STAX SRD-7 + SR-X MK3

Pre owned. In good condition. Shows some signs of wear, visible in the photos. Without original packaging.

The Stax SR-X Mark III are high-end vintage electrostatic headphones, paired with the dedicated Stax SRD-7 amplifier, which still outperform most conventional dynamic headphones, such as the Sennheiser HD 800 or Ultrasone Edition 8, delivering extremely clean, open, and natural sound. Despite their age, these headphones represent an exceptional value on the second-hand market, available for roughly a tenth of their original price.

Introduced in 1975, the SR-X Mark III were the highest-performance headphones in the world at the time, unmatched until the even more sophisticated SR-Sigma models arrived in 1977. Unlike conventional dynamic headphones, which use a coil inside a magnet on a heavy diaphragm, the SR-X Mark III employ electrostatic technology, far more effective at reproducing sound with extremely low distortion and no unwanted resonances or coloration.

The design of the headphones may appear dated or almost “military,” but the electrostatic technology demonstrates its superiority during extended listening sessions. These headphones are intended for listeners who want to capture every nuance of the music, every detail and harmonic, without artificial enhancement or exaggeration; if you are looking for dramatic effects for short demo sessions, these are not the right choice.

Stax, the inventor of electrostatic headphones in 1959, had sixteen years of experience optimizing them when the SR-X Mark III series was introduced. The design recalls the first SR-1 from 1959, with a sober and functional black appearance, while earlier SR-X or SR-3 models from 1971–1975 showed more visual differences than performance differences.

The Stax SRD-7 amplifier works differently from traditional Stax amplifiers: it is not a standalone amplifier but connects to a regular speaker amplifier, drawing current from it and converting it into the high-voltage power needed to drive the electrostatic headphones. This provides a cost-effective alternative to more expensive dedicated electrostatic amplifiers while maintaining excellent performance.

The SRD-7 “normal bias” model has two headphone outputs with 6-pin Stax connectors, compatible with most vintage Stax headphones. The “pro-bias” version supports higher-voltage Pro-series Stax headphones. The unit is compact (approximately 12 x 7 x 20 cm) and can be conveniently placed next to a computer or in small spaces. The front panel features a power knob that allows easy switching between headphone and speaker setups, routing the amplifier signal to the speakers when the headphones are not in use.

On the rear panel, there are two cables and four binding posts for speaker connections. The black cable is the AC power supply, while the gray cable connects to the main amplifier; Stax includes a legend to facilitate proper connection. Power consumption is remarkably low at 0.1 W, making the SRD-7 efficient and unobtrusive during daily use.

Together, the SR-X Mark III headphones and the SRD-7 amplifier form a complete electrostatic system capable of reproducing every musical detail with transparency, naturalness, and engagement, making it an ideal solution for audiophiles seeking the highest fidelity in analog playback.

SR-X MK3 Technical Specifications:

• Transduction type: Electrostatic (Normal Bias, compatible with Stax energizers/power stages 230 V)
• Impedance: approx. 130 kΩ at 10 kHz
• Electrostatic capacitance: approx. 120 pF
• Frequency response: approx. 30 – 25,000 Hz (some sources report up to 20 – 70,000 Hz)
• Sensitivity: approx. 97 dB SPL with 100 V RMS input at 1 kHz, bias 230 V
• Maximum sound pressure: approx. 110 dB SPL (some sources indicate 115 dB)
• Diaphragm thickness: approx. 2 microns
• Electrode gap: approx. 300 microns
• Cable: black cloth-covered cord, low microphonic, approx. 2.5 m
• Connector: Stax 6-pin Normal Bias
• Weight: approx. 290 g without cable; approx. 357 g with cable and connector

SRD-7 Technical Specifications:

• Type: Electrostatic Transformer/Adaptor
• Bias voltage: 230 V DC (Normal Bias)
• Frequency response: 10 Hz – 30,000 Hz (±1.2 dB)
• Distortion: <0.02% THD (1 kHz)
• Inputs: Speaker-level input from main amplifier
• Outputs: Two 6-pin Normal Bias sockets
• Maximum input power: 30 W (1 kHz instantaneous)
• Dimensions: approx. 90 x 66 x 194 mm up to 73 x 120 x 215 mm
• Weight: approx. 1.7 kg