I just bought a pair of Stax SR-80 electret headphones along with the SRD-4 energizer (SR-84 system) and would like to avoid using them with a full-size (loudspeaker) amplifier if I can avoid it, since I have a couple of decent headphone amps. (Creek OBH-1 and Audio Alchemy class A headphone amp.) EDIT: some incorrect assumptions here, which are addressed later in this thread Apparently the SRD-4 unit provides a polarizing voltage via the 5th cable, but it's secondary purpose is to step up the voltage provided by an amplifier to that needed by the high impedance (150 kOhm) driver itself. I believe it basically consists of a transformer with 1 ohm input impedance with a series resistor to pad the input impedance up to 8 ohm for conventional amplifiers -- and this padding allows you to use the volume setting in a normal range without sending too much power to the drivers. Back of the envelope calculation: amplifier delivers 1 watt into 8 ohms with 2.8v signal (P=(V^2)/R). After padding, the transformer sees 0.35v. If the turns ratio is 400:1 (just a guess!), the Stax electret driver will get 140v, which will produce 130mW power and should be plenty loud. (The rated sensitivity is 95dB/100v). Most headphone amps will of course have trouble delivering a watt into 8 ohms. If you remove the padding resistor, the amp only has to deliver 130mW but is now driving a 1-ohm load, which is very different from the usual 30-300 ohm headphones in terms of damping factor and current requirement. But it seems easy enough to replace or modify the transformer to have a lower turns ratio. If we reduce the turns ratio by a factor of 20, the headphone amp will see a 20 ohm load and should be able to supply the needed voltage and power. Does anyone else have insight into this? I'm curious why Stax designed their energizer units for speaker-level signals instead of headphone-level signals in the first place?