STAX SRM-500T

The STAX SRM-500T is a driver amplifier specifically designed to power STAX electrostatic headphones and EarSpeakers. It replaces the previous SRM-006tS and introduces an updated architecture, with a low-noise FET input stage and an output stage based on a pair of 6FQ7/6CG7 vacuum tubes. This configuration combines the fast, linear response of solid-state devices with the tonal characteristics of vacuum tubes, while maintaining a low noise level and a good ability to follow signal dynamics.

The power supply design has been revised compared with the previous generation and uses a specially developed transformer to improve circuit efficiency and stability. The metal chassis has also been reinforced to help reduce vibrations and interference, while the component layout and signal path are designed to limit interaction between the audio and power supply sections.

The input stage uses selected FET transistors characterized by low noise and linear operation, while the output section employs two 6FQ7/6CG7 vacuum tubes. This configuration provides a smooth, information-rich reproduction while maintaining the response speed required by electrostatic headphones.

The SRM-500T is a genuine STAX Driver Unit rather than a conventional amplifier for dynamic or planar headphones. STAX electrostatic EarSpeakers require a dedicated bias voltage and a high signal-voltage swing. The unit therefore features a 5-pin Pro Bias output with a 580 V bias voltage, compatible with STAX headphones using this standard.

The combination of solid-state circuitry and a tube output stage is designed to preserve the characteristics of electrostatic technology, with particular attention to transparency, response speed and the reproduction of the most subtle signal variations. The low frequencies remain well controlled, while the midrange benefits from the presence of the 6FQ7/6CG7 tubes, and the high frequencies are reproduced with extension and continuity. The circuit's low noise level also helps preserve low-level information without introducing excessive listening fatigue during extended sessions.

Its ability to accurately follow the information contained in the recording also contributes to a detailed spatial presentation. The different elements of the musical program remain clearly distinguishable, while the soundstage maintains good separation between its various layers, enhancing the characteristics of the connected electrostatic headphones.

The rear panel features both balanced XLR and unbalanced RCA inputs, allowing the driver to be connected to DACs, streamers, preamplifiers and other analog sources. An RCA Line Out pass-through is also provided, allowing the signal to be routed to another component in the system without interrupting the connection between the source and the SRM-500T.

The construction follows the typical design approach of STAX equipment dedicated to EarSpeakers, with particular attention to circuit layout, power supply stability and chassis rigidity. The design of the power transformer and the careful control of the tubes' operating conditions contribute to stable operation, while the mechanical structure reduces the influence of vibrations on the circuit's more sensitive sections.

The SRM-500T can be used with STAX headphones featuring a 5-pin Pro Bias connection, including models such as the SR-L500 MKII, SR-L700 MKII, SR-007MK2, SR-009S and SR-X9000, allowing users to take advantage of the dedicated driver system developed by STAX.

Technical Specifications:

• Type: Hybrid Driver Amplifier for STAX electrostatic headphones (EarSpeaker)
• Circuit technology: FET input stage; vacuum tube output stage
• Tubes: 2 × 6FQ7 / 6CG7
• Compatibility: STAX Pro Bias headphones (580 V)
• Bias: Pro Bias 580 V
• Frequency response: 1 Hz – 90 kHz (-3 dB)
• Gain: 60 dB
• Maximum output voltage: 300 Vrms
• Harmonic distortion: Less than 0.01%
• Analog inputs: 1 × stereo RCA; 1 × balanced XLR
• Outputs: 1 × STAX 5-pin Pro Bias; 1 × RCA Line Out
• Volume control: High-precision analog potentiometer
• Power supply: Redesigned high-stability linear power supply
• Power consumption: 38 W
• Dimensions (W × H × D): 195 × 102 × 376 mm
• Weight: 3.4 kg