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Stax - International SRM-500T Vacuum Tube Driver Unit For Earspeakers
Complete model change for the first time in 11 years
The SRM-500T, which follows the numerical sequence of the SRM-700 series, is the successor model to the vacuum tube driver unit SRM-006tS and is 11 years in the making.
STAX custom-made low-noise DUAL FET is used for the first stage, and high-voltage double triode 6FQ7 / 6CG7 is used for the output stage. While following the traditional circuit design which is perfect for STAX vacuum tube drivers, we have improved the efficiency of the power supply and further improved the sound quality.
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-500T. By adopting this low-voltage winding, the efficiency of the power supply has improved, and compared to conventional vacuum tube driver units such as the SRM-006tS, power saving and improved sound quality have been achieved.
In addition, a new chassis with increased rigidity compared to SRM-006tS has been adopted. The hybrid structure, which uses a lot of non-magnetic aluminium, further suppresses the effects of physical external noise 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 ampli er con guration that the signal circuit includes no coupling condensers is one of the biggest traditional features.
The clear and highly transparent sound, a feature of the vacuum tube circuit and the warm and wide sound texture are maintained and the thickness of the low-mid range is brushed up.
Therefore we recommend the SRM-500T in combination with the advanced lambda series earspeakers (SR-L300/SR-L500 MK2/SR-L700 MK2).
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Professional Reviews 88/100
Nicely Roasted
Sound Quality
87/100
Build Quality
89/100
Features Usability
84/100
Value for Money
94/100
The SRM-500T brings tube-like musicality to electrostatic listening, pairing beautifully textured mids and a spacious, precisely layered stage with clean, natural treble. AUDIOKEY REVIEWS found its hybrid circuit delivers impressive transparency, transient snap and controlled bass, making it a refined, compelling gateway to compatible 580V-bias earspeakers.
What Reviewers Are Saying
Pros
Natural, grain-free treble and beautifully rendered midrange timbre, tone and texture.
Convincing three-dimensional imaging, strong transparency and fast transient response from the hybrid tube/FET design.
Compact, well-executed chassis with balanced XLR, RCA and parallel outputs.
&
Cons
Does not deliver the deepest bass or full large-scale impact; powerful dramatic passages can feel restrained.
Requires compatible 580V-bias electrostatic earspeakers and cannot drive conventional dynamic or planar headphones.
SOUND QUALITY PERFORMANCE 87/100
The SRM-500T is described as a musical and transparent electrostatic driver with particularly natural treble, expressive midrange timbre, and convincing spatial presentation. Its main limitation is ultimate bass extension and large-scale authority, which the review finds less capable than Stax's higher-tier driver units.
Transparency Resolution
90/100
AudioKey Reviews calls the amplifier's transparency very good and praises its detailed, controlled presentation. Treble resolution is especially strong: cymbals are said to sound convincingly like cymbals rather than becoming grainy or splashy.
Tonal Balance Musicality
92/100
The hybrid design is praised for combining tube-like musicality and dimensionality with solid-state bass control. Midrange timbre, tone, and texture are described as beautifully rendered, while the treble is natural and free from harshness or sibilance.
Dynamics
85/100
Transient speed is described as very strong for its class, and the presentation has good dynamics and controlled bass. However, the review notes that powerful, dramatic passages can be left wanting, limiting its sense of large-scale impact.
Noise Floor
80/100
The review does not specifically discuss hiss, hum, or background silence. It does report a clean, grain-free treble presentation, but provides little direct evidence about residual noise with sensitive electrostatic earspeakers.
Headphone Compatibility
88/100
As a 580V-bias electrostatic driver, the SRM-500T is intended for compatible Stax-style earspeakers rather than conventional dynamic or planar headphones. The reviewer found it capable of delivering good bass response and musical performance across the electrostatic headphones used, though it does not match the extension and authority of Stax's more expensive units.
Assessments of Build Quality, Usability, and Overall Propositions.
Build Quality
89/100
AudioKey Reviews describes the compact chassis as well executed and built to last. The front panel's large volume control and dual five-pin sockets suggest a practical, purpose-built layout, though the review offers limited detail on internal construction or long-term control durability.
Features Usability
84/100
The SRM-500T provides both balanced XLR and single-ended RCA inputs, two electrostatic headphone outputs, and a parallel RCA output for feeding another system. It remains a focused analogue driver with no DAC, remote, gain selection, or digital connectivity, while the warm-up indicator usefully signals when the tube circuit is ready.
Value for Money
94/100
At the reviewed price of $1,340, AudioKey Reviews considers the SRM-500T a particularly compelling entry into electrostatic listening, stating that substantially more spending is required to approach or improve upon it. The reviewer highly recommends it both as a first electrostatic amplifier and as a driver that many owners could keep for the long term.
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