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STAX SRM-400S Electrostatic Headphone Amplifier

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STAX is proud to introduce a new semiconductor driver unit SRM-400S as the successor model of SRM-353X. A STAX custom-made low noise DUAL FET is installed on First Stage and additional FETs are also installed on last stage. SRM-400S follows the design and the manufacturing method of the high end model SRM-700S, in which the amplifier high quality FETs are installed in all stages. The characteristics of the right and left FETs are measured strictly and the selected FETs installed in 400S are also featured in SRM-700S. In addition to the conventional power transformer for the high voltage circuit, the newly designed power transformer with winding for a low voltage circuit is installed in SRM-400S. The low voltage winding enables to improve the power efficiency and achieves further power saving and better sound quality than what was featured in older driver units such as SRM-353X.

The new chassis which is more rigid than the conventional model is installed. The chassis has a hybrid structure using a lot of non-magnetic aluminum and further reduces the effect of the environmental noise factors such as vibrations. The custom made concentric dual volume knob which is common to STAX products and RCA PARALLEL OUT functions are installed same as the conventional driver units. All electric components are strictly selected considering the environment.

SRM-400S presents the high resolution sound which is unique to the semiconductor circuit design and dynamic low range sound reproduction. From the lows to the highs, the frequency response is well harmonized throughout the entire spectrum. Not only for the combination with the advanced lambda series, SRM-400S is also ideal for the upgrading from the entry level SRS-3100. This is a promising new model which covers beginners to intermediates among the renewed driver unit line-up.


• All-stage FET Configuration DC amplification

• Newly Designed Output Stage

• Custom-Made Low Noise Dual FETs

• Pair of RCA input terminals

• Pair of XLR 3-pin input terminals

• RCA parallel output terminals

• 30W Power Consumption

Technical Specs

Frequency Response :DC - 90kHZ (with one SR-L500 MK2)

Harmonic Distortion :0.01% or less (1kHZ/100Vrms output)

Maximum output voltage :400Vrms (1kHz)

Bias Voltage :DC 580V

Operating Conditions :32 to 95 degrees fahrenheit @<90% humidity

Dimensions :(H)4.0" x (W)7.68" x (L)14.8"

Weight :3.4 lbs

About STAX

STAX produce exclusively electrostatic headphones, or “earspeakers” as they call them, in Japan, since 1938. STAX chose the electrostatic technology for its ability to produce transparent sound, and low weight for comfort/ergonomics, both due to the driver design not employing fixed magnets and moving coils, as opposed to other common headphone driver designs. The minimal weight of the driver ensures a very quick response to the signal, creating a more faithful sound reproduction.