STAX SRM-400S Electrostatic Headphone Amplifier

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