Headphones amplifiers — Stax SRM-400S

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

Key specs that matter

The SRM-400S is an all-FET DC amplifier. Frequency response reaches from 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 the bias voltage is DC 580 V. Input impedance is 50 kohm on RCA and 50 kohm per leg on XLR, and only one of the two inputs can be used at a time. An RCA parallel output passes the input signal on to another component. Power consumption is 30 W, and the unit measures 195 mm wide, 102 mm high and 376 mm 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, so they require a dedicated energiser rather than an ordinary headphone amplifier. The signal circuit is a full DC amplifier with no coupling capacitors. Its input stage uses a custom low-noise dual FET with matched left and right devices, a technique carried over from the SRM-700S. Alongside 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 that of the SRM-353X and uses more non-magnetic aluminium to cut vibration and eddy currents. The concentric dual volume knob found on Stax units adjusts left and right together or separately. Two 5-pin Pro-bias outputs take two headphones at once, letting two people listen to the same source together. Silver and black versions are available.

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 loudspeaker 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.