Усилители для наушников — Stax SRM-700T

The SRM-700T is a hybrid driver unit for Stax electrostatic earspeakers that places a vacuum tube output stage behind a FET input stage, and it launched in 2020 together with the solid-state SRM-700S in an identical chassis. Stax builds it in Japan and lists it at JPY 330,000 including tax, while the US price is USD 3,400.

Key specs that matter

The output stage runs two 6SN7 GT tubes supplied by JJ Electronic, and maximum output is 340 V rms at 1 kHz. Gain is 60 dB, and harmonic distortion measures 0.01% or less at 1 kHz with 100 V rms delivered. Response is flat from DC to 100 kHz when driving one SR-009S. Bias voltage is Pro 580 V. The RCA input presents 50 kohm, and the XLR input 50 kohm on each leg. Consumption is 54 W. The case is 240 mm wide, 103 mm high and 393 mm deep, and the unit weighs 5.7 kg. The rated operating range is 0 to 35 degrees C with humidity under 90% and no condensation.

Engineering and use context

Stax describes the 6SN7 as the original form of the 6FQ7 used in its smaller tube drivers, and chose it for a simple, powerful output stage with a high slew rate. The input FETs are a custom low-noise dual type selected as matched pairs. There are no coupling capacitors between input and output, so the amplifier is direct-coupled, and the film capacitors inside come from the German maker WIMA. Load resistors are non-inductive wound VISHAY parts. The volume control is the two-axis, four-channel unit Stax has long used, fitted in a shielded case shared with the SRM-T8000. New large aluminium feet combine vibration isolation and load capacity with a high friction coefficient, so the heavy case stays in place without being held while cables are connected.

Pairing

The back panel offers one unbalanced RCA input next to one XLR input, suiting sources with either type of output. A LEVEL CONTROL MODE switch with an EXTERNAL (BYPASS) position removes the internal volume from the circuit, so the driver can work as a fixed-gain earspeaker amplifier placed after a preamplifier or after a source that has its own variable output. RCA parallel output terminals feed the same signal to a second component. The Pro-bias sockets accept the current Stax earspeakers, including the SR-009S used to measure the response.

Alternatives

For a driver without tubes in the same series, Stax makes the SRM-700S, which uses FETs in every stage, a J-FET second stage and a maximum output of 450 V rms.