The STAX SRM-400S is a semiconductor driver unit designed as the successor to the SRM-353X. It employs the same all-FET configuration found in the higher-tier SRM-700S, with a custom-made low-noise DUAL FET used in the first stage and carefully matched FETs in the output stage. Each unit is assembled with selected components that have been measured and paired for optimal performance.
A key feature of the SRM-400S is its dual power transformer design. In addition to the high-voltage transformer, a dedicated low-voltage winding improves overall efficiency, reduces power consumption, and contributes to a cleaner signal path compared with the previous SRM-353X.
The chassis uses a rigid hybrid construction that combines non-magnetic aluminum with traditional materials to minimize the effects of external vibration. Inputs are provided on both RCA and XLR connectors (used exclusively), while a custom concentric dual volume control and RCA parallel outputs are retained for system flexibility. All signal paths are fully DC-coupled, with no capacitors in the audio circuit.
Sonically, the SRM-400S delivers the high-resolution character typical of STAX semiconductor drivers along with strong, well-controlled bass. Compared with its predecessor, it offers improved overall balance and smoother integration across the frequency range. It pairs effectively with both the advanced Lambda series and entry-level systems such as the SRS-3100, making it a versatile choice for listeners moving beyond starter setups.
