Hello
I'm not an electrical engineer, the only thing I know for sure about electronic devices, is that you should never take them into the bathtub with you.
That having been said, a source like a CD player, or a microphone outputs a modulated electrical current at very low voltage. This signal is directed to the preamplifier which acts like a center on a basketball team, it sends the signal to the right place. For instance, you have a number of sources - CD player, record player, FM tuner, the preamp allows you to select the source and direct the signal from the source to the amplifier.
This low voltage signal is insufficient to drive speakers which are essentially mechanical devices so the pre amp sends the low voltage signal to the amplfier which amplifies the signal - that is, the low voltage signal enters the amplifier, which then amplifies the signal across the frequency range of the signal. Low voltage in, higher voltage - proportional across the frequency range - out. An amplifier has a power supply which provides the amp with the power to bump up the signal.
The ideal audio amplifier would instantly convert the low voltage current input to a proportionally higher current output across the entire frequency range. In other words, the ideal amp would "disappear" in your system, and act as if a larger current had been produced by the source in the first place.
Do not take any amplifier into the bathtub with you, especially if it is plugged into the wall socket.
