Koss Electrostatic Line Overview
Koss started making headphones in 1958 with SP-3 stereophone being the first headphone they made. In 1968 Koss introduced ESP/6, one of the first electrostatic stereophones in production. ESP/6 were an unusual design for an electrostatic headphone because it utilized step-up transformers inside earcups. ESP/6 were also terminated in regular TRS plug, but included an adapter that allowed them to be hooked-up to the amplifier or receiver speaker-outs.
Koss continued making ESP line headphones through 1970's, 1980's and 1990's with one or two new electrostatic stereophone models produced throughout each decade. The latter models abandoned the "transformers in cups" design in favor of more common energizer adapter boxes that allowed connection to the speaker out of amplifier or receiver.
The current Koss flagship electrostatic stereophone model is ESP/950. It's the first ESP model to be released with a full functional amplifier and is positioned as a transportable system that consists of headphones, amplifier, various cables, battery / wallwart adapters in a nice leather carry bag.
Koss ESP (Electrostatic Stereophone) Models
Stereophile 1971/1972, Stax SR-3 and Koss ESP9 reviews