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previous: Dani Anyone here know what to change in ... -- 8/16/2000 8:52 PM View Thread

Re: opamp wah

8/17/2000 6:54 PM
Mark Hammer
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Re: opamp wah
I don't know the DOD unit, but when a pedal only allows you to vary one resistor or one voltage divider, there are only so many types of filters you can implement. So, I'm going to take a wild guess and assume it works like the one below.  
 
http://hem.passagen.se/robin2/im/dualfil.gif  
 
This is Craig Anderton's Dual Filter project from Electronic Projects for Musicians. What you'll see is two op-amps serving as bandpass filters. Each filter has a resistor between the inverting input and output, and two same-value capacitors connected to those same pins at one end, and connected to each other at the other end. Where they connect to each other, there is a resistor going to ground. Vary that resistor and you re-tune the filter. Stick that resistor in a footpedal and you have a wah. Change those capacitors for other values, and you shift the range of the wah. Smaller values get you a higher range, larger ones get you a lower range (double the value, go down an octave). Increase the resistor between the output and inverting input, and you increase the Q and gain of the filter.  
 
Based on this info, you *could* wire up a 6 position rotary switch to select between 6 sets of matched capacitor pairs (best to have one set of the smallest value hardwired, and select additional pairs in parallel). However, this configuration has a fairly wide range compared to a standard wah, so I doubt that a 6-position switch is necessary. Indeed, a 3-position toggle switch is probably sufficient to cover a very broad range.  
 
If you can identify the comparable components on your FX17, and if it is what I think it is, you should be fine.