Seven-pin XLR connectors are used to connect some valve (tube) condenser microphones to their power supplies (carrying signal, polarisation voltage, heater, and HT).
Used by several models of Le Maitre and Ultratec fog machines for remote control.
An obsolete use for seven-pin XLR connectors was analog lighting control signals, as well as for wired intercom in broadcast studios, specifically with Ward-Beck intercoms.
