E. J. James described a two-knob tone control in 1949 with easily selectable boost/cut depths as well as cutoff frequencies. This design was later popularized by P. J. Baxandall to provide negative feedback in active circuits, and was subsequently popular in many high-end amplifiers. Here, the authors analyzed the circuit in the s-domain, preserving parametric control of bass and treble potentiometer values. Poles and zeros were found using Ferrari’s solution to a quartic equation, followed by bilinear transformation to the z-domain, and finally lumping into second-order sections to produce a computationally efficient and faithful emulation of this classic tonestack.
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