A headphone extension cable is one of many kinds of interconnects. Audio systems use interconnects to connect one element of a home audio system to another. Make your own interconnects to learn basic soldering and electronics skills, and have exactly what you need to wire your home theater. Headphone extension cables have a simple construction due to having only three wires. One wire transmits the left channel, another transmits the right channel, and the third connects to ground.
Strip 1/2 inch of insulation from both ends of a three conductor cable. Strip the three wires inside once exposed. Plug in the soldering iron.
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Slip two pieces of heat shrink onto the cable. Place them in the middle so that they don't prematurely shrink from the heat of the soldering iron.
Unscrew the male and female 3.5mm jacks, exposing the terminals inside. Slip the housings onto the cable. The longest terminal is the "sleeve" and connects to ground. The middle length terminal is the "ring" and connects the right channel. The shortest terminal is the "tip" and connects the left channel.
Choose a wire and wrap the bare copper around the male 3.5mm jack's sleeve. Wrap the other end of the wire around the female 3.5mm jack's sleeve. Then wire the male's tip to the female's tip and the male's ring to the female's ring.
Melt some solder onto the soldering iron. This checks the temperature and helps solder flow.
Place the soldering iron underneath the male jack's sleeve, and feed a small amount of solder on top of it. Once you remove the soldering iron, there should be a shiny joint. Solder each wire to each terminal.
Wrap any bare copper connected to the tips and rings of both jacks with electrical tape to prevent short-circuits. Screw the housing back onto the jacks.
Slip a piece of heat shrink over the housing of each jack. Heat them with the heat gun.