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Stax - International SRM-400S Semiconductor Driver Unit
STAX SRM-400S Semiconductor Driver Unit
STAX introduces a new semiconductor driver unit SRM-400S, the successor model of the SRM-353X. STAX custom-made low noise DUAL FET is installed on First Stage. And FET is also installed on Last Stage. The characteristics of right FET and left FET are measured strictly. Selected FET are installed as well as SRM-700S. SRM-400S succeeds to the design and the manufacturing method of the high end model SRM-700S, the amplifier which FET are installed in all stages.
In addition to the conventional power transformer for the high voltage circuit, the newly designed power transformer with the winding for the low voltage circuit is installed in SRM-400S. The low voltage winding enables to improve the power efficiency and achieve further power saving and better sound quality than the conventional 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 inputs are RCA and XLR, the mutual exclusion inputs. 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. Also the sound quality and the performance is improved. The complete DC amplifier configuration that the signal circuit includes no coupling condensers is one of the biggest traditional features.
SRM-400S takes over the high resolution sound which is unique to the semiconductor circuit and dynamic low range sound reproduction. Simultaneously, the good sound balance is further pursued than SRM-353X. From the low range to the high range, the sound of each range is well harmonized. 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 the promising new model which covers from beginners to intermediates among the renewed driver unit lineup.
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Stax SRM-400S Semiconductor Driver Unit
Stax's SRM-400S is a focused, high-output electrostatic energizer that pairs a precise solid-state topology with pro-bias compatibility. Its FET-based circuitry delivers 60dB gain, up to 400Vrms output and <0.01% harmonic distortion for a neutral, controlled presentation with fast dynamics and excellent midrange clarity. The all-metal chassis, large transformer and tactile controls underline excellent build quality, and the SRM-400S handled the SR-X1, SR-009S and Audeze CRBN2 in review sessions while remaining competitive in A/B comparisons, per W. Jennings at ecoustics. Connectivity is purposefully simple: 5-pin DIN earspeaker outputs, a rear RCA/XLR selector (mutual exclusion) and RCA parallel outputs for daisy-chaining. Ideal for dedicated electrostatic listeners who want transparent performance and ample headroom.
Pros
- Very low distortion and tightly matched channels thanks to the FET-based design, supporting a transparent, neutral sound.
- High output and headroom (60dB gain, 400Vrms) that easily drives pro-bias electrostatic headphones, even demanding models.
- Robust, all-metal construction with a large transformer and tactile controls—mechanically solid for desktop use.
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Cons
- Specialized unit: only energizes pro-bias electrostatics and lacks DAC, preamp or non-electrified line outputs.
- RCA/XLR input selector is on the rear panel, which can be inconvenient for frequent source switching on a desktop.
- Non-standard chassis dimensions may make stacking or integration into some systems awkward.
Sound Quality
91/100
The unit provides ample gain (60dB) and very high maximum output voltage (400Vrms), and the reviewer notes it 'worked admirably' and is 'very capable' of driving pro-bias electrostatic headphones, implying excellent headroom, transient control and punch with demanding electrostatics.
Noise Floor
86/100
STAX implemented a newly designed winding for the low-power circuitry to reduce noise and the review highlights tight FET matching to maintain channel balance; the reviewer reports no audible noise concerns, indicating a very quiet unit suitable for sensitive electrostatic headphones though the review stops short of 'black background' phrasing.
Headphone Compatibility
86/100
Specifically built as a pro-bias electrostatic driver (DC 580V bias, 5-pin DIN earspeaker ports) and explicitly tested with multiple Stax models (CRBN2, SR-009S), it drives single or two electrostatic headphones simultaneously with ease. It is excellent for electrostatics but intentionally specialized — not a general-purpose dynamic/planar amp — which limits cross-type compatibility.
Build Quality
92/100
Hefty, all-metal chassis with a large transformer and tactile controls; reviewer describes a solid, well-crafted unit with secure XLR connections and a well-behaved volume knob, reflecting high-quality construction and attention to mechanical detail.
Features Connectivity
84/100
Baseline desktop driver with RCA and XLR inputs (mutual-exclusion), front 5-pin DIN earspeaker outputs and RCA parallel outputs for daisy-chaining; features include balanced XLR input (+2), RCA parallel output (+1) and multiple headphone outputs (+1). No DAC, streaming, or preamp volume-controlled line-out functionality, keeping the feature set focused and appropriate for its role.
Value for Money
90/100
At the ~$1,299 price the reviewer calls it a 'good compromise between price and performance' and suggests it extracts nearly all the potential from high-end Stax headphones, implying a justified premium and strong value within its niche.
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