STAX SRM-400S – refined solid-state control for electrostatic earspeakers

The STAX SRM-400S is a solid-state driver unit created specifically for electrostatic earspeakers and positioned as the successor to the SRM-353X. It brings several design and manufacturing principles from the higher-tier SRM-700S into a more accessible part of the STAX range, including an amplifier architecture that uses FET devices throughout every stage. This makes the SRM-400S a strong system-building choice for dedicated music listeners using advanced Lambda-series earspeakers, while owners of an entry-level SRS-3100 set can consider it as a purposeful amplification upgrade. Its tuning aims to combine the fine resolution associated with semiconductor circuits with controlled, dynamic bass and an even tonal relationship from the lower frequencies through to the treble.

STAX equips the SRM-400S with a complete DC amplifier configuration, meaning that the signal circuit contains no coupling capacitors. A custom low-noise Dual FET is fitted to the first stage, while selected FET components are also used in the final stage. Devices intended for the left and right channels are carefully measured and matched, following an approach also applied in the SRM-700S. The published specifications include a DC-to-90 kHz frequency response when used with one SR-L500 MK2, 60 dB of gain and a maximum output voltage of 400 Vrms at 1 kHz. Bias voltage is DC 580 volts, while total harmonic distortion is specified at 0.01% or below at 1 kHz with a 100 Vrms output.

Power delivery has also been revised compared with earlier STAX driver units. In addition to the transformer serving the high-voltage section, the SRM-400S incorporates a newly designed transformer arrangement with a winding for the low-voltage circuit. This is intended to improve operating efficiency, reduce unnecessary power use and support consistent sound quality, with total consumption rated at 30 watts. The rigid hybrid chassis makes extensive use of non-magnetic aluminium to limit the influence of vibration and surrounding interference. RCA and balanced XLR inputs are available as mutually exclusive options, accompanied by an RCA parallel output and STAX’s familiar concentric dual volume control. Match it with a suitable 580-volt STAX earspeaker, allow adequate ventilation and consider the SRM-400S as the foundation for a carefully developed electrostatic listening system.