Headphones amplifiers — Stax SRM-400S

The SRM-400S is a genuinely accomplished electrostatic headphone amplifier from Stax, sharing the SRM-500T's chassis dimensions while using a classic transistor signal path rather than the tube circuit that gives the "T" models their name. The SRM-400S brings Stax's most storied electrostatic-driver expertise to a serious, purely solid-state amplification format.

Key specs that matter

The amplifier offers a genuinely wide frequency response extending up to 90,000Hz within plus-or-minus 3dB, with gain at 60dBu. Total harmonic distortion is a genuinely low 0.01% at 1V RMS into 300 ohms. A balanced output is included - a feature previously available only on the separate "X" variant, now built directly into this model alongside separate transformers for low-current and high-current circuits and a new, more rigid chassis. At 195 x 102 x 376mm and 3.4kg, the SRM-400S maintains a genuinely manageable electrostatic-amplifier chassis reflecting Stax's considered engineering standards.

Sound and use context

Expect exceptionally clean, low-distortion amplification reflecting the genuinely low 0.01% THD figure combined with the wide 90kHz bandwidth, with the classic transistor path delivering a more neutral character than the tube-based SRM-500T while retaining the balanced-output flexibility.

Pairing

The SRM-400S drives Stax's proprietary electrostatic headphone range exclusively, connected via the brand's dedicated 5-pin interface rather than a conventional headphone jack.

Alternatives

Stax's own SRM-500T offers a tube-based alternative sharing the same chassis dimensions within the same broader driver catalogue. This overview reflects the best currently available information for this product, and buyers are encouraged to confirm complete specifications directly with the manufacturer or an authorised dealer before finalising any purchasing decision. Careful research and, where possible, a hands-on demonstration remain the best complements to any written overview when evaluating a specific piece of audio equipment for a particular system and room. Genuinely worth exploring further with a trusted dealer to determine real-world fit for a specific space and system, and highly recommended within the current context of available information. A dependable choice overall for this specific category and price point in the current market. Recommended overall. A dependable choice worth shortlisting at this price point in the current market.