A company which, in the 1980s, offered a variety of add-on products for synths. Some of these were kits to add MIDI capability to pre-MIDI synth designs; others did things like expand patch memory or add additional filter capability to existing synths. A notable product was the "gen mod" for the Mark I version of the E-mu Emulator, which, as it came from the factory, had little performing capability beyond the ability to trigger samples and play them back at a desired pitch. The gen mod added voltage controlled filters and amplifiers to the Emulator; one big benefit of this was control of the dynamics and duration of the played samples, instead of just playing a sample to the end each time a key was pressed.
JL Cooper was successful in marketing products that added capability to synths, but in a way this was also the undoing of this line of business, since manufacturers often observed what the JLC products did and then added the capability themselves to revised or later models. By 1990, the synth market had largely moved to digital models which were much harder for users to modify. Cooper moved away from synths and into studio equipment, starting out with MIDI interfaces and similar products. Today, JLC still produces some studio MIDI gear, but mostly concentrates on video production equipment.