Resonant low-pass filter with variable peak, maybe passive?

[Balthazar]

Hi

Last night, I got a really awesome sound out of my telecaster by running it through a wah-wah at a constant position, and playing on the low strings with a little overdrive. It got really punchy and cool, but it is a bit tricky to find the right position on the wah. So I got an idea: How about making a stomp box which does the same thing, except with a regular knob to turn with the fingers instead of a pedal for the foot? Then I took the idea a little further. As I understand, a wah-wah pedal is a variable low pass filter with resonance. It would be really cool if that low pass filter had another knob to adjust the resonant peak. So that in one position, there is no resonance (regular low pass filter), while in the opposite position there would be a high, sharp peak. I did find the resonance a tiny bit too much on my wah. Searching a little, I found the schematic for the exact same pedal on the forum:

http://www.diystompboxes.com/smfforum/index.php?topic=98514.0

Would anyone here be able to tell what adjustment would have to be made to this circuit to make the resonance variable, like described above? Is that simple, or hard? This is what is called variable Q on analog synthesizers, but electronic textbooks I've glanced at says Q is the width of the resonant peak, not the height.... so I'll avoid using that term.

But then I took the idea even further in my head, and here is where I think the experts will say it's doomed, but it's worth to ask!! The coolest thing for me, would be if I could simply build it into my guitar, by replacing the tone and volume control of my guitar with two controls: Low pass cut-off, and resonance of the filter...... then I would not need an extra stomp box. I couldn't build a whole wah pedal into it, but I thought like this: The electronics in the wah is active. The tone and volume controls in the guitar are passive. That often requires much fewer components. What if my tone and volume was replaced with a PASSIVE resonant filter with a variable resonant peak? It would only be OK if the peak could be taken out completely, so I don't have to always have the peak. Would this be at all possible, in the little space I have in the control cavity of the tele, and are there reaons this might be a silly idea? With my extremely limited knowledge of electronics, I think that a resonant filter would require an inductor? And this might be too big and/or pick up too much hum? Or not? But the pickups themselves are inductors, aren't they, could that be exploited somehow? Or is there already a resonance in the tone filter, maybe? This is too hard for me to figure out, I know very little about electronics, but have good soldering skills. Does any of the forumites know if this can be done, and if so, how?

[nocentelli]

Resonant filters do not necessarily need an inductor, but I assume they would need power (i.e. cannot be passive), since the resonance I think you are describing is a boost at the frequencies just below the point of the LPF cut-off frequency, i.e.

It's perhaps possible you could do this passively (I know there is a line of Cave passive pedals that do alllsort of stuff without power, but since goddamed physics says we can't get something for nothing, I suspect the losses would make the circuit impractical without a gain make-up stage after it anyway. There are a number of modified (active) wah circuits with a "peak" or "Q" control, but I suspect any circuit that requires a power source will require some routing of your guitar's cavity to get it inside the guitar itself.

If you're looking for a resonant filter circuit to put in a pedal, I really like the Eric Archer LPF filter here: http://ericarcher.net/devices/diy-lpf/

It has a mix control as well as resonance and frequency, and sounds very cool - It can easily go into self-oscillation, and the slight distortion is quite acceptable in a guitar application: I'm building a vero layout right now, and I'll post it when it's verifed.

[WaveshapeIllusions]

I'm a big fan of the state-variable filter. One dual pot to vary cutoff and a single to change Q. Good resonant peak at high Q. Gives LP, BP, and inverted HP simultaneously. The downside is that it takes 3 opamps, 4 really since you need to buffer the guitar first. It'd take one quad, which isn't too big though. Hope this helps.

[Jdansti]

Also, checkout R. G.'s article on the technology of wahs. He touches on variable Q in this article.

Click on GEOFEX in the menu at the top of the forum page, then in the upper left corner: >>FX TECHNOLOGY OF THE... >>Wah Pedals.

[ashcat_lt]

There's already a passive resonant LPF in your guitar!  The combination of pickup inductance and cable capacitance make it happen.  It's tough to get a good variable version though.

Two big problems with passive filters:

1) They are lossy.  Yes, you can get real gain in the resonant peak without active components, but it comes at the expense of attenuation across the rest of the spectrum.

2) They are extremely dependent on the impedances around them.  If anything changes at input or output (Change pickups, adjust V or T control, plug into a different amp or pedal...) both cutoff frequency and resonance will be affected.

[Balthazar]

Thanks for all your help. Yes, nocentelli, that's what I meant with the peak. Waveshape, perhaps this might be an idea then: http://www.ti.com/lit/ds/symlink/uaf42.pdf
It's pretty cheap. It includes an extra op-amp. They have a computer program to test different setups as well. We'll see if I am smart enough to figure it out....

[Gus]

IIRC there are fixed wha effects, a wha circuit with a knob control on the box.

search for boss PQ-4

There are inductor-less wha circuits