Усилители мощности моно — Air Tight ATM-3211

The ATM-3211 is a valve mono power amplifier produced by A&M Limited of Osaka, Japan, and sold under the Air Tight name. It is built around a pair of 211 triodes in push-pull, a step up in output from the single-ended ATM-211 that Air Tight introduced in 2001 to mark the new millennium, and the company says the expertise gained on this model later fed into the redesigned ATM-2211. The ATM-3211 took five years of development before its release, and its American list price is USD 99,975 for a pair.

Key specs that matter

Rated output is 120 W into 6 ohm at 2 percent THD. Frequency response is 10 Hz to 50 kHz within plus or minus 1 dB, with distortion of 0.06 percent at 1 kHz and 30 W. Input sensitivity is 1 V and input impedance 100 kohm. The damping factor is 6, measured at 1 kHz and 1 W. Each chassis measures 400 x 405 x 450 mm and weighs about 44 kg.

Engineering and use context

Every monoblock holds one 12AX7, three 12BH7 and two 211 output triodes. In the input and driver section, the 12AX7 works with two of the 12BH7 tubes, and the second of these operates as a cathode follower so the grids of the big 211s are driven from a low source impedance. Two large Tamura output transformers are mounted on top of the chassis and run without mechanical hum in use. The output transformer offers a second tap for 16 ohm loads besides the standard one. A volume attenuator is located at the rear, which allows the amplifier to be fed from a source with fixed output when needed. The front panel is thick aluminium, and the chassis carries an enamel finish. With one channel per chassis, the power supply and output stage serving the left speaker are kept fully separate from those serving the right. Selecting the tap that matches the nominal impedance of the speaker lets the 211 stage see its intended load.

Pairing

The low damping factor means the output impedance is high enough to interact with a speaker's impedance curve, so loudspeakers with a fairly flat impedance and no deep dips suit it better than complex multi-way designs that fall to very low values. The input sensitivity is easy to meet from almost any preamplifier, and the higher-impedance tap gives a better match for vintage or horn-loaded speakers with higher nominal impedance. Two chassis of this mass need a sturdy rack or floor platform with space around the valves for ventilation.

Alternatives

The Air Tight ATM-2211 is a single-ended monoblock from the same maker that uses one 211 triode to deliver 32 W.