How to Wire a Headphone Jack Into an Older Home Radio | It If you are using a closed circuit jack where the speaker disconnects when the headphone is plugged in, there is a pair of unmarked leads on the side of the jack. Solder a wire from the positive speaker output on the amplifier to either lead on the jack and another wire from the other lead on the jack to the positive speaker pole. Headphone Jack Schematic Diagram | Electrical Wiring With a headphone jack, insert a headphone plug with known wiring scheme and use an ohmmeter or continuity tester to match the jack connections to the plug. Note: Headphone jacks are sold in open-circuit and closed-circuit versions. The open-circuit jack is general purpose and has 3 pins for the left and right channels and ground.