Усилители для наушников — Stax SRM-700S

The SRM-700S is the solid-state member of the SRM-700 pair of driver units from Stax, introduced in 2020 alongside the tube hybrid SRM-700T and sharing its case. Every amplifying stage uses FETs, and it was the first Stax driver unit to put a J-FET in the second stage. It is made in Japan and priced at JPY 330,000 including tax in its home market and USD 3,400 in the US.

Key specs that matter

Maximum output is 450 V rms at 1 kHz, and gain is 60 dB. Frequency response extends from DC to 100 kHz, measured with a single SR-009S earspeaker as the load. Harmonic distortion is under 0.01% at 1 kHz for an output of 100 V rms. The diaphragm bias follows the Pro standard at 580 V. Input impedance is 50 kohm for RCA and 50 kohm twice for XLR. The unit draws 46 W, measures 240 mm wide, 103 mm high and 393 mm deep, and weighs 6.3 kg. Stax rates it for use between 0 and 35 degrees C at up to 90% humidity without condensation.

Engineering and use context

Custom low-noise dual FETs, paired for matched characteristics, handle both the input and second amplification stages, and an emitter-follower output circuit capable of high current feeds the earspeakers. The whole path from input to output is DC coupled, with no coupling capacitors. German WIMA film capacitors are used throughout. The main circuit board is laid out symmetrically left to right around a large central heatsink, with dedicated cooling for the power transistors. The level control is the traditional Stax two-axis, four-channel volume unit housed in a shielded case of the same design as in the SRM-T8000, which blocks outside noise. The unit stands on large aluminium insulators chosen for their vibration damping and high friction coefficient.

Pairing

A rotary switch on the back selects between one RCA and one XLR input, so both a single-ended and a balanced source can stay connected. A LEVEL CONTROL MODE switch on the rear offers an EXTERNAL (BYPASS) setting that takes the internal volume control out of the circuit, letting a preamplifier, a DAC with its own volume or an external attenuator set the level. A set of RCA parallel outputs sends the same input on to other equipment. The Pro-bias sockets drive Stax earspeakers such as the SR-009S, which Stax used for its own measurements.

Alternatives

The tube hybrid alternative in the same Stax series is the SRM-700T, which combines a FET input stage with two 6SN7 output tubes and delivers up to 340 V rms.