Усилители мощности стерео — Air Tight ATM-300R

The ATM-300R is a two-channel 300B single-ended triode amplifier from Air Tight, the brand of A&M Limited in Japan, and the letter R stands for Reference. The line began with the ATM-300 in 1999, which used WE300B valves and became closely tied to the brand's identity. A limited ATM-300 Anniversary model marking 30 years of the company followed in 2016, and after a period of intensive development the ATM-300R arrived in 2018, described by the maker as the destination of its 300B amplifier work.

Key specs that matter

Rated power is 9 W per channel at under 10 percent THD. At 1 kHz, 1 W and 8 ohm, distortion stays below 1 percent and the damping factor is 7. Frequency response extends from 30 Hz to 40 kHz within minus 1 dB at 1 W. Input sensitivity is 290 mV for the full 9 W. The chassis is 430 mm wide, 275 mm deep and 245 mm high, and the amplifier weighs 24.5 kg. It is offered without power valves, with Electro-Harmonix 300B tubes, or with Takatsuki 300B tubes, the latter matched to within 1 percent.

Engineering and use context

The tube line-up is two 300B output triodes, two 12AU7, two 12BH7 and a single 5U4GB rectifier. The circuit has three triode stages, and the 300B in each channel runs self-biased in class A with its heater fed from DC to lower hum. Tamura output transformers provide secondary taps for 4, 8 and 16 ohm loads. A slow-start circuit protects the 300B valves when the unit is switched on, and a meter lets the owner monitor output tube current. Wiring is point to point on a pure copper sub-chassis board, with a copper-plated chassis used to reduce magnetic distortion. The power transformer and choke use bobbin-less interlaminar construction and are impregnated with lacquer under vacuum to limit vibration. Film coupling capacitors and wirewound resistors are fitted, and the unit also carries a damping selector and an attenuator on a direct CD input. Compared with the Anniversary model, the R version gained newly designed transformers and a higher power rating.

Pairing

With such modest power, the maker states that high-efficiency speakers are needed, which in practice means horn designs or wide-band drivers rather than low-sensitivity multi-way boxes. The choice of three output taps allows the transformer to match the speaker's nominal impedance. Because a normal line-level signal drives it to full output, a CD player or DAC can feed the attenuated direct input without a preamplifier.

Alternatives

The Air Tight ATM-1 is a stereo push-pull model from the same maker using four EL34 pentodes for 35 W per channel.