Усилители для наушников — Stax SRM-400S

The SRM-400S is a driver unit for electrostatic headphones from Stax, the Japanese company that has specialised in electrostatic listening since the 1960s. It succeeds the SRM-353X in the middle of Stax's range, carrying over circuit design and manufacturing methods from the top SRM-700S, and it is aimed at the Lambda series headphones and at owners upgrading from the SRS-3100 set.

Key specs that matter

The SRM-400S is an all-FET DC amplifier. Frequency response is DC to 90 kHz with one SR-L500 MK2 connected, gain is 60 dB and THD is 0.01% or less at 1 kHz and 100 Vrms output. Maximum output voltage is 400 Vrms and bias voltage is DC 580 V. Input impedance is 50 kohm on RCA and 50 kohm per leg on XLR, and the RCA and XLR inputs are mutually exclusive. An RCA parallel output passes the input signal on to another component. Power consumption is 30 W. The unit measures 195 x 102 x 376 mm wide, high and deep, with a weight of 3.4 kg.

Engineering and use context

Electrostatic headphones need a high polarising voltage for the diaphragm and hundreds of volts of audio signal on the stators, which is why they require a dedicated energiser rather than a conventional headphone amplifier. The signal circuit is a complete DC amplifier with no coupling capacitors. Its input stage uses a custom-made, low-noise dual FET with left and right devices matched, a technique taken from the SRM-700S. In addition to the high-voltage transformer, a new transformer with separate windings for the low-voltage circuits improves efficiency and lowers noise. The chassis is stiffer than the SRM-353X's and uses more non-magnetic aluminium to reduce vibration and eddy currents. The concentric dual volume knob, common to Stax units, sets left and right levels together or separately. Two 5-pin Pro bias outputs allow two headphones to be connected at once, so two listeners can share one source. The unit is available in silver and black.

Pairing

The SRM-400S is designed for Stax Pro-bias headphones such as the SR-L300, SR-L500 MK2 and SR-L700 MK2. The balanced XLR input suits DACs and preamplifiers with balanced outputs, and the parallel output keeps the source available for a speaker system. Older Normal-bias Stax headphones with 6-pin plugs cannot be used, because both outputs are 5-pin Pro-bias sockets.

Alternatives

Stax's SRM-700S is the larger semiconductor driver unit with a more elaborate circuit.