For EMG pickups only. This system features a pre-wired balance control (blend), master volume, separate bass and treble pots along with the concentric/stacked mid-control. All the EQ controls and balance control feature center detents. This active EQ circuit for bass guitars works by rotating the control clockwise to boost, counterclockwise to cut. There is a center detent for flat response. The mid-range control has a variable frequency knob that allows you to sweep through the mid-frequency range from 100Hz to 1KHz. This selects the frequency to boost or cut.