BOSS PQ-4 PARAMETRIC EQUALIZER

Boss Guitar Effects Guide Book vol.9 (1993):
For the first time, a compact pedal with concentric knobs for presence and three frequency bands. Each frequency band and equalization curve is ideally suited for guitar use, and each control adjusts over a +/-18dB range. The PQ-4 allows users to get a variety of sounds comparable to using several guitar amps, or different types of pickups and guitars.