http://wiki.faust3d.com/wiki/index.php?title=ESP950_Electrostatic_Stereophone
The ESP/950 operates on the same principle as Stax headphones and uses a 600V bias voltage. Modern Stax systems use a 580V bias voltage. As a result, the ESP/950 can be made part of the wide-ranging Stax family of direct-drive amplifiers and transformer boxes with a simple custom made adapter. Many users prefer one of the many Stax amplifiers to the Koss E/90, although some users with access to third-party direct-drive amplifiers with adjustable bias voltage claim the ESP/950 gains by the use of bias voltages up to 620 volts.
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Modifications
As mentioned above ESP/950 can be used out of Stax amp or Stax compatible amps. There several ways to modify the headphones to acomplish this.
1. Replace the cable with Stax cable
2. Re-terminate the cable with Stax plug
3. Re-terminate provided adapter cable with Stax plug
The benefits are different for each approach. Using a better quality Stax cable should improve the overall quality of sound, but will void the warranty. Re-terminating original Koss cable will eliminate additional connection that will be there if adapter is used but will also void the warranty. if you are not concerned with voiding the warranty you can do either 1 or 2, some users prefer to have excellent life time warranty thus choose to make an adapter cable.
ESP/950 to Stax plug adapter details
Below you can find Koss and Stax plug pin-outs and picture of the ESP/950 to Stax plug adapter.
[Bilder]
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