Capacitor Question

Amplifier power supply

The capacitor between + & - is usually used to fill in the gaps in the "DC" waveform that appear on the output of a rectifier in a power supply that runs from the mains. The gaps will still be there, but very much smaller (called ripple)

If you are deriving your power from the mains then the power supply you are using should have a capacitor - Add one if it's not already there, without it you will get a prominent 100hz buzz in the speakers. Ensure the capacitor has a working voltage higher than 12 volts (shouldn't be a problem) - the bigger the capacitor the less ripple.