Cross-Fading Capacitors

[MoltenVoltage]

I picked up a used cry-baby and can't wait to hack it, as it can't yet hold a candle to my old Vox wah.

There is a capacitor mod where you swap the sweep cap - .01 cap for a .022, but according to Joe Gagan, this dramatically changes the center frequency.
http://www.diystompboxes.com/smfforum/index.php?topic=95255.0

What I'd like to do is use those values, but vary the capacitance between the two extremes.

My gut tells me you can't just hook up a pot and cross fade the two caps since you are also introducing resistance, but I am guessing there is a way to accomplish what I am imagining.

Thanks!

[Fender3D]

You may build and adapt a capacitance multiplier with any op-amp.
Check in the application notes...

[earthtonesaudio]

Crybaby already is a variable capacitance multiplier.

With the .01 the freq range is from ~328 to 2000 Hz,
With a .022 it becomes ~220 to 1370 Hz.

So you could simply double the gain so that it sweeps lower with the existing .01 and it will still sweep approximately as high as before (1900 Hz).  But this does depend on how much of the potentiometer you are using.

[PRR]

> gut tells me you can't just hook up a pot and cross fade the two caps

Switch.

[R.G.]

Note that the frequency of an LC circuit changes as the square root of a component change. This is one reason that the range of the LC based wahs can't be made very wide. Doubling or halving the cap only changes the range by 41%.

If what you want is to experiment to find the perfect range, do that by subbing. If you really, no-fooling want to adjust the value and dial it in on the fly, that's going to be tough. You're correct that introducing a resistor will be a problem, as this will add damping directly to the LC resonance and kill the peakiness at some settings.

The only good ways I can think of to do this is either a floating capacitance multiplier, which is a bit tricky to do well and more complicated than the entire wah circuit, or a PWM'ed capacitor, duty cycling the cap downwards from a max value in the same way a resistor can be increased by PWM. This carries the usual issues with switching audio in terms of sampling rate, clean switching, clock bleed, etc, etc.

I did once see a wah made from one of those huge old rotary plate radio tuning caps. It worked fine - but boy was it bit and ugly.