Stax SRM-400S

STAX SRM-400S

Discrete FET electrostatic amplifier for STAX Earspeakers

The STAX SRM-400S is a high-quality solid-state driver amplifier for electrostatic STAX Earspeakers. As the successor to the SRM-353X, it brings the classic STAX transistor architecture to a more modern level, combining high resolution, fast transient response, and strong low-frequency control.

Inside, the SRM-400S uses a fully FET-based DC amplifier circuit. A custom STAX low-noise Dual FET is installed in the input stage, while the following amplification stages also rely on FET technology throughout. The result is a particularly clean, direct, and low-noise presentation with the speed and transparency electrostatic systems are known for.

The power supply has also been redesigned compared with earlier STAX driver units. In addition to the transformer for the high-voltage circuit, the SRM-400S uses a newly developed power transformer with a dedicated low-voltage winding. This improves efficiency and contributes to cleaner, more stable signal handling.

The more rigid chassis, using non-magnetic aluminum, helps reduce vibration and external noise influences. Combined with a maximum output voltage of 400 Vrms, the SRM-400S delivers a controlled, dynamic, and well-balanced sound that pairs beautifully with the advanced Lambda series and also represents a major upgrade from entry-level systems such as the SRS-3100.

With RCA and XLR inputs, RCA Parallel Out, and two STAX PRO Bias outputs, the SRM-400S integrates easily into high-quality headphone systems. Sonically, it is ideal for listeners seeking the transparent, fast, and precise STAX character with excellent tonal balance and solid low-end authority.

Technical Specifications