Stax SRD-P
Stax Earspeaker Pro Energizer
According to one web site this earspeaker energizer was introduced in about 1979-1980
The SRD-P is for the all Electret earspeakers like the SR-30, SR-40 and SR-Lambda Jr and for Pro electrostatic earspeakers like the SR-Lambda Pros, SR-001, SR-003, SR-007, SR-303, SR-404 and many others. These are not for non-Pro electrostatic earspeakers like the SR-Gamma, SR-Lambda, SR-Sigma, SR-5 and many others. Basically if it's got a 6-pin connector it can not be used on the SRD-P energizer. If it has a 5-pin connector then it will work with the SRD-P energizer.
The pictures below are of my personal unit. I was curious to see what type of amplifier circuit it had inside so I took it apart and took a bunch of pictures. This unit is slightly different from the SRD-P listed in the Stax brochure I have. First, there is only 1 output socket. The brochure shows two. Second, the brochure shows a input jack on the front. Mine has no input on the front, only a pair of RCA's on the back. Third, the brochure shows it powered by an AC adapter (wall wart) or by 8 C-size batteries. Mine can only be powered by plugging it into the wall socket. The battery compartment is filled with a small power transformer
Web page for the Stax CA-Y Preamplifier