Stax SRM-700T
Stax valve amplifier dedicated to electrostatic headphones, with transformerless OTL output stage and balanced circuit.
Description
The Stax SRM-700T is a valve amplifier designed specifically to power the Japanese brand's electrostatic headphones. Unlike a conventional headphone amplifier intended for dynamic transducers, this unit delivers the high voltages required for electrostatic elements to operate, via a proprietary Stax connection.
The output stage is based on an OTL (Output TransformerLess) architecture, meaning without an output transformer, fitted with two 6SN7 dual triodes. The circuit is of balanced construction throughout the signal path, which limits interference and background noise. The 60 dB gain and harmonic distortion of 0.01% or less, measured at 1 kHz for a 100 Vrms output, reflect accurate signal reproduction, even at high output levels. The maximum output voltage reaches 340 Vrms at 1 kHz, a value consistent with the requirements of Stax electrostatic headphones. Paired with an SR-009S, the amplifier displays a frequency response extending from DC up to 100 kHz.
Volume and balance are adjusted via an electrically shielded four-gang potentiometer, a component prone to wear on this type of unit. The connection panel offers a switchable input, balanced via 3-pin XLR connectors or unbalanced via RCA, as well as an unbalanced RCA output and two headphone outputs, allowing two electrostatic headphones to be connected simultaneously.
This model is aimed at owners of Stax electrostatic headphones seeking dedicated amplification, capable of making the most of the finesse of these transducers. It will suit quiet, fixed-position listening, in a system where sound reproduction quality takes precedence over versatility.
Technical specifications
| Frequency response | DC - 100 kHz (with an SR-009S) |
| Gain | 60 dB |
| Harmonic distortion | 0.01% or less (1 kHz output / 100 Vrms) |
| Input impedance | 50kΩ (RCA), 50kΩ x 2 (XLR) |
| Maximum output voltage | 340 Vrms (1 kHz) |
| Dimensions (H × W × D) | 103 x 240 x 393 mm |
| Weight | 5.7 kg |
What to check before buying used
The four-gang potentiometer is worth testing through its full rotation to detect any crackling or balance imbalance, common with age on this type of component. The two 6SN7 triodes are wear parts in their own right: hum, audible micro-vibration or a loss of gain on one channel may indicate tired valves, which are replaceable but should be checked. The proprietary Stax connection and its high-voltage socket should also be examined, with any play or signs of overheating at this point to be ruled out.
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