A connection found on many guitar and bass amps that is designed to accept the output from either an effects processor or an external preamp. The power amp input connection feeds the unit’s internal amplifier, which in turn drives any speakers that may be connected (either internal or external). In most cases, a power amp input is found complementing a preamp output – the two connections, when used together, form a simple effects loop.
Despite the somewhat confusing nomenclature, a power amp input connection should NEVER be connected to the output of an external power amplifier.