STAX SRM-400S Semiconductor Electrostatic Driver

£1,245.00 inc. VAT

Trade-in available.

An ideal upgrade from their entry-level SRS-3100, the SRM-400S is a semiconductor-type driver unit by STAX Audio, the perfect headphone amplifier for all Lambda series earspeakers.

Featuring improvements in the output stage, chassis design, power transformer and overall performance. Visit us and listen to the difference.

Available in Black finish with Silver faceplate only.


* Usually ships within 1-3 business days. See exceptions

SKU: USV-STAX-SRM400S Categories: , ,



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.


  • 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), 50kΩx 2 (XLR)
  • Maximum Output Voltage: 400 V RMS (1 kHz)
  • Bias Voltage: DC 580V
  • 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

Full STAX Audio Range Available

Unilet Sound & Vision are official retail partners for STAX, providing sales, advice and assistance on their entire range of high-quality drivers, earspeakers, parts and accessories. Please do get in touch if you would like a demonstration or wish to purchase.

More Information



Founded in 1938, STAX brought the world’s first electrostatic headphone to market in 1960 and today continues their long tradition by offering the finest products for both professional and audiophile applications.

All STAX headphones are referred to as "earspeakers" – the company states that such ultra-high fidelity devices can only be compared with the best cost-no-object loudspeakers, not other headphones. Their unique amplifiers, earspeakers and drivers are highly sought-after products, and extremely well considered by aficionados in the audio industry.

Unilet Sound & Vision are one of the oldest STAX dealers in the UK, and one of the very few official outlets for their products.

STAX | Unilet Sound & Vision

Additional information

Weight 3.4 kg
Dimensions 37.6 × 19.5 × 10.2 cm

Black / Silver

Purchase Options

Click & Collect, Trade-In Available


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