Source direct, also known as tone defeat or bypass is a sound engineering term. It describes a feature on a piece of audio equipment which allows the audio effect such as bass and treble controls to be bypassed, leaving the sound unaltered by the equipment. This has two functions:
It allows the effects unit to be bypassed without the need to unplug it
It allows the isolation of part of a piece of equipment, such as an audio amplifier's tone controls, without affecting the rest of the functions.
Read more: Wikipedia
