The BQC System HZ is a complete control system for bass guitars using passive pickups or EMG HZ pickups. Active Balance, Master Volume and EQ circuits are included. The EQs 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