[sdiy] Pot-based SVFs

[Aaron Lanterman]

Tomorrow's lecture is on State Variable filters.

I have examples to show from the Oberheim SEM and the EML-101, which I  
understand fairly well.

I'd also like to show a couple of real-world examples of non-voltage- 
controlled SVFs - i.e. ones controlled with regular pots - before I  
dig into the voltage-controlled ones. I'm looking for something with a  
real-world world schematics, i.e. not idealized textbook schematics.  
Any ideas?

I ran across this design by M. Danielson:

http://home.swipnet.se/cfmd/synths/schematics/svf1.pdf

But something doesn't look right to me. Both feedback loops should be  
negative, if I understand the universe right; going through two  
integrators and then into the - input of the op amp maps sense to me,  
but taking the output of the first integrator and running it to the  
negative doesn't, since that looks like positive feedback. I think it  
should be going to the positive terminal of the first op amp.

The initial input is going into the negative terminal, but that's  
incidental.

Is there something I'm missing?

- Aaron