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Stax - International SRM-700S Semiconductor Driver Unit
All semiconductor driver unit that has adopted custom-made low-noise dual FET at the first stage, and for the first time in STAX driver unit history J-FET has been employed at the second stage. The output of J-FET excellent in low-noise amplification as well as large current emitter follower circuit brings out the full performance of earspeakers. Even the music source with much high-frequency ingredients can easily be driven. The driver unit pursues an exact reproduction of musical details.
Stax's SRM-700S driver unit is all JFET, and it is so transparent I swear that in the quiet passages I could hear its JFETs flowing current. Which suggests the 700S is a simple, naturally clean design with minimal feedback.
Listening to Serge Gainsbourg's Histoire de Melody Nelson, my attention kept focusing on how Serge and Jane Birkin pronounced their words. With the 700S powering the 007S, each word had its own shape and internal structure.
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Professional Reviews 91/100
Deliciously Crispy
Sound Quality
95/100
Build Quality
91/100
Features Usability
85/100
Value for Money
89/100
The SRM-700S makes compatible electrostatics feel startlingly alive, combining transparent resolution, propulsive sub-bass and lightning-fast transients with a vast, precisely layered stage. AUDIOKEY REVIEWS admired its natural musicality and commanding scale, while SoundStage Solo found low noise, effortless dynamics and crisp, non-fatiguing treble. This focused all-FET driver is a formidable centerpiece for a serious PRO-bias electrostatic system.
What Reviewers Are Saying
Pros
Exceptionally transparent, detailed and musically natural presentation with fast transients.
Deep, tight bass with convincing slam, rhythmic drive and strong large-scale dynamics.
Expansive, highly precise imaging and layering, supported by a notably quiet background.
Useful analogue connectivity includes balanced XLR, RCA and RCA pass-through outputs.
&
Cons
Works only with compatible five-pin PRO-bias electrostatic headphones, not conventional dynamic or planar models.
Its uncompromising resolution can expose recording flaws and make weaker material less enjoyable.
SOUND QUALITY PERFORMANCE 95/100
Reviews portray an exceptionally transparent, spacious and musically engaging electrostatic amplifier with powerful bass control, striking transient speed and expansive imaging. It is praised for extracting fine detail from quiet passages while retaining composure and force during large-scale crescendos.
Transparency Resolution
96/100
AudioKey calls the SRM-700S exceptionally transparent, with “wholesale excavation of detail and nuance,” natural timbre and superb treble extension. SoundStage likewise heard crystal-clear highs, rich midrange detail and precise, expansive imaging.
Tonal Balance Musicality
95/100
Despite its solid-state design, reviewers found the presentation highly musical, natural and engaging rather than sterile. Treble is described as vibrant and accurate without shrillness, while the midrange is weighty, natural and richly detailed.
Dynamics
96/100
SoundStage reports huge amplification, ample dynamic range and effortless retrieval of faint pianissimo passages before thunderous climaxes. AudioKey found it commanding, potent and explosive on orchestral material, with quick transients and convincing rhythmic drive.
Noise Floor
93/100
SoundStage specifically praises the amplifier's very low noise, while AudioKey highlights profound silence between notes and instruments. The available listening reports point to a notably clean, quiet background with electrostatic headphones.
Headphone Compatibility
94/100
This is purpose-built for PRO-bias electrostatic headphones rather than conventional dynamic or planar models. AudioKey reports that it will drive any compatible five-pin electrostatic headphone and found no practical concerns around impedance or sensitivity within that intended ecosystem.
Assessments of Build Quality, Usability, and Overall Propositions.
Build Quality
91/100
AudioKey describes the SRM-700S as very well executed and built to last, with a simple and clean layout. The substantial transformer-based power supply and internally managed heatsinks support the impression of a serious desktop component, though long-term ownership evidence is limited.
Features Usability
85/100
The SRM-700S provides both RCA and balanced XLR inputs, RCA parallel/pass-through outputs, two five-pin electrostatic outputs, and a large volume control with separate channel-balance adjustment. It is straightforward to operate, but it is a focused analogue energizer without a DAC, digital connectivity, remote control or broad headphone-output support.
Value for Money
89/100
At roughly US$3,400 and AU$5,900 in the reviewed markets, it is a substantial specialist purchase. Still, SoundStage considers its performance very difficult to better, while AudioKey highly recommends it and awarded its Golden Keynote Award.
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Authorized dealers for Stax - International SRM-700S Semiconductor Driver Unit
- 3mA Audio — online and in-store — Houston, Texas, United States (website)
- Acoustica — online and in-store — Chester, England, United Kingdom (website)
- Ambiance Concert — in-store — Angers, Pays de la Loire, France (website)
- Analogue Seduction — online and in-store — Whittlesey, England, United Kingdom (website)
- Arctic Audio — in-store — Fargo, North Dakota, United States (website)
- ATR - Audio Trade — in-store — Eltville am Rhein, Hessen, Germany (website)
- AUDIO 2000 — in-store — Wuppertal, Nordrhein-Westfalen, Germany (website)
- Audio 46 — online and in-store — New York, New York, United States (website)
- Audio Evolution — online and in-store — Volla, Campania, Italy (website)
- AUDIO EXCLUSIVE — in-store — Wels, Oberösterreich, Austria (website)
and 85 more verified dealers