I measure 271Vdc with using a Fluke 80K15 probe having nominal 1000M ohm input resistance. That measurement should be hardly loaded by the input resistance. It is very consistent, whether phones are plugged in or not, and whether ground is taken from any stator drive (with no signal) or input ground. The bias rises very quickly after E90 energizer is turned on, and falls a little more slowly when E90 is turned off.
Straight into the Fluke 8060 meter which has 10M ohm input resistance, I get around 178Vdc. AC line voltage is 123V rms, above nominal.
Koss claims 600V in the specifications. One poster at Head-Case determined that the bias supply is derived from the rail voltage of 600V, but through a voltage divider consisting of two 10M ohm resistors in series to ground. He then figured the bias voltage to actually be 300V. His measurement into 10M DVM was 202V, consistent with 300V with 5M ohm impedance. His measurements seem slightly higher than mine for some reason. But in either case, it appears that the nominal bias voltage is around 300V, and not 600V as Koss claims.
This is important because many people who use the Koss phones with Stax or other energizers find, before long, that they are getting a squeal. Once it develops the squeal with another energizer, it may or may not have the squeal also with the E90. The poster above believes that the Koss phones may be damaged by higher than 300V bias. It's also possible, however, that the Koss squeal comes about because typically the phones are rewired to use with other energizers, and the rewiring could introduce dust into the electrostatic driver. One company that does this removes the back pad when they do so, that could allow dust in.
Not all people using Stax or other alternative amplifiers with the E950 phones has experienced the squeal problem. And it also does happen, but possibly more rarely, with the E90. It's possible that Koss has redesigned the bias lower to reduce the number of lifetime warranty repairs.
To be safe, I'm going to use no higher than 300V with my Stax amp. For the purpose of using the Stax amp, I've just mistakenly ordered 3 of the required Cooper (formerly Amphenol) plugs. It's a 6 pin plug which has to have the middle pin removed (with needlenose pliers) to be used with Stax Pro amplifiers. An early poster on the above thread suggested that one. However, at the very end of the thread, electrostatic amplifier designer Kevin Gilmore says it doesn't have thick enough pins to mate well with actual Stax amplifiers. He was in the process of desiging a new Teflon version, but in the meantime, he suggested the Stax connector available from Moon Audio.
A description of how to wire an adapter cable is here.
A description of how to wire an adapter cable is here.
