Technically speaking, a pickup is an audio voltage source followed by a second-order lowpass filter. The tone (colour) of the pickup is characterised by its cut-off frequency and its resonance contour — the so-called Quality factor or simply Q.
The Resonant Filter allows you to dial in the characteristics of virtually any pickup with two high quality dual-ganged Bourns® potentiometers. The Resonant Filter offers independent control of the cutoff-frequency and resonance (Q). The Resonant Filter is a single opamp VCVS second-order low-pass filter design with variable frequency, and variable resonance with constant gain compensation. The circuit utilizes the audiophile grade OPA209, a modern precision operational amplifier with very low voltage noise density of 2.2nV/√Hz, and a rail-to-rail output swing with a supply voltage from 4.5 to 36 volts — maximizing the dynamic range. The Resonant Filter module utilizes high quality gold-plated pin connectors.