If I understand correctly, a wah effect is just a variable filter, right? I was just thinking, couldn't you build a wah effect very easily out of a dual pot? One pot would turn clockwise to increase a high-pass filter while the lower pot would turn clockwise to lower a low pass filter. And vice versa. Wouldn't that accomplish the same thing minus a 9v?
Yeah but a wah is a booster, it not only takes away certain frequencies it also adds amplitude to the selected ones. You can do it - There was an old company whose name I can't remember that made a passive wah, like a tone control. They sound ok with a booster before and after, but are better for leaving in a fixed position.
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I would think you could build a wah with a single pot... since that's what a wah uses.