AW: Good, simple filter cct required

[Don Tillman]

   Date: Tue, 14 Oct 1997 23:56:49 +1000 (EST)
   From: Paul Perry <pfperry at melbpc.org.au>

   At 02:59 PM 14/10/97 +0200, you wrote:
   > >I'm trying to get hold of a simple bandpass or lowpass
   > >VCF, as a circuit that can be built by first year undergrads. 
   > > No specialist ICs, just opamps. 

   a switched resistor system (not to be confused with a switched capacitor
   filter ) is conceptually simple, and you get to show them pulse-width
   modulation while you are at it... and it tracks perfectly...
   think the ETI synths had something like this.

When I do any teaching I work hard to pick examples that are free of
weird side effects and complications which would distract from the
main concept, that are historically significant or actually used in
certain products so the students can relate to them better, and are of
fundamentally good design so the student isn't shown how not to do it.

So this would pretty much rule out the PWM resistor filter.  :-)

A 3-opamp biquad filter with a stereo pot for tuning would be real
fine for a lot of reasons.

  -- Don