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An audio amplifier setup will have a power amplifier and usually a preamplifier. 

The preamplifier takes the weak signal from an input device, for example a microphone, and increases its voltage level so that the power amplifier can use it. 

Some preamplifiers simply switch inputs without changing the signal.

The power amplifier increases the current of the signal so that it can drive the speakers.

If the signal is amplified too much, it becomes distorted and it may need to be filtered through an equalizer before going to the speakers.