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Good, affordable entry level electrostatic headphone
The Koss ESP/95X is very comfortable and very lightweight for extended listening sessions. The plastic construction is somewhat fragile. The headband clamping force is light and the large, oval circumaural ear pads are comfortable. Earcup position is less sensitive than that of dynamic driver headphones.
The overall sound signature is polite bass, natural mids, smooth highs, and large sound stage. The ESP/95X is non-fatiguing, easy listening for extended periods. It lacks the punchiness of dynamic drivers, but the articulation of the mid bass through lower treble range is very good. The upper treble is slightly laid back. The sound stage is big with good width and height, but constricted in depth and dynamic range. These shortcomings could be due to the energizer unit rather than using a true amplifier (at significant additional cost) as with my Stax Lambda Pro and SR-507 electrostatic headphones. I have not tried using an adapter cable to drive the ESP/95X using the rather bulky Stax Class A amplifier unit.
One issue I encountered with my unit is susceptibility to AC hum/EMI interference due to the unshielded plastic energizer case. I had to replace another system component's wall wart power adapter to eliminate AC hum picked up and amplified by the ESP/95X energizer unit. Hum was never an issue with this other component until paired with the ESP/95X.
The earcups do not swivel about the vertical axis, but it was a nonissue for me. The headband adjustment sliders do not have detents and the friction is too loose for my preference. The dual concentric volume control knob is a major disappointment. It is loosely ganged to provide balance control as well as volume, but it is too loose. I wrapped some electrical tape around both knobs so they rotate together without having to fiddle with (or simultaneously gripping) each knob when changing volume.
The ESP/95X is a good, affordable entry level electrostatic headphone. They complement, but do not replace a comparably priced set of dynamic driver headphones. Lifetime warranty is a major plus.
Would recommend to a friend.
The Koss energizer is a "true" amplifier.
Using two push-pull differential class AB output stages per channel.
Each channel uses twelve FET's in the output stages, for twenty four FETS's total.
Each channel can output up to 2,200 Volts Pk-Pk from +-600 volt supplies.
I modified mine to reduce the supply voltages to prevent static discharges in humid air (what people are calling scratching and hissing).
Since the output volume available is ear damaging, this only drops the maximum volume by less than 6 dB.
Have you encountered any bottoming-out with enough volume? If I play a tone at max volume, I can only rotate the pot to about 80% before it starts clipping. My right cup is also significantly quieter than the left.
Then you should send it to Koss for warranty. I had left and right balance difference issue and noise issues so I had it sent for warranty. When it came back, there was no noise and left right balance is perfect.