Koss · Electrostatic · 1990–today
Koss ESP/950
Koss made the first self-energizing electrostatic headphone, the ESP/6, in 1968, and returned to the idea in 1990 with the ESP/950. Its diaphragm is a polyester film just 1.5 microns thick with a semi-conductive coating, driven by the matching E/90 energizer. Koss pitched the pair as a portable audiophile system. The E/90 can run from a box of six C batteries, and the kit came with a calf-skin carrying case. It's still on sale, and the newer ESP/95X versions use the same energizer. A used one is only worth buying as a complete set, because the headphone ends in a custom 5-pin plug made for the E/90.
ESP/950 (1990)ESP/95XESP/95X Plus (velour cushions, same energizer)
Spec at a glance
- Maker
- Koss
- Type
- Electrostatic
- Made
- 1990–today
- Impedance
- Not rated (an electrostatic; the E/90's input is 100,000 ohms)
- Sensitivity
- 104 dB at 100 V rms (manual)
- Driver
- Electrostatic, 1.5 micron film
- Bias
- 600 V DC, from the E/90 energizer
- Weight
- 353 g headphone; 482 g E/90
- Also listed as
- ESP950, ESP-950, ESP 950, E/90
Before you plug in
What it needs, and what wears out
- Drive it fromNeeds an energizer
- It works only with its E/90 energizer, which supplies the 600-volt bias and takes a line-level signal through RCA jacks or a 2.5 mm input. The E/90 runs from an AC adapter or a box of six C batteries.
- Pads
- Foam-filled leatherette pads around the ear. Koss still sells genuine ESP/950 ear cushions.
- Cable
- Wired in, 47 in. long, ending in a custom 5-pin rectangular plug for the E/90, with a 72 in. extension cord in the box. It isn't a Stax plug.
Should come with it
- The E/90 energizer
- The AC adapter or the battery box
- The extension cord and the input cables
- The carrying case
Check the listing photos against this list, and ask the seller about anything you can’t see. A missing piece should come off the price.
What to check
The main risk is an incomplete kit, a headphone without its E/90 or its power supply. Hum can come from a nearby source of mains interference, and one long-term owner had the plastic holders give out after about 30 years.
- 01Energizer: Check that the E/90 energizer is in the photos and the listing. A headphone on its own needs an E/90 found separately.
- 02Power: Ask whether the AC adapter or the battery box is included and whether the E/90's LED lights up. A steady LED with fresh power is what you want.
- 03Both sides: Ask the seller to confirm both sides play at the same volume with no crackle. Electrostatic faults show up as noise or a quiet side.
- 04Pads: Look at the leatherette pads for cracks and flaking. Genuine replacements are sold, so worn ones are an easy fix.
- 05Plug and cord: Look at the 5-pin plug for bent pins and the cord for splits. Both are wired in, so damage means a repair.
- 06Holders: Look at the plastic parts that hold the cups to the headband. They should be uncracked, with both cups secure.
New to used headphones? How to check a used pair covers pads, cables, channel dropouts and fakes, and impedance and your amp explains what will drive it.
Find one on eBay
We haven’t been through this model’s listings yet. Search eBay and run every listing through the checklist above. Ask for close photos of the pads, the headband and both ends of the cable, and pass on any seller who won’t send them.
Search “Koss ESP/950” on eBay ↗eBay searchCompare with
Stax
ElectrostaticStax SR-Lambda Pro
- Years made
- From 1982
- Impedance
- 129,000 ohms
- Drive it from
- Needs an energizer
$750–$1,125 typical ask
Stax
ElectrostaticStax SR-Lambda (Normal bias)
- Years made
- 1979–1982
- Impedance
- 122,000 ohms
- Drive it from
- Needs an energizer
$550–$1,025 typical ask
Koss
Closed-backKoss Pro/4AA
- Years made
- 1970–late 1980s, and again since
- Impedance
- 250 ohms
- Drive it from
- Wants a headphone amp
$45–$90 typical ask