A VU meter is an essential device that lets you monitor the audio levels in a circuit. This tells you if the volume of the audio is too loud for your equipment to handle, which would cause distortion, a usually undesirable effect caused by overly loud audio. Based on old analogue technology, the meter has a set of numbers written on it. In the negative range (say -48 to 0) the audio is at the appropriate level. Above 0, in the positive range, the signal is too loud and distortion, also known as clipping, will result.
Turn off and unplug your amplifier. If the amplifier has been on, wait about a half-hour for it to cool down.
Locate the screws holding the amplifier cover in place. Refer to your instruction manual for their exact location.
Remove these screws and remove the cover.
Locate the output circuit on your amp. This circuit will be the connection between the amplifier and the output jack, so the last set of wires you see running to the output are the cables you are looking for. On a unit that is built into a speaker, the circuit is the connection between the speaker itself and the amplifier, rather than the output.
Find the last two wires leading directly to the speaker output. One will be grounded, and the other will be live. To tell the difference, look to see if one of the cables to soldered to what appears to be nothing, like the case. This one is the ground.
Connect the VU meter to the amplifier by connecting the live wire to the terminal marked with a negative sign and the ground wire to the terminal marked with a positive sign.
Connect the output from the meter back to the speaker output, connecting the positive output to the ground and the negative output to the speaker.
Never work on electronics while they are still plugged in, even if you've already turned off the power.