Original French Version
by Jean Giguére
Magazine Son & Image, May-June 2006
If we compare sound reproduction with aviation, their equipment can both be separated into several different categories. There are regional jets which provide comfort and luxury, there are charter flights which get you where you want to go economically, there are antique planes for nostalgia buffs, fighters which combine performance, speed and thrills and then there are the ultralights. These, with their minimal framework and motor of a few horsepower, offer the feeling of flying like a bird. This is the kind of plane to which I would compare triode amplifiers like the one reviewed here. With a handful of components and a few watts of power, they offer reproduction of astonishing transparency and purity. Just as with the ultralight planes, though, this performance doesn't come without some limitations.
A bit of history
A & M Limited is a company situated in the city of Osaka in Japan. It was founded by Atsushi Miura, who worked until 1985 for Luxman Corporation. General manager at his departure, he contributed to the development of products like the MQ-60 and the CL-35, which established the reputation of the brand. At the time he retired the transistor was perceived to be the solution to everyone's problems; however Mr. Miura thought it was too bad that tubes, which are exceptional amplifying devices, should be sidelined. With his associate, the engineer Masami Ishiguro, he developed the first Air Tight amplifier, the ATM-1. More recently, some faithful American customers asked him to make an amplifier which used the famous 300B tube. One of the challenges he took up was the conception of a unique circuit, different from conventional ones which used the 300B.
300B, 300B… what's so special about the 300B?
In this digital era it may be difficult to understand why the tube, an anachronism dating back to the thirties, should generate so much passion. One of the essential qualities of an amplifying device is linearity. A linear device, like a pantograph, reproduces a larger-scale, undeformed copy of the original signal. The linearity of the 300B is exceptional considering its power handling. Another of its properties is the maintenance of harmonic shadings at any frequency or power level. At the time of its development, the internal construction of tubes was simple and the filament served as the cathode, so it is a "direct heated cathode" type. Up until the end of the Second World War, nothing equaled the power and the longevity of the 300A and 300B. Who would have thought that a part developed for the talking pictures would find itself, seventy years later, at the heart of many high end audio systems?
The ATC-2 preamplifier
I fell right away for the retro "look" of this first piece; it recalls the best of the Japanese electronics of the late seventies and early eighties. When you touch it and use the controls, the unit suggests quality and inspires confidence. Usually, the kind of gear we call "purist" has no more than three inputs, a volume control and a power switch. That's not the case with this preamp, whose owner will get a great deal of flexibility in terms of connection and control. It has seven line level entries, including two tape loops. This means that, along with all the possible sources, the modern user can connect a digital recorder like the Marantz PMD560 (which uses a Compact Flash card to memorize music in PCM or compressed format). There is also a muting control, a mono/stereo switch and a balance control. You have to admit that's pretty complete for a minimal-circuit "purist" high-end component.
On the back panel there are two rows of sturdy, high-quality Cinch-branded connectors which will have no trouble accepting the most exotic interconnect cable. Among them are two sets of outputs, convenient for biamping, or for connecting a headphone amplifier; there is no headphone jack on the ATC-2. For phono, Air Tight offers the vinyl lover the ATE-1, an external phono stage of the same size and appearance as the preamplifier. Its circuit uses three 12AX7 tubes and it is driven from a dedicated power connector on the preamp.
Inside the unit, the power supply provides six constant-voltage sources. It is a "hybrid" type, to use the term in fashion. High voltage is rectified by a 6X4 tube while semiconductors on two circuit boards look after the rest. To my disappointment, one detail clashed, in such a meticulous construction. The ground terminal of the IEC C14 AC power input is connected to the chassis by a wire soldered to the fuse holder screw. This does not conform to Canadian standards. The audio circuit is all-tube, obviously, and uses only thirteen passive components per channel. These are PCB-mounted. The audio circuit is made up of two amplification stages and uses a Sovtek 12AX7 tube and two JJ 12AU7s. These last two are covered with cylindrical RF shields.
The amplifier
Next comes the amp, starring the famous 300B triodes, the most famous being the Western Electrics, of course. Our ATM-300 came with Electro-Harmonix tubes. They are placed in full view at the front of the chassis and their sockets are elegantly ringed with brass. On the chassis front is a dial which lights up when you push the "Bias test" button. Its needle indicates whether each of the 300Bs is correctly biased. However if a tube does turn out to be wrongly biased there's nothing you need to do; biasing is automatic and requires no adjustments. In use, then, the dial's function is to indicate the condition of the output tubes and if they should be replaced. The last two controls are input attenuators, usable as volume controls if you connect your CD player directly to the amp. I was happy to have them as a means of adapting the preamp's output to the amplifier's input sensitivity. What's more, the ATM-300's input sensitivity varies according to another user-adjustable setting. This is a three-position control which adjusts the damping of the speakers' back-EMF. In position 1, the damping factor is 2.6 and sensitivity is 230 mV. In position 3 damping is a bit higher, at 6.4, and sensitivity drops to 450 mV. Two massive custom-made Tamura output transformers dominate the back left of the chassis. Behind them are two sets of WBT posts for speaker cables. The output impedance is set at 8 ohms. The output transformers have taps for 4 and 16 ohms but if you need them, the services of a technician will be required to resolder the connections.
Inside the ATM-300, all parts are point-to-point soldered. Amplification is in three stages. The first two use twin triode tubes connected in parallel. The signal goes to a 12AU7A and then to a 12BH7A, which drives a 300B running single-ended in pure class A. A 5U4G tube rectifies input voltage, which is then filtered by a coil and capacitor circuit. The tube filaments are fed by direct current to keep noise low. A thick copper plate covers the chassis bottom and protects the circuitry from interference.
A sensitive question…
Since the amplifier's rated power is a thundering eight watts per channel, speaker selection is critical. In fact it is their sensitivity which needs to be right. With this amplifier the best match would be a speaker with a sensitivity of 90 dB/watt/metre or higher. Two metres from such speakers, sound pressure should come up to 100-decibel peaks without distortion. This is an appropriate level for acoustic music, and symphonic music in a concert hall rarely goes over it. The system would be running at its upper limit at that level, though, and this is why speakers more efficient than the average are preferable with class A tube amplifiers.
Listening
Among the highly efficient speakers available today, there is one which has been on the market since 1946! Designed on the horn principle, the Klipschorn has a sensitivity of 105 decibels. I was offered access to a brand new pair just installed in Montreal, and I jumped at the chance; so for the listening test of our Air Tight set we will be using these high fidelity legends. I first listened to a classic of the audiophile repertoire in order to find the best possible setting for damping: James Newton Howard and Friends, on Sheffield Lab. It had life and impact. Listening to the kick drum, position 2 of the damping control seemed to provide the most natural effect.
As disc followed disc, the transparency of the system became more and more evident. Music had clarity and the high frequencies were reproduced with unusual precision and refinement. I had had some doubts about this before we started, since we were using horn speakers, but my doubts turned out to be unfounded. The amplifiers also had very high resolution and the kind of articulation that can be tiring when you listen to transistorized gear. Not here.
With this setup and choice of albums, it was World Beat that turned me on the most. Groove alla turca, with Natasha Atlas singing, was especially interesting. I had first heard her Middle-Eastern-flavoured voice on Jean-Michel Jarre's Métamorphose. Groove alla turca is a blend of traditional Turkish music and funk. The seventh track starts with a bass solo and percussion accompaniment. The bass line was detailed and I could follow the phrasing with ease. The tam-tams in the background were distinct, lively and energetic. Another good moment was Ali Farka Toure and Ry Cooder's African blues-styled Talking Timbuktu. On Ai Du, the large, airy soundstage filled the room. Once more the highs were rich and delicate, the cymbals natural. The plaintive voice had striking realism, appearing in the foreground without losing its connection with the musicians behind. The complexity of the many strings was light work for the Air Tights and their power of articulation. Horn speakers are notorious for aggressiveness, but during the test this notion never even crossed my mind. The ATC-2 and ATM-300 are obviously capable of sweetness without obscuring detail. As a final note, it should be mentioned that even with speakers as sensitive as the Klipschorns, these Air Tights are very silent; there was no noise perceptible from the listening position.
Measurements
It was only after the listening sessions were over that I took the measure of the equipment, using an Audio Precision Portable One analyzer. I'll try not to be too technical and stick to the essentials. Looking at the ATC-2 to start with, its gain was 19.5 dB, harmonic distortion was 0.13% and the noise level was -79.7 dBu. Frequency response was very flat. At the extremes, it was -3dB at 20 Hz and -2dB at 20 KHz. However to maintain this bass response the impedance of the preamp's load needs to be over 8.2 kilohms. With such a load the maximum output level is 23.6V RMS at 0.9% THD. The ATC-2 can thus be paired with any amplifier of high input impedance, even DC coupled types, since no DC is present at its outputs. As for the ATM-300, its unique damping control means it is a bit like three amplifiers in one. Curiously, some of its characteristics, including output power, vary according to the damping control setting. At 0.9% THD, output in position 1 is 3.5 watts RMS per channel, at position 2 this figure is 5.8 watts and it is 6.2 watts in position 3. At any of these levels, frequency response is always within 3 dB from 20 Hz to 20 KHz, and is perfectly flat in positions 2 and 3. Clipping started at 7.1 watts and at rated output of 8 watts the distortion level was 7%. Maximum output was obtained at 8 ohms and diminished below this value. This is normal for a unit with an output transformer, and it is often the Achilles heel of a tube amplifier. In the case of the ATC-300, though, there was no downside. Even at full output, frequency response was maintained and bass distortion did not increase. Harmonic distortion was 0.16% at 1 watt, a value typical of normal listening levels.
Still there?
To wrap up, you may have guessed that I fell for the Air Tights' "Made in Japan" quality. The preamp behaves like a normal piece of audio gear. Forget it uses tubes. Its faultless performance and its flexibility mean that it can become the heart of any high-end audio system. The quality of its construction will provide its owner with complete satisfaction for many years. I have to say that the amplifier, though, is not made for everyone. It will appeal to a small circle of audio purists whose unwillingness to compromise means they can accept its limits. The ATM-300 almost needs its own listening room to be happy, where it can take the place of honour enthroned between two large speaker enclosures, well away from tiny fingers and other assaults. Given the investment it represents, I'm sure that those who choose it will welcome it appropriately. It is true that the famous 300B sound has something special and the ATM-300 will reveal it in all its subtlety. The Holy Grail of 300B tubes is the Western Electric type. The ATM-300 is of a quality to do it justice.
Magazine Son & Image May-June 2006