Variation of the famous Baxandall circuit is shown in the passive circuit in figure 1. Smoothly increasing +- 6 dB/octave slope of boost or cut is the feature of this circuit. Although the “shelves” are outside the audible range with these component values, the bass and treble filters have a shelving response. The threshold and shelving frequencies is predicted by the filter equations in figure 1a.The wiper shorts out the .033uf capacitor, when the bass control is rotated for maximum boost. A frequency dependent voltage divider that determines the shelf frequency of the boost are formed by R3 and C4. The wiper bypasses the 10K resistor. C1 and R2 form a high pass filter, when the treble control is set for maximum boost.
The circuit uses commonly available parts to simplicity. Radio Shack sold even the 100k log taper pots, which are usually hard to find, as part number 271-1732 (in a stacked configuration for stereo). The pot will have about 10K on one side and 90K on the other at the midway point.The side with 10K parallels R2 for the treble control. The side with 10K parallels C4 for the bass control. Note that this circuit could be divided for an individual bass or treble control (R5 may then be omitted – it helps isolate the bass from the treble circuit when the two are put together). To avoid loading the network and affecting the response curves, the passive Baxandall must see a low impedance source and drive a high impedance load. [Circuit’s schematic diagram source: Rudolph Graf and William Sheets]