Q factor MOD - Wah pedal

[Chrismoi]

Hello everybody,

I have made a successful replica of the classic Vox V847 crybaby, but it seems I cant variate the Q by adding a 100K pot in series with the 33k resistor.
Sliding through the pot doesnt make any significant audible changes. I am not sure if this is by design, i mean if there is an upper Q limit, further which I cant go.

Any thoughts on that?

Thanks,
Chris

[Marcos - Munky]

Welcome to the forum.

According to RG's awesome article on wah pedals (http://www.geofex.com/article_folders/wahpedl/wahped.htm),
"This resistor is the primary determiner of the Q, or sharpness of the bandpass/resonance effect of the filter Values lower than 33K make the filter less sharp, reducing the quality of the wah effect. Values up to 100K contribute to sharper, peakier, more resonant tones. If it gets too sharp, the wah effect can be lost because it may not hit harmonics to emphasize."

Since you added a 100K pot in series with the 33K resistor, your Q resistor is going from 33K to 130K. You're getting only the upper part of the variation and going past the limit. You should remove the 33K resistor and replace it with a 100K pot, so you can use the full effective Q range.

[PRR]

The 68k, and the input of Q1, also shunt the coil. So making the 33k larger may have little effect.

Trying to hit HIGH Q in a 1 or 2 transistor thingie is tough. What opamp Wahs are there?

[ElectricDruid]

I reckon variable Q in wahs is over-rated anyway. It *sounds* like a good idea, but in practice, the range of stuff that's usable is fairly small. Too low a Q doesn't give you enough "quack" and too high a Q doesn't catch a broad enough bit of signal and just gets thin and whistley.
I'd probably swap the 33K for a 22K or 15K or something, and then use a 47K/50K pot. Dunno. Experiment, but my bet is you'll never use the extreme settings.