All-stage FET amplification for STAX electrostatic headphones

The STAX SRM-400S is a solid-state electrostatic headphone driver unit that succeeds the SRM-353X. Drawing on the design and manufacturing methods of the SRM-700S, it combines selected FETs throughout its amplification stages with a revised power supply and a more rigid chassis. RCA and XLR inputs connect your source, while a 580V DC bias supports compatible STAX electrostatic headphones.

Selected FETs throughout the signal path

A custom low-noise dual FET handles the first amplification stage, with FETs also used in the output stage. STAX strictly measures and selects these components for their left- and right-channel characteristics, applying the approach used in the higher-end SRM-700S. This all-stage FET architecture supports high resolution and dynamic bass reproduction.

Direct-coupled amplification

The SRM-400S continues STAX’s tradition of complete DC amplification, with no coupling capacitors in the signal circuit. Its frequency response extends from DC to 90kHz when measured with one SR-L500 MK2, and harmonic distortion is rated at 0.01% or less at 1kHz with a 100Vrms output.

A revised power supply for greater efficiency

The newly-designed power transformer adds a dedicated winding for the low-voltage circuit alongside the high-voltage supply. This arrangement improves power efficiency and contributes to STAX’s stated improvements in sound quality over earlier driver units such as the SRM-353X. Power consumption is rated at 30W.

A more rigid aluminum hybrid chassis

A revised hybrid chassis makes extensive use of non-magnetic aluminum. Its increased rigidity helps reduce the influence of vibration and other environmental noise factors on the amplifier circuitry.

RCA and XLR source connections

Connect a source through the RCA or XLR input pair, using one input type at a time. RCA parallel outputs provide an additional connection to other audio equipment, and STAX’s concentric dual volume knob provides individual left- and right-channel level adjustment.

Balanced sound across the frequency range

STAX developed the SRM-400S to retain the resolution and dynamic low-frequency reproduction of its semiconductor amplifiers while refining the tonal balance of the SRM-353X. Designed to pair with the advanced Lambda series, it also offers an upgrade path for listeners moving beyond the driver unit in an SRS-3100 system.

Product highlights

• All-stage FET amplification
  
• Custom low-noise dual FET input stage
  
• Strictly measured and selected FETs
  
• DC amplification with no signal-path coupling capacitors
  
• Revised transformer with a dedicated low-voltage winding
  
• Rigid hybrid chassis using non-magnetic aluminum
  
• RCA and XLR inputs, used separately
  
• RCA parallel outputs
  
• Concentric dual volume control
  
• 580V DC bias

Specs

• Type: All-stage FET DC amplification electrostatic driver unit
  
• Frequency response: DC–90kHz, measured with one SR-L500 MK2
  
• Gain: 60dB
  
• Harmonic distortion: 0.01% or less at 1kHz, 100Vrms output
  
• Input impedance: 50kΩ RCA, 50kΩ × 2 XLR
  
• Maximum output voltage: 400Vrms at 1kHz
  
• Bias voltage: 580V DC
  
• Operating temperature: 32–95°F (0–35°C)
  
• Operating humidity: Below 90%, non-condensing
  
• Inputs: One RCA stereo pair, one XLR stereo pair, mutually exclusive
  
• Line outputs: RCA parallel output terminals
  
• Power consumption: 30W
  
• Dimensions: 7.68 × 4.02 × 14.80 inches (195 × 102 × 376mm), W × H × D, including protrusions
  
• Weight: 7.5lb (3.4kg)

What’s included

• STAX SRM-400S electrostatic headphone driver unit