2nd-order allpass VCFs?

Don Tillman don at till.com
Tue Nov 23 16:40:04 CET 1999


   From: "Curtin, Steven D (Steven)" <sdcurtin at lucent.com>
   Date: Tue, 23 Nov 1999 09:28:02 -0500

   What's interesting about about many wah-wah pedals is that they
   don't vary the cutoff frequency of the filter, since this would be
   too expensive.  Instead they vary the Q of the filter, by changing
   the feedback resistance.  

   I found this out when looking through the schematics on the web for
   such classics as the Crybaby.  

Not true, take a closer look at the schematics.  The wah-wah pedal's
pot and simple amp work like a capacitance multiplier.  Changing the
pedal position varies the effective capacitance, and with the fixed
inductor, varies the resonant frequency of the filter.  

Pretty clever, eh?  You can picture the designers saying "How can we
tune this LC filter over a wide range with a pot?".

  -- Don



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