This shelving filter was obtained serendipitously by combining an op-amp integrator stage (bottom) with an all-pass filter stage (top). The top stage output is the filter output.
I have not done the analytical determination of the transfer function down to the resistance and capacitance of each component. If you wish to do so, This could be done either through FACT – fast analytical circuit techniques, or automatically with some Ltspice adjunct program like SLICAP.
Thus the behavior was mostly determined empirically by sweeping all resistances and capacitances, with resistances replaced by potentiometers.
It exhibits resonance. Most shelving filters do not exhibit this feature. It may be desired in some cases.
It can also switch between a high shelf and a low shelf behavior.
Control is a bit touchy as the filter shows a certain degree of inter-dependence with respect to some potentiometer effects. (Certain potentiometers affect several characteristics at once)
It requires the TL072 model for the op-amps.
There is a wave input if you wish to test it on a wave file. The filename is input.wav and the output filename is output.wav. Connect the “in” net instead of the V5 source and change the simulation to .tran
Don’t forget to set the transient analysis to the length of the input wave file.
The zip below contains the ASC file and some screenshots of the frequency response while stepping some potentiometers.