[sdiy] David's sortof biquad filter

[Aaron Lanterman]

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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Dr. Aaron Lanterman, Asst. Prof.
and Demetrius T. Paris Junior Prof.  Voice:  404-385-2548
College of Electri. and Comp. Eng.   Fax:    404-894-8363
Georgia Institute of Technology      E-mail: lanterma at ece.gatech.edu
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Atlanta, GA 30332                    Office: Centergy 5212