the low-level amplifying stages in a guitar amplifier This is where the tiny signal from the guitar pickup is amplified and shaped for the desired tonality before being sent to the power amplifier, which generates the high power signal needed to drive the speakers
An amplifier that is used before the main amplifier in a system Usually this is an amplifier that takes microphone level signals and brings them up to line levels
An amplifier circuit that takes the signal from an instrument (or in the case of an onboard preamp, a pickup or set of pickups) and alters its impedance and/or amplitude, thereby preparing the signal for a power amp Preamps may also include EQ sections, compressors, effects loops, etc See Bass Amp Basics, July '92