[sdiy] David's sortof biquad filter

[Harry Bissell Jr]

Either the schematic is wrong or he's
done something really strange on the
non-inverting input to OTA2... the
470 ohm is in the wrong place imho...

H^) harry



--- Aaron Lanterman <lanterma at ece.gatech.edu> wrote:

> 
> One of my students, David Hembree, put together this
> webpage describing 
> his effrorts:
> 
>
http://www.prism.gatech.edu/~gtg066g/filterhembree.html
> 
> I didn't demand that he put in a more complicated
> expo control source 
> since he wound up spending a huge amount of time
> fighting the effects of 
> DC offsets and I took pity on him. He'd change his
> control current, and 
> the DC of the output would swing all over the place.
> I kept suggesting 
> kludges at him. By the time we decided to call it
> done enough for the 
> semester, he had poked at it to the point I'm not
> sure it's a biquad 
> anymore, but it still sounds cool.
> 
> The biquad is supposed to be adjustable bandwidth
> instead of adjustable Q 
> (i.e. adjust bandwidth independent of freq, insteaqd
> of Q indep of freq). 
> But, from David's design we never got any really
> dramatic frequency bump 
> effects. It sounds smooth though.
> 
> David wants to keep playing with it past the
> semester (yay David!) - any 
> suggestions for him?
> 
> One thing I suggested (I didn't make this clear in
> my explanation to him) 
> was that each OTA needs its own separate DC adjust
> at the + pins.
> 
> - Aaron
>
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