Усилители мощности моно — Air Tight ATM-2211

The Air Tight ATM-2211 is a monaural valve power amplifier built around a single 211 triode, with one 12AX7 and one 12BH7 ahead of it, and stereo use calls for two units. It follows the ATM-211 of 2001 and the push-pull ATM-3211, and Air Tight launched it on the eve of the brand's 35th anniversary as its next single-ended 211 design.

Key specs that matter

Rated output is 32W at under 5% THD, with 500mV input sensitivity for full power and 100k ohm input impedance on both inputs. Frequency response is 20Hz to 20kHz within 1dB, measured at 3.5W. Every chassis offers a single RCA socket plus a single XLR socket, with pin 2 hot on the balanced socket. Power consumption is 250VA, the chassis measures 400 x 350 x 225 mm and it weighs 25.5 kg.

Engineering and use context

Air Tight does not take overall negative feedback from the output transformer secondary. Instead, a large amount of feedback runs from the plate of the 211 back to the first stage, and the operating points of the circuit were set more strictly to raise output. The 211 runs with fixed bias and a DC-heated filament. A large vertical edgewise meter shows plate current and bias, and a dial lets the owner switch between the conventional 211/VT-4C type and vintage 211 valves rated for 75W plate dissipation. High-voltage and low-voltage supplies now use separate power transformers instead of the single combined unit of earlier models. The filter choke is wound in layers without a traditional bobbin to withstand high voltage and is made by hand in Japan, while the output transformer is a high output type from Hashimoto Electric. The main chassis follows the brand's monocoque construction, with a thick pure copper sub-chassis suspended inside to carry the amplifier board and the power supply with its large block capacitor. Screws are fixed by threaded studs pressed into the chassis, so no screw heads show, and the thick aluminium front panel is machined from solid. At switch-on a timer relay applies plate voltage with a delay, protecting the output valve and extending the life of the smoothing capacitors.

Pairing

Separate WBT speaker terminals serve the 8 ohm and 4 ohm taps, and a 16 ohm tap can be fitted at the factory as an option. With its modest output from a single triode, the ATM-2211 suits loudspeakers of reasonable sensitivity, and the XLR input suits preamplifiers with balanced outputs.

Alternatives

Air Tight's ATM-211 is the earlier single 211 monoblock, rated at 22W into 8 ohms.