Koss ESP/950 electrostatic headphone

For the duration of a couple of weeks I had the opportunity to test the Koss ESP/950 inclusive energiser E/90 and get an impression of the sound characteristics of this combo. Product description and some technical information you will find here: http://www.koss.com/koss/kossweb.nsf/p?openform&pc^es^ESP950#spec
 

At first sight, the supplied energiser E/90 in the small plastic box will give you an idea about what you have to expect in quality. It is supplied with an external 9V mains adapter. The gain is 63dB with a linear frequency response over the complete frequency range. The -3dB-border ist reached at about 60khz. The maximum output voltage is 770V rms. The sound of this amplifier is far away from a high-end product. The bass is nearly not present, the mids sound fuzzy, the high frequency is missing. All in all sounding thin, without sound volume.

The headphone itself, driven by a high-quality amplifier, shows quite sound potential. The sound gets more structured and precise. But, compared to the Omega I, still has a lack of bass and sound volume.
The hedphone stesses the amplifier with 160pF/stator, that means a load of 100kohms at 10khz. In contrast to other sources in the internet, where BIAS voltage is stated to be 600V, I have measured a BIAS voltage of 300V, using a measuring bridge that has no load to the voltage source. Unfortunately I could not find any manufacturer’s data to verify this item. The frequency response, measured with ATB PC from Kirchner http://www.kirchner-elektronik.de/gb-atb-pc-pro.php looks like this:

Conclusion:

A headphone with potential for sound quality that is wasted by the supplied amplifier.

Pin configuration of the socket (view plug side):