Product introduction CP-40 is a preamp and D.I. Box for acoustic guitars. When using this D.I. Box, you must use the INPUT knob of this D.I. Box in order to avoid overloading your acoustic guitar E.Q. If overload occurs, please reduce the volume. 2.Turn on the ANTI FEEDBACK Foot switch, and adjust the ANTI FEEDBACK Knob to avoid volume feedback. 3.Turn on TREBLE BOOST Foot switch, to increase the brightness of the tone.