The most common form of active bass and treble tone control circuit, based upon British engineer P.J. Baxandall’s paper “Negative Feedback tone Control — Independent Variation of Bass and Treble Without Switches,” Wireless World, vol. 58, no. 10, October 1952, p. 402. This is the formal name for the grandfather of the type of EQ controls found in most analog equipment in use today. The Baxandall design is distinguished by having very low harmonic distortion due to the use of negative feedback in the circuits, which among other things helps keep more precise control over the behavior of the op amps. Negative feedback basically works by sending a polarity-reversed copy of the output signal back to the input of the amp. If this signal is filtered by components that change the relative frequency content you have an active tone control.