[sdiy] Pot-based SVFs

[Jim Patchell]

That schematic is incorrect.  The loop for controlling the Q should 
return back to the + terminal just like you said.  Most likely just a 
little slipp up....

-Jim

Aaron Lanterman wrote:
> 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
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