In 1958, John C. Koss teamed up with engineer Martin Lange, Jr. to develop a portable stereo phonograph player with side-wing speakers. The pair demonstrated their new stereo record player at a Wisconsin audio show. One of the product's unique features was a privacy switch, which allowed listeners to hear music by plugging in the world's first SP/3 Stereophone. The stereophones were intended to be an accessory to demonstrate the high-fidelity stereo sound that the portable phonograph player delivered.
The revolutionary sound of the Koss SP/3 Stereophone became the hit of the show.