Mad Professor BLUEBERRY Bass Overdrive Bass Effects PedalCONTROLS VOLUME: controls output. DRIVE: sets the amount of distortion and sustain, as well as the type of distortion. lower settings give dynamically controlled overdrive, the unit distorts more the harder you strike the strings and there is no definite clipping point, but rather different shades of distortion as the amplitude changes. Higher settings will give more compressed sounds. a good starting point with most basses is fully CCW, turn the knob