I read about active filters at Elliot Sound website, and the SVF caught my attention.
I opened LTSpice and began running simulations.
I followed the guidelines, added an output buffer in order to use a quad opamp, and after some tweaks here and there... voila!, a wah wah with two modes as shown below.







At first I was getting Mode 2 curves, but I thought "what if I change only one resistor, the other fixed at some value", and suddenly I had Cry Baby-like curves.
Since the change is very easy to do with a single switch, I decided to keep both modes. The switch is not shown in the schematic but you can figure it out how to place it
Another thing I noticed is the wide range possible with only 50k.
C1 and C2 do not need to be equal, use them to tune to taste. Any value in the range 27n to 47n is fine.
BTW, I drew a simple mechanism to rotate a dual gang pot about 90°.  A thin zinc sheet works fine here, and it's easy to cut with a normal scissor.
LED/LDR can also be used.

Enjoy!

mac

ps: my laptop mic stopped working, windows you know, so no sound sample yet.

Great work Mac
I'll have to look up the data sheet for that op amp.
Thanks for sharing

SVFs make excellent wahs.  Thirty odd years ago, I installed the EPFM Super Tone Control - a SVF - in a wah shell.  I imagine the dual-ganged pot needed to sweep it did not have both halves tracking each other especially well, but it sounded great in spite of it.  All modes - lowpass, highpass, notch, and bandpass - sounded terrific and were musically useful.

QuoteI'll have to look up the data sheet for that op amp.

Use any quad, double or single opamp, TL071/72, RC4558, LM741, etc.
I placed a LT1001 because it is the first one in LTSpice opamp list 

QuoteI imagine the dual-ganged pot needed to sweep it did not have both halves tracking each other especially well, but it sounded great in spite of it.

Tolerance is not critical here. LDRs, dual mini pots, they are ok.


Is the mechanism easy to understand?

mac

Thanks for this. So the ouput taken from U3 would give a LPF filter?

I have an expression pedal with a 10k pot, and would like to use it to control this wah circuit. Can anyone suggest a simple (i.e. one or two transistors or an opamp or two) circuit that could use a 10k pot to control the brightness of two LEDs simultaneously to allow vactrol control of R4 and R5? Ideally, it would have a trimmer to tune the respponse of the LEDs to the pedal.

The power of whisky compells me, but the Lovetone Meatball's expression pedal fakes it pretty well. Plus it has LP, BP, and HP out.

Quote from: nocentelli on June 15, 2014, 04:00:48 AM
Thanks for this. So the ouput taken from U3 would give a LPF filter?

I have an expression pedal with a 10k pot, and would like to use it to control this wah circuit. Can anyone suggest a simple (i.e. one or two transistors or an opamp or two) circuit that could use a 10k pot to control the brightness of two LEDs simultaneously to allow vactrol control of R4 and R5? Ideally, it would have a trimmer to tune the respponse of the LEDs to the pedal.

Have you 'searched'?   There have been many little snippets designed to allow you to control an LED (or 2) using your wah/expression.   At root, they're just an opamp or transistor that illuminates the LED depending on the position of your pot (of course).
 
Only tricky part is probably being able to trim the range to fit the pot's travel, as you mentioned (trimmers....).

BTW: Mac - GREAT project, and one I am sure I will be experimenting with - I see your nice "1k, 5k, 10k" etc labels on the plot.  Did you add those after, or is there a command in LT Spice that will do that for us?  I haven't found one after much looking.  Thanks.....

http://tech.thetonegod.com/rockncontrol/RockNControl_2Pole_Schematic.gif

This looks like a good place to start, might try it tonight.

QuoteI see your nice "1k, 5k, 10k" etc labels on the plot.  Did you add those after, or is there a command in LT Spice that will do that for us?  I haven't found one after much looking.

I used Photoshop, and OS 9 Charcoal fonts converted to XP

Quotehttp://tech.thetonegod.com/rockncontrol/RockNControl_2Pole_Schematic.gif

This looks like a good place to start, might try it tonight.

403 Forbidden  %&$#%@#$

QuoteThanks for this. So the ouput taken from U3 would give a LPF filter?

Yes.
http://sound.westhost.com/articles/active-filters.htm#s5

mac

The EBS bass wah pedals are very similar. They just use the SSM2164

http://tech.thetonegod.com/rockncontrol/rock.html

Quote from: mac on June 15, 2014, 01:20:26 PM

QuoteI see your nice "1k, 5k, 10k" etc labels on the plot.  Did you add those after, or is there a command in LT Spice that will do that for us?  I haven't found one after much looking.

I used Photoshop, and OS 9 Charcoal fonts converted to XP

Thanks, well done