ESP/950 Electrostatic Stereophone

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ESP/950 Electrostatic Stereophone System - Overview

Koss ESP/950 and E/90 amplifier
Koss ESP/950 and E/90 amplifier

ESP/950 is one of the few non-Stax electrostatic headphones currently in production and has the largest diaphragm surface area of any electrostatic headphone being made. It is the first Koss electrostatic to be sold with a direct-drive amplifier rather than an elaborate "energizer" (step-up transformer box with included bias supply). Confusingly, Koss still calls the E/90 amplifier an "energizer". The E/90 requires 12VDC, allowing battery operation.

The Koss ESP/950 shares a frame with the Koss A/250, an out-of-production top-of-the-line dynamic headphone which was part of Koss' Auditor Series of the 1990s which included the A/130 (still in production) and A/200. It's possible that the A/200 shares the A/250 frame as well, but due to the A/200's rarity no one has been able to confirm this.

Per Koss' website:

Image:ESP950 1.jpg

The ESP950 Electrostatic Stereophone System is the pinnacle of audio reproduction. The standard upon which all other electrostatic headphones can be measured. A professional listening system so distortion free, it literally pushes the limits of the world's most sophisticated instruments that attempt to measure its performance. The result is an experience that no other stereo system can approach. A revolutionary design in electrostatic transducers which includes cross matched mechanical damping and semi-conductive coating. The result is an ultra-flat frequency response across the entire range: deep, clean bass with real power that other attempts at electrostatic transducers can only envy. You would expect nothing less from the people who introduced the world to private listening with its first SP3 stereophone in 1958.

The ESP includes the Electrostatic Stereophone, energizer, battery pack, leather carrying case, connecting cables, and 120 VAC adapter. This unit is equipped with soft, vinyl closed ear cushion design for isolation. The Electrostatic Stereophone features pivoting ear cups and adjustable, detachable headband for comfort and convenience. The ESP950 has a straight, dual entry 4 foot cord/6 foot extension.

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.

General Description of the Sound

My impressions taken from this comparative review:

ESP/950 plugged in SRM-1Mk2Pro is a great headphone. It can do everything a almost any Lambda can but it has more bass slam and seems to be even faster with better attack. The soundstage is also a bit different, as Koss is even more forward in the mids than SR-Lambda. Think of Koss as SR-Lambda on steroids. Well balanced and somewhat laidback, with great tone, but forward mids. With Koss possessing so much of SR-Lambda’s best qualities it brought it up to LNS level but with it different presentation of soundstage and details that gave Koss a nice edge over SR-303 and especially SR-Lambda. I still think LNS is a more neutral and accurate headphone, but Koss seems to be better when it comes to very technical music as it can really become a speed monster no Lambda can.

Pricing & Other Data of Interest

Since its introduction in the early 1990s the price of ESP/950 system has been decreasing steadily from about $1200 to $900 and currently $700 - $600.

At the time of writing, a good online-auction price for a used ESP/950 in better-than-average condition is $350 - $400.

Pictures of Koss ESP950 complete system

Pictures (by AudioCats) of ESP/950 driver and earcup construction

Technical Specifications

ESP/950 Headphones:

Frequency Response: 8-35,000 Hz

Impedance: 100k ohms

Sensitivity: 104 dB SPL/1mW

Distortion <0.01%

Diaphragm radiating surface area: 46.6 sq. cm

Diaphragm: Ultra-low mass 1.5 micron C grade polyester film electron beam coated with proprietary semiconductive material 250 Å thick

Acoustic design principle: Circumaural open air design incorporating minimal acoustic damping

E90 (amplifier for ESP950):

Frequency response: 1.6 Hz to 50,000Hz (-3dB points) 100 vrms differential output.

Input Impedence: 100k ohms

Input level: 1 Vrms (for full power output)

Voltage Amplification: 60 dB

Audio output voltage: 600 Vrms differential, 2300 Vrms push-pull with soft limiting

Channel Separation: - 80 dB at 1 kHz, 100 Vrms differential output

Signal to Noise: -100 dB at100 Vrms output

Power consumption: 320 mA DC current at normal listening levels


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.

ESP/950 Cable and Plug Connection details

. . 1. . 2 . .

3 . . 4 . . 5

1 - right side, outward stator (away from your ear)

2 - left side, outward stator (away from you)

3 - right side, inward stator (near your ear)

4 - diaphragm bias

5 - left side, inward stator (near your ear)

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