Stax SRM-400S
Amplifier dedicated to Stax electrostatic headphones, with selected Dual-FET stages and Class A operation without feedback, derived from the SRM-700S model.
Description
The Stax SRM-400S is a dedicated amplifier, designed exclusively to power Stax brand electrostatic headphones. It is not a conventional headphone amplifier: electrostatic technology requires an output voltage and bias level far higher than those of a dynamic headphone, making this unit essential for the operation of Stax headphones fitted with electrostatic connectors.
The unit shares the architecture of the flagship SRM-700S model: a low-noise input stage using Dual-FETs custom-made by Stax, and an output stage also using FETs. The left and right FET transistors are measured and matched before assembly, a method intended to limit discrepancies between the two channels. The output stage operates in Class A, DC-coupled and without negative feedback, a design that prioritises signal linearity throughout the amplification chain.
In terms of performance, the frequency response extends from DC up to 90 kHz, with a gain of 60 dB and total harmonic distortion of 0.01% measured at 1 kHz at 100 V. Input impedance is 50 kOhms on the RCA inputs and 100 kOhms on the XLR input. On the output side, the maximum voltage reaches 400 VRMS, with a DC bias voltage of 580 V, values characteristic of electrostatic operation. The SRM-400S also incorporates a dedicated power transformer with a low-voltage winding, in addition to the conventional high-voltage transformer, a design intended to improve energy efficiency compared with previous models such as the SRM-353X.
Connectivity comprises a stereo XLR input, a stereo RCA input, and two electrostatic headphone outputs in RCA format, allowing two Stax headphones to be connected simultaneously or switching between two sources.
This model is aimed at users already equipped with a Stax electrostatic headphone, or wishing to move towards this technology for demanding stereo listening in a quiet environment, typically at a dedicated listening station rather than on the move. It will particularly suit enthusiasts seeking an amplifier capable of fully exploiting the definition and speed characteristic of electrostatic transducers.
Technical specifications
| Architecture | DC, Dual-FET input stage, FET output stage |
| Frequency response | DC – 90 kHz |
| Gain | 60 dB |
| THD | 0.01% (1 kHz/100 V) |
| Input impedance | 50 kOhms (RCA), 100 kOhms (XLR) |
| Maximum output voltage | 400 VRMS |
| Bias voltage | 580 V DC |
| Dimensions (W × H × D) | 195 x 102 x 376 mm |
| Weight | 3.4 kg |
| Connectivity | 1x stereo XLR input, 1x stereo RCA input, 2x electrostatic headphone RCA outputs |
What to check before buying used
When buying used, it is advisable to check that both electrostatic headphone outputs are working correctly, as well as the condition of the RCA and XLR connectors, which are used every time the source or headphones are changed. The high bias voltages involved (580 V DC) also warrant particular attention to the overall condition of the unit and the absence of any background noise at idle, which could indicate ageing components within the FET stages or power supply.
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