The BQC Control is an active EQ circuit for bass guitars. They allow you separate control over bass, midrange, and treble all via 2 concentric/stacked pots. Rotate 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. The BQC works with active or passive pickups.