At the most basic level, power amplifiers convert a weak input signal into a more powerful signal – with minimal distortion. On its own, a weak audio signal (from a mixing board, preamp, etc.) produces a modest electrical current. But this current isn’t powerful enough to move the cones within loudspeakers; these cones are vital for translating electrical signals into audio. Power amplifiers, then, act as a middle man; they boost weak input signals to line levels that are compatible with speaker systems. For example, a power amp might take a 700 microwatt input signal, and convert it to a 700- or even 7,000!- watt signal. High-quality power amplifiers do this without distorting, or otherwise jeopardizing, the original audio signal waveform.
A phono preamplifier is an electronic device that amplifies and equalizes the analog output of the cartridge in a phonograph turntable. Typically built into an A/V receiver, phono preamps also come as external devices for units without phono inputs as well as for audiophiles who want the highest-quality restoration.