Stax SRM-400S Semiconductor Electrostatic Driver

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STAX introduces the SRM-400S, their new semiconductor driver unit and the successor model of the SRM-353X.

Custom-made STAX low noise DUAL FET are installed on the first stage, and FET on the last stage too. The characteristics of right and left FET are carefully matched, and SRM-400S has benefited from the design and manufacturing method of the high-end SRM-700S model.

In addition to the conventional power transformer for the high voltage circuit, SRM-400S has a newly-designed power transformer with windings for the low voltage circuit, which improves the power efficiency and achieves better sound quality than conventional driver units. The new chassis is more rigid than the conventional model, with a hybrid structure of non-magnetic aluminium. It further reduces the effect of environmental noise factors and vibrations.

The inputs are RCA and XLR, both mutual exclusion design. The custom-made concentric dual volume knob is common to STAX products, and RCA Parallel Out functions are installed. All electric components are strictly selected with consideration for the environment, sound quality and overall performance is improved.

SRM-400S takes over the high-resolution sound unique to the semiconductor circuit, with dynamic low range sound reproduction and excellent sound balance. From the low range to the high range, the sound of each range is well harmonized.

A great choice for any of the advanced lambda series earspeakers, SRM-400S is an ideal upgrade from the entry-level SRS-3100.

More Information
Manufacturer Stax
Free warranty 3 years
Delivery Charge Free
Delivery Time Normally 1 - 5 Working Days (Covid-19 and Brexit Delays Possible, contact us for more information)
Card Surcharge No
See more Stax products.


  • Full update to the mid-level STAX semiconductor driver unit.
  • Successor to the SRM-353X.
  • New custom-made STAX low noise DUAL FET.
  • Newly-designed power transformer.
  • Non-magnetic hybrid rigid chassis.
  • Ideal combination with advanced Lambda series earspeakers.


  • Type: All-stage FET configuration DC amplification driver unit
  • Frequency Response: DC – 90 kHz (with one SR-L500 MK2)
  • Gain: 60 dB
  • Harmonic Distortion: 0.01 % or less (1 kHz / 100 V RMS output)
  • Input Impedance: 50 kΩ (RCA), 50 kΩ x 2 (XLR)
  • Maximum Output Voltage: 400 V RMS (1 kHz)
  • Bias Voltage: DC 580 V
  • Operating Temperature / Humidity: 0 – 35 °C / less than 90 % (non condensing)
  • Input Terminal: RCA x 1, XLR x 1 (mutual exclusion input)
  • Output: RCA parallel output terminal
  • Power Consumption: 30 W
  • Dimension: 195 (W) x 102 (H) x 376 (D) mm (maximum protruding portion included)
  • Weight: 3.4 kg