company's now iconic product was introduced in the early 1960s and is still, in successive versions, in the company's catalog.

▌ ATM-2 Plus

WHAT AIR TIGHT SAYS ABOUT ITS NEW amplifier - because it's a brand new circuit with different input tubes, different output circuit, changed components, only using a very similar chassis, is that:

Air Tight released the original ATM-2 in 1988. This KT-88 based Push-Pull power amplifier received honor of many awards and has became our longest running power amplifier model for over 30 years. Today, we are renovating this celebrated model to redefine KT-88 push-pull amplifier with Air Tight`s 30 years of technical expertise.

ATM-2 Plus is a completely new design while keeping the legendary AirTight style & outlooks. All circuity including 1st& 2nd amplification and internal constructions are renovated. Unlike a time when we released the original ATM-2, vintage KT-88 tubes are almost impossible to obtain, and quality of modern tubes are different. Our focus was to lower plate voltage of the tube while keeping output power for longer tube life as well as stable operation of the amplifier.

⸜ ATM-2Plus. Stereo power amplifier, →www.AIRTIGHT-ANM.com, accessed: 22.04.2024.

The ATM-2 Plus is a classic tube amplifier with solid-state power supply. The input stage features three dual triodes, 12AX7 EH and 6CG7/6FQ7 models from Electro-Harmonix, made - unfortunately - in Russia. The output uses a pair of KT88s per channel, in an ultralinear push-pull mode in class AB. They, too, come from Russia. All tubes are selected, broken-in and paired, as evidenced by the printing on their bulbs: "Air Tight Platinum Selection." We will get 70 watts per channel from them. That is, 10 watts less than with the ATM-2 model. As we read in the press materials above, the idea was to make the tubes last much longer than in the basic model.

The unit is nominally a power amplifier and appears under this name in the manufacturer's catalog. The signal is fed from the preamplifier to a pair of sockets on the rear panel, and output to the speakers via speaker sockets - separately from the 4 and 8 ohm taps; if desired, it is possible to solder the cables at the output of the transformer for a 16 ohm load. But, like the ATM-2, it is equipped with a second line input with attenuators at the input. So it can, if it has to, work as an integrated amplifier, with two inputs, direct and ariable. The knobs are delightful, but they work separately, so you have to turn them separately. Well, and there is no remote control.

The amplifier is damn heavy, it's 32 kilograms in a small body, it features powerful transformers and is wonderfully made. It makes a man all happy inside when he looks at it.

▌ SOUND

HOW WE LISTENED • The Air Tight ATM-2 Plus was compared to my reference, the Soulution 710 solid-state power amp, and drove Harbeth M40.1 speakers. Preamplification was handled by an Ayon Audio Spheris III tube preamplifier. I also listened separately to a Leben CS-600X amplifier, running EL34 tubes.

The signal between the preamplifier and power amplifiers in both cases was carried by Crystal Cable Absolute Dream interconnect. The tested amplifier was powered by Harmonix X-DC350M2R Improved-Version cable, and the signal to the speakers was provided by Crystal Cable Da Vinci speaker cable (test → HERE ˻ PL ˺). The signal source was an Ayon Audio CD-35 HF Edition SACD player.

I conducted a separate test using the adjustable input, that is, without the use of external preamplifier, straight from the output of the Ayon Audio player, set as unregulated ("Max"). The front panel RCA jacks are recessed in the chassis, so they require cables with narrow plugs. The Crystal Cable Absolute Dream interconnects meet this requirement, but proved too short to connect the player and amplifier. So I had to turn the Air Tight amplifier around to shorten the distance between the player's output and the amplifier's input.

» ALBUMS USED FOR THE TEST ⸜ a selection

⸜ THE MODERN JAZZ QUARTET, The Sheriff, Atlantic/Warner Music Japan WPCR-25129, „Atlantic 60th”, CD ⸜ 1964/2007.
⸜ THE DOORS, The Best of The Doors, Elektra/Audio Fidelity AFZ5 206, Limited Edition #0115, SACD/CD ⸜ 2015
⸜ BILL EVANS TRIO, Sunday at the Village Vanguard, Riverside Records/Craft Recordings/Universal Classics & Jazz UCGO-9058, „Bill Evans Remaster Series”, SHM-SACD ⸜ 1961/2023.
⸜ THE JIMIE HENDRIX EXPERIENCE Electric Ladyland, Reprise Records/Sony Records Int'l SICP 30823, BSCD2 ⸜ 1968/2015.
⸜ LAURIE ANDERSON, Homeland, Nonesuch Records 524055-2, CD ⸜ 2009.
⸜ ROBERT KANAAN, Music Therapy: Whispers of the Five Oceans, Soliton SL 821-2, Gold Master CD-R ⸜ 2018.
⸜ ENYA, Enya, BBC Entertainment BBC CD 605, CD (1987).

»«

OHH..., the KT-88... EVEN IF IT'S HARD TO BELIEVE, but it's now a "vintage" tube, and in the double sense of the word. EL-34 pentodes have a similar status, and only companies that know how to handle it reach for them. But the EL34 has always been one and the same, and its more powerful versions have never gained as much popularity. With the KT-88 the case is different, because its more powerful cousins are available, and they are the object of most manufacturers affection nowadays, even those who used to build their amplifiers with the "eight-eight". That's why I refer to them as "double vintage."

And yet it is an excellent tube. High power is useful, even in small rooms and with high-efficiency speakers. That's when power transfer is best, and that's when the sound is usually the most natural, unforced. But, it seems, a power output of 70W from two KT-88 tubes per channel is perfectly adequate, especially since this is, after all, quite a bit more than the 36 watts obtainable from EL34 tubes in the → ATM-1 ˻ PL ˺, 32 watts from the → LEBEN CS-600X or 34 watts from the → KONDO OVERTURE II, and these are, after all, in my opinion, some of the coolest EL34 tube amplifiers in audio history.

The problem with KT-88s is that everyone seems to know them. So devices were created that were either too warm/slow or too bright, which had nothing to do with the sweetness and naturalness of the EL34. Air Tight ATM-2 was an exception, and the Plus version is better than the original in this respect (if my memory serves, of course). This manufacturer's new amplifier listened to in the High Fidelity reference system, replacing here not only the Soulution 710 power amplifier, but also the long parade of other units I tested, it played a perfectly balanced and energetic sound. And at the same time a sound with a beautiful balance between openness and sweetness, energy and relaxation, bandwidth equalization and saturation.

ATM-2 Plus shows sound events effortlessly, with the lightness of a man who knows he has the power (money, background, education, etc.). Listening to THE MODERN JAZZ QUARTET'S The Sheriff, released in 2007 by Warner Music's Japanese division in the "Atlantic 60th" series, I „saw” a dense yet fast sound image before my eyes. The tape noise, quite high at times, was shown not as a hiss, but a broadband "background", which is how it is actually heard. Which immediately "set" my perception of this amplifier. This was because it meant a rich, balanced, presentation with high energy, but also with saturation.

Because it's hard to single out any one part of range in this sound, they are all synchronized and work together like clockwork. But it's also not that it's bland playing. It is not! That's the thing I like so much about the best products from Japan. On the one hand, they are technically perfect, and on the other hand, you can feel the human touch in them. The same is true here as well. So I had both strong, dense, heavy brass, but also saturated double bass. And it was the latter that caught my attention, as it was not only clean and selective, but also powerful and dense.

The amplifier seems to "not notice" that it has to drive something. It does it effortlessly, but not because it "doesn’t care", but because it "embraces" everything and has nothing to fear. The stereo panorama of this album, artifactual because with instruments assigned to the left and right channels, was with the ATM-2 Plus significantly more natural than usual. And that's because the instruments took up a lot of space, filling the space with their presence. It was not "left-nothing-right," but "left" and "right," meeting in the middle.

Perhaps that's why the ˻ 5 ˺ Riders on the Storm by THE DOORS, as included on the compilation The Best of The Doors, remastered in 2015 by Steve Hoffman and released on SACD by American label Audio Fidelity, sounded so great. I was able to play this track very, very loud, and it was not tiring. Excellent, "rolling" bass, heard in the left channel, drums more to the right, and on axis, and all the time, underneath the music, a storm. That's where this track begins, and although it's almost monophonic, not counting the reverb, Air Tight filled the space with it in a focused, powerful way.

As we remember, Morrison's vocals are doubled there and under the main one there is also his strong whisper. You can always hear it, but in this case it was not only clear, but also shifted on the stage, as if it had a different reverb. You could also hear something that usually escapes you, which is that it's not just a quiet whisper and that the singer had to make an effort to make it sound this way, and not other way. You can literally "see" him flexing his muscles. As it seems to me, this is a continuation of the advantages we get from the ATM-2 Plus along with the bass being shown this particular way. It's the foundation on which both bass instruments, space and instruments bodies are laid.

Unlike a large part of amplifiers featuring EL34, and 300B tubes, the ATM-2 Plus does not warm up the sound. That is, it seems sweeter than most push-pull amplifiers, but it is not noticeably warm, as with - for example - Audio Research or Jadis devices. Indeed, when I listened to another track from the aforementioned album, ˻ 6 ˺ Touch Me, when the Philadelphia crowd screamed, which was captured in the opening track ˻ 1 ˺ What Do you Love, it was open and resonant sound. No rounding or smoothing was noticeable.

And yet the whole thing was interestingly arranged internally. It was a holistic message, not a point-to-point one. That is to say, I didn't pay attention to the details, the particulars, because the energy of the presentation came to me immediately from all points of the panorama. But when the bass entered in the song in question, it was clear, contoured, powerful with the tested amplifier. So was the organ in the left channel and the guitar (played like a slide guitar) in the right channel. It was a fast, dynamic, explosive sound. But also saturated, which gave a big picture and a touch of aggression, because that's what it's all about.

And, importantly, I got a very similar, even identical, set of qualities when listening to another concert, only nine years older, this time from the small Village Vanguard club: the BILL EVANS TRIO and Sunday at the Village Vanguard. I got, of course, a different sound, because it's a different recording and different music. But the energy, the broadband playing, the dynamics, the slight emphasis - upon reflection I came to that conclusion - on the midrange, all made the three instruments that were on stage that day, June 25th 1961, very tangible, very natural and very enjoyable.

The most important feature of the tested amplifier seems to me to be the combination of vitality and attentiveness. High power results in the former, because that's how it works, but it takes a great deal of skill and effort to produce a such a high quality sound as in this case. A sound that, on the one hand, almost "jumped" out of my powerful Harbeth M40.1s, and on the other, was under control all the time. This combination of high volume and saturation resulted in a beautiful presentation, as it was both natural and neutral - a rare thing.

» DIRECT CD INPUT • Eliminating the active preamplifier from the audio signal’s path and leaving only the attenuator in it changes the sound, that much is clear. Although I am a declared proponent of preamplifiers I approached Air Tight's proposal with an open mind, trying not to forcefully seek what I know to be true from so many other comparisons.

The first thing that I could hear was the moving sound away from the foreground. It was still tangible playing, the ATM-2 Plus just plays like that. But the energy of the sound was located a bit further away, no longer in the membranes themselves, but just behind them. Shaped in this way, the presentation was more "carrying". That is, it seemed less focused on the sound sources and more on the atmosphere, the space. Both of these elements were clearer, more unambiguous with the Direct CD input.

The focus I mentioned was also very cool, it was one of Air Tight's inherent features. But also the panorama spread out in front of me was wider and deeper. But less precisely defined. Likewise, so was the sound itself. Perhaps because the bandwidth extremes in this configuration were softer, more pastel. The Ayon's preamplifier tightened them up, both bass and brass, concentrating them more firmly at one point. Its "removal" from the system yielded a more resolving sound, still dense and rich, but no longer as unambiguous.

Also, the crowd at The Doors concert seemed larger than with the preamp, and that's because the space was more spacious and "blown out" without it. But both versions were excellent, it was not a "painful" change, even for me. I think I liked it because without the preamp in the setup nothing was brightened or somehow particularly slimmed down, which is what you get by eliminating the active gain stage from the signal path. Yes, the center of gravity of the sound was set higher than with the Spheris III in the signal’s path, but it wasn't brightened by that. The overall sound was a bit less energetic, the kind of energetic that one associates with weight and mass, although it was still vital and fast.

▌ Summary

AIR TIGHT IS ONE of the most interesting Japanese manufacturers. And it is not, after all, the biggest, nor the best known. Its products are superbly made and sound great. I have a great weakness for the ATM-1 model with EL34 tubes, because I am a fan of these tubes. KT88 beam tetrode tubes make my heart beat faster much less often. But this time - they did.

The ATM-2 Plus performed wonderfully, and this was due to its control over the speakers combined with natural tonality. This is neither a warmed up nor brightened sound, but, I should repeat, a natural one. The amplifier excellently energizes the listening room, but not by playing brightly, but playing in a full, dense, and saturated way. The bass is excellent with it, both in terms of clarity and mass, as are the highs.

What seems most important to me, however, is how the device built up the events in front of me. And it did it excellently, because I got with it a wide and deep panorama with strong sound sources having a very cool timbre. As I say, it's an amplifier that doesn't emphasize any sub-band, it's well-balanced. But it's not bland either, which is why it seems that the midrange is the most important element here. A very cool, extremely pleasant to listen to amplifier with excellent mechanical and electrical design.

▌ DESIGN

» LOOKS • AIR TIGHT ATM-2 Plus IS a power amplifier featuring an adjustable input sensitivity. From the outside, it will be difficult to distinguish it from the earlier version, because both models look, to me, very similar. They are extremely compact devices with a very rigid chassis, in which dark gray elements contrast with silver and dark titanium-colored ones. The only colorful elements are a blue LED, indicating power status, and an orange backlit bias „window”. Even the company's logo badge is now covered in black chrome, rather than being gold-plated. Which, in my opinion, looks very good.

The device sports two line inputs, both with a nominal sensitivity of 500 mV. They are therefore 50% more sensitive than those of the ATM-2, which will allow it to be driven more easily. The sockets on the front are associated with two knobs connected to rotary potentiometers. With them we can set the input sensitivity (volume) separately for the left and right channels, and these are scaled differently than usual (the start and end are in different places than in 99.99% of amplifiers). Next to them there is another knob, which we can use to select which of the two inputs is active: front or rear.

On the right side, we see a circular window with a meter, we can use when adjusting the bias current of the output tubes; this is a fixed bias type amplifier. There is also a mechanical power switch. The backlight of the window has two levels - it is slightly darker when the unit is in daily operation, i.e. not indicating anything; on the other hand, it is brighter when user sets the bias. This is done manually, using four potentiometers, access to which is on the top panel, just in front of the mains transformer.

On the back there are gold-plated speaker connectors, for 4 and 8 ohm speakers, a stereo RCA input, but without the ground terminal present in the ATM-2, and an IEC power inlet. The speaker terminals were manufactured by the American company Charming Music Conductor (CMC). The chassis was made of thick steel sheets, and the bottom (also steel) wall was covered with a thick layer of copper - another thing typical of Japanese manufacturers.

» TUBES • Tubes are placed classically for this type of product, i.e. on the top panel, in front of the transformers, and grouped by channel. The input uses dual 12AX7 triodes, here in the 12AX7-EH version from Electro-Harmonix (previously they were Sovetek's 12AX7LPS). Anyway, all the tubes in this amplifier come, unfortunately, from Russia. This is a big problem for manufacturers around the world, and Air Tight itself mentions it in its materials. By the way, I'll say that there's a chance that in the future we'll get new tubes from a Polish company - Fezz Audio is already researching and looking for sub-suppliers of individual components.

The pre-amplified signal is sent to a second double triode, the 6CG7/6FQ7. This is a triode with considerable gain, the model replacing the 12AU7EH used here previously, as well as the last double triode, the 12AB7. The latter, operating in a cathode follower mode, drives output tubes KT88 beam tetrodes in a push-pull Class AB circuit, in an ultra-linear mode. All tubes have been inspected and selected for performance, hence the presence of the "Air Tight Platinum Selection" inscription and the company logo.

The output transformers were shielded separately; they were made by a Tokyo company → TAMURA SEISAKUSHO. Next to that company's logo, however, you'll find another one, Tamradio, indicating that the company customized the transformers specifically for this amplifier model, presumably by flooding them with vibration-damping and heat-dissipating mass. The power transformer has only one shielding element screwed on top. The transformer plates are exposed and painted black.

» INSIDE • One of the hallmarks of Air Tight amplifiers is point-to-point assembly. That is, one in which there is no circuit board, and the components are soldered directly to each other, or to auxiliary solder points. The latter are simply metal pins, here, however, insulated with a very cool-looking (Bakelite-like) material.

All components in this amplifier are top shelf. These are TKD and Dale resistors, AuriCap XO coupling capacitors, that is from the American company Audience, or the great-looking Axial Capacitors in the power supply. Some of the components are of Japanese origin, such as Alps potentiometers, which regulate input sensitivity, or Cosmos potentiometers, which we set the bias with. All circuits are assembled not directly on a steel chassis, but on additional steel sheets covered, no surprise here, with a thick layer of copper.

Let's also add that the amplifier's power supply is extremely elaborate and has several secondary windings. The anode voltage uses a large choke and capacitors to form a "Pi" type filter - the diodes in the rectifier were chosen for fast switching, that is, low noise (Shottky diodes). The input tubes use another capacitor bank, as does the rectified heater voltage.

Although it's a point-to-point assembly, a lot of interconnect cables were used inside the ATM-2 Plus. The company reports that they were all made of high-purity copper. There are three connections from the output transformers to the speaker jacks - one for ground, one for a 4 ohm load and one for an 8 ohm load. However, there is another tapping, unsoldered, for 16 ohm - it can be very easily connected to any of the sockets if necessary. The transformers themselves, as I said, are huge. They were made using special cores called Orient, have a power output of more than 100 watts and minimal losses of 0.26 dB.

And one more thing - if you want to be absolutely purist, even if you have an external preamplifier or use the volume control on your CD or file player, it's worth using the front pair of RCA jacks, because they are connected to the potentiometers and these to the input tubes via short, unshielded cables; much longer, shielded interconnects run to the rear jacks.

I'll repeat what I wrote when I tested the ATM-2 model, because I can find no other words to summarize the construction of the new Plus model: "This is a beautiful example of a modern approach to the classics - large power supply, copper, high-end components, precise assembly. It is simply beautiful, even stunning". :) ●

▌ Technical specifications (according to the manufacturer)

Nominal output: 2 x 70 W (TDH <5%)
Frequency range: 20 Hz - 60 kHz (-1 dB/1 W) ⸜ 20 Hz – 50 kHz (-1 dB/50 W)
Output loading: low 4 Ω, high 8 Ω (optionally 16 Ω)
Tube complement: 2AX7 x 2, 6CG7/6FQ7 x 4, KT88 x 4 (or 6550), Fixed Bias
Input sensitivity: 500 mV (32 W)
Input impedance: 100 kΩ
Power consumption: 250 W
Dimensions (W x D x H): 415 x 380 x 230 mm
Weight: 33 kg

THIS TEST HAS BEEN DESIGNED ACCORDING TO THE GUIDELINES adopted by the Association of International Audiophile Publications, an international audio press association concerned with ethical and professional standards in our industry, of which HIGH FIDELITY is a founding member. More about the association and its constituent titles → HERE.

→ www.AIAP-online.org