An audiophile preamp accepts audio inputs (mostly analog), switches the inputs and raises the level of the selected input from line level to a point where a power amplifier can boost the sound to a more meaningful volume.  Audiophile preamps can take digital inputs. Audiophile preamps can be solid state or tube designs. Preamps can also be merged with power amps to make an integrated amp.

To read reviews of many of the best affordable as well as audiophile grade stereo preamps - check out this audiophile resource page: