varitone solution uses only passive filters with a fixed number of capacitors and then
it’s a discrete solution, I think because the difficulty to get a variable capacitance to the audio
frequencies or even because the capacitance changes without keeping the Q factor constant.
They are used in old radio receivers but are pretty bulky.
The solution that could fit for you is in the circuit I’ve attached below, with an active filter, which uses
an operational amplifier to emulate a variable inductance and then get the same result of shifting the
resonance frequency.I’ve tried with different fixed capacitors value
together with the variable inductance, 10nF, 2nF, and 470pF and I’ve found these results.At
low frequency there’s a quasi constant x-shift of filter response.
You can run the simulation with the 9.1 PSpice student version.
I hope it can be useful.