88%
Stax - International 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.
WATT RECOMMENDS
Save your favorites. Compare them anytime.
Create a free HiFi Hub account to save products to your Wishlist and compare your favorites side by side.
Professional Reviews 88/100
Nicely Roasted
Sound Quality
88/100
Build Quality
89/100
Features Usability
85/100
Value for Money
92/100
The SRM-400S brings clean, resolving electrostatic drive and a pleasingly dry, controlled balance to Pro-bias headphones. W. Jennings at ecoustics found it assured with models from the SR-X1 to SR-009S and CRBN2, delivering performance competitive with far pricier amplification. Dual Earspeaker outputs, balanced input and RCA pass-through make this a remarkably capable energizer.
What Reviewers Are Saying
Pros
Drives a broad range of Pro-bias electrostatic headphones confidently, including the Stax SR-009S and Audeze CRBN2.
Clean, drier solid-state presentation was judged competitive with the substantially more expensive HeadAmp BHSE.
Dual Pro-bias outputs and RCA pass-through support two electrostatic headphones and integration with a separate conventional headphone amplifier.
Solid all-metal construction with tactile controls, secure XLR connections and a smooth volume control.
&
Cons
Exclusively an electrostatic energizer: it has no DAC, preamp function, conventional headphone output or variable pre-out.
Rear-mounted RCA/XLR selector is inconvenient for listeners who switch sources frequently.
Non-standard component dimensions may complicate equipment stacking.
SOUND QUALITY PERFORMANCE 88/100
The SRM-400S was reported to work admirably with three different Pro-bias electrostatic headphones and delivered performance the reviewer found difficult to distinguish from the far more expensive HeadAmp BHSE. Its presentation is described as slightly drier than a tube alternative, while its electrostatic drive capability appears assured within the intended Stax Pro-bias ecosystem.
Transparency Resolution
91/100
In direct A/B listening against the reviewer’s HeadAmp BHSE, the SRM-400S was not felt to lose a step despite that amplifier’s substantially higher cost. This strongly suggests a clear, resolving presentation, although the review offers little separate discussion of fine-detail retrieval or spatial separation.
Tonal Balance Musicality
89/100
The reviewer characterizes the SRM-400S as a drier-sounding alternative to the tube-based BHSE, rather than as thin or fatiguing. Listeners who prefer a cleaner, less tube-inflected balance may find that character especially appealing.
Dynamics
84/100
The review found the amplifier fully successful with all three electrostatic headphones used and describes it as easily driving any Pro-bias model. There is little specific commentary on transient attack, slam, or macro-dynamic contrast, however.
Noise Floor
82/100
Stax uses a separate low-power transformer winding intended to reduce noise and improve efficiency. The review does not provide direct listening observations about hum, hiss, or background silence, so its real-world noise behavior with particularly sensitive electrostatics is not explored in detail.
Headphone Compatibility
94/100
Within its intended role, the SRM-400S is reported to drive a variety of Pro-bias electrostatic headphones with ease, including the Audeze CRBN2 and Stax SR-009S. Its two Pro-type sockets can run two electrostatic headphones at once, but it is exclusively an electrostatic energizer and cannot power conventional dynamic or planar headphones.
Assessments of Build Quality, Usability, and Overall Propositions.
Build Quality
89/100
The all-metal chassis is described as well crafted and solid, with tactile controls, securely locking XLR connections, and a smoothly operating volume knob. The review gives a favorable account of fit and finish, though it does not address long-term reliability.
Features Usability
85/100
The SRM-400S provides RCA and balanced XLR inputs, a rear input selector, dual Pro-bias electrostatic outputs, and RCA pass-through for integrating a separate conventional headphone amplifier. It intentionally omits a DAC, preamplifier function, and conventional headphone outputs; the rear-mounted input selector is also inconvenient for users who frequently change sources.
Value for Money
92/100
At the stated $1,245-$1,595 price range, the reviewer considers it a strong price-to-performance compromise among electrostatic driver units. The review also argues that spending considerably more may bring little audible improvement with headphones such as the CRBN2 and SR-009S.
Reviews
Online Reviews & Mentions
Forum Mentions
No forum mentions found yet
Verified Dealers
Online Dealers
All Dealers
Zoom out the map to see more dealers
Authorized dealers for Stax - International SRM-400S 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