In the hi fi world normally the power amplifier works in pair with the preamplifier.
The preamplifier allows to connecting many audio sources as: cd, every kind of players, turntables, microphones, radio receivers and others, and it has the selector's function of the sources connected.
In some models of preamps it's also possible to make the tone's regulation as well as the volume.
The hi-fi amplifier receives the audio signals already adjusted from the preamp to amplify them without generate any kind of noise or audio imperfections.
Thus, an hi-fi audio system which has an preamplifier and and a final amplifier, divides the amplification stadiums which need a lot of electrical power to generate the power acoustic to transfer to the speaker system, from the high gain stadiums that they work in electrical voltage which are connected all the audio devices.
The hi fi stereo system which adopts this type of signals amplification's choice allows a cleaner audio signal without imperfections.