Stax SRD-7
Not a headphone. It's the little transformer box that let a plain power amp drive Stax's electrostatic earspeakers, and the quiet workhorse behind the SR-X system.
Overview
The SRD-7 is the energizer unit that sits between an ordinary amplifier and a pair of Stax electrostatic earspeakers. It has no faceplate to admire and no tubes to glow. It steps up the drive from a conventional power amp to the high bias voltages an electrostatic element needs, and it takes that drive from the speaker terminals rather than from a dedicated electrostatic amplifier. In the UK, Wilmex Ltd. distributed Stax and listed the SRD-7 as the power unit for the SRX electrostatic headphone. They quoted a frequency response of 10 Hz to 30 kHz ±2 dB, distortion of 0.02% at 1 W/1 kHz input, a maximum input of 5 W, and a 200 to 240 V ac polarising source drawing 0.25 mA. Those are respectable numbers for a transformer box of the period. They also explain why the SRD-7 became the default way into Stax ownership for people who already had a good stereo.
The unit is a selector as much as a transformer. A control marked ES/SP lets the listener switch between the electrostatic earspeakers and ordinary loudspeakers, so the energizer can stay wired into the system permanently. Stax's own advertising of the era made the same point. Period ads in Audio (October 1975, p.47) noted that SRX III headphones without the SRD-7 adaptor were available separately for direct Stax preamp coupling, which positioned the adaptor as the bridge between a conventional power amp and electrostatic earspeakers. The SRD-7 is a passive box that borrows the user's existing amplifier instead of demanding a dedicated one.
Physically it's a modest thing: roughly 73 x 120 x 215 mm and 1.7 kg, per the UK distributor listing. That's small enough to tuck beside a power amp, and heavy enough to suggest real transformer iron rather than a token circuit. Matched with the SRX earspeakers, it formed a complete electrostatic system. Stax sold the combination, and period advertising repeatedly paired the SRX III with the SRD-7 as a $230 system, with the headphones available separately for direct connection to a Stax preamp.
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