Abstract: A parametric equalizer has an audio filter with a plurality of electronic components. A first control knob (32) controls a center frequency of the audio filter. A second control knob (34) jointly controls signal level and bandwidth of the audio filter. The second control knob uses a potentiometer (120) with variable resistive elements on a common shaft (122). The variable resistive elements (56, 110) are coupled to nodes within the audio filter to jointly control the signal level and bandwidth. The control knobs are mounted to a control panel (30) for user access. A musical instrument, such as an electric bass guitar, generates audio signals, which are routed to a pre-amplifier (16) containing the parametric equalizer. The equalizer conditions the audio signals, which are boosted with a power amplifier to drive a speaker system.