Boost: The Boost effect was created to increase the volume (gain) of the guitar’s signal before it hits the input stage on your amp. It does this without adding distortion to the signal or making any EQ adjustments. The output from the boost pedal is clean, or some would call transparent. A common use for the boost pedal is to increase the volume of a guitar solo to stand out above and beyond the other instruments. Many effect pedal manufacturers have added a separate boost switch into their other effect pedals to create an easy to use pedal combination.
Simple Definition: Boost adds volume.
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