Resonance peak control for guitar?

[shapein]

Hi Spicers,

I build electric guitars and would love to have a control that works by changing the resonant peak of the pickup, not just by bleeding frequencies to ground like tone controls do.

A changing resonant peak (that is, a curve that stays basically the same, but which can be displaced horinzontally along the x-axis) will create sounds with different characters; if I am not wrong, a regular “tone” pot affects the HEIGHT of the resonant curve (its peak), affecting the tone alright, but also creating a flatter curve, a sort of an “unimpressing” sound, lacking character (that’s why many guitarists left the tone pot at “10” and equalize on the amp instead, which is not perfect, but is “less worst” :).

I know about the “varitone” controls, which use a rotating switch wired to several, differently valued capacitors. I am looking for the continuous (no pot detents) version of that, like the one you would get by using a variable cap.

Thanks in advance for any advice you could give! 

[studio2100]

Hi shapein,

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.
greetings

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