Усилители мощности стерео — Air Tight ATM-2Plus

The ATM-2Plus is a KT88 push-pull stereo power amplifier from Air Tight, the Japanese manufacturer founded in 1986 by Atsushi Miura. It revives the ATM-2 of 1988, which became Air Tight's longest-running model, with a completely new internal design while keeping the original's appearance and proportions. The circuit is hand-wired throughout.

Key specs that matter

Output is 70 W per channel into 8 ohms from KT88 or 6550 output tubes in push-pull. The driver stages use 6CG7 or 6FQ7 double triodes in place of the original's 12AU7, and the input stage uses 12AX7. Frequency response is 10 Hz to 60 kHz at -1 dB and 1 W, and input impedance is 100 kohm. Output transformer taps serve 4 and 8 ohm loads, with 16 ohm available as an option. The chassis measures 415 x 380 x 230 mm and weighs 33 kg.

Engineering and use context

Air Tight redesigned every stage, from the first and second amplification stages to the internal layout. Plate voltage on the KT88s was lowered while output was maintained, because modern KT88 production differs from the vintage tubes available when the ATM-2 appeared, and a lower operating point gives longer tube life and more stable operation. The 6CG7 and 6FQ7 drivers deliver wider signal swing than the 12AU7, with lower distortion, which suits the needs of the KT88 output stage. The negative feedback loop was redesigned to give a damping factor appropriate to modern speakers. Output transformers are new Tamura designs of 100 W class, larger than the amplifier's rating, to handle low frequencies with margin. The power transformer is custom-made, hand-wound and paper-wrapped by a Japanese craftsman. The new chassis hides its fasteners with stud bosses and provides firm grounding of the frame. Fast-acting fuses protect the amplifier if an output tube fails. Level controls set the input gain, and with them fully open about 1 V of input is enough to reach rated power. Bias is adjusted manually, and a bias tool is supplied. The standard tube set comprises four KT88, four 6FQ7 and two 12AX7.

Pairing

The ATM-2Plus suits speakers of reasonably high sensitivity with an even impedance curve, and its high input impedance is an easy load for valve preamplifiers such as Air Tight's ATC-5. The heavy chassis needs a sturdy shelf and good ventilation around the tubes.

Alternatives

Air Tight's ATM-1S is the smaller EL34 push-pull stereo power amplifier of the range.