Conference tekvax::electro_hobby

>   Wah-Wah pedals use, I believe, a high Q passband filter with a
>   sliding center frequency which is controlled with a pedal.

I would have guessed a variable lowpass filter.  Maybe some peaking
around the cutoff point might have an interesting effect too.  But I'm
not a musician.


>   In particular, two things "bother" me : 
>   - the lack of symmetry (One inductor) in what seems otherwise to be
>     to be a Twin-T type filter (but I'm only guessing - is it a PI type?)

There's more to this circuit than meets the eye.

The filter doesn't seem to be a twin-T.  Re-draw it.  The input to the
filter is through the 470K resistor.  I think maybe it's more like a
low-pass filter??  Or maybe bandpass?  It's part of the feedback
branch around the first transistor.

Then there's another feedback branch by way of the second transistor
(emitter-follower), which seems to have that potentiometer in series
with its base, perhaps as a sort of crude gain control?

>   - the fact that a variable resitor is able to modify the center frequency 
>     in what seems to be a L (and hence F depends on L and C) based circuit.

The transistors have a lot to do with how this circuit works; it's
more than just a passive filter with transistors added for buffering
or gain.  By combining signals having different phase shifts and
amplitudes, and varying their relative amounts, that is apparently how
they achieve the desired effect (F depends on R).

I haven't figured out how the circuit works, these are just some
guesses.

Andy

>>   Wah-Wah pedals use, I believe, a high Q passband filter with a
>>   sliding center frequency which is controlled with a pedal.

In general, no.  

>I would have guessed a variable lowpass filter.  Maybe some peaking
>around the cutoff point might have an interesting effect too.  But I'm

In general, yes.  Though newer effects pedals may do the more complex
filtering of variable Q, variable center frequency, the traditional
Wah-wah pedal was just a foot operated version of the passive tone control
that would be mounted on the guitar itself.  This is a variable cut-off
low-pass filter.

Try the NAPALM::GUITAR notes file for more current informed opinion.

- tom]

    
    The NAPALM::GUITAR guys seem to indicate that Wah's are sliding
    frequency bandpass filters.
    
    I guess I just have to build one and test what the circuit 
    actually does !
    
    Thanks
    Bill
    

    The circuits I've seen for WahWahs (many years ago) was that it was a
    variable frequency notch filter, cutting out a portion of the frequency
    band rather than enhancing it.  I suppose the opposite would do
    something interesting as well, though.

    That makes sense actually. Building it will certainly help 
    solve the mystery ...
    
    Thanks
    Bill

If you've ever shopped for a guitar special effects device, you will be aware
that if a transformation can be applied to a sound signal, someone will
have tried it.
I built my first fake wah-wah pedal in college by copying the passive 
tone control circuitry in my guitar and connecting it to a foot pedal (made 
from a bread loaf tin and a drilled out piece of wood).
I talked through a design with a colleague that would have linked
variable Q and variable center frequency that would have used two
pots on a single foot pedal (one up and down, the other left and right).
We never built it.
So you can call it a "wah-wah" if it changes the tone,
but it's up to you whether it's a notch filter or a low pass
or whatever.

- tom]

    Hi,
    
    I think .1 and .6 are right.
    
    I ran a PSPICE simulation (I can provide the source) of an RLC 
    passive filter in which I managed to change the "apparent" value of L
    (and hence the center frequency) by re-injecting into the L part of the 
    signal with a phase change (RC circuit with variable R).
           
    Thanks a lot everybody (and PSPICE !)