[sdiy] Synthancon VC-Resonance

[harry bissell]

The queerest thing about the sallen-key filter is that when the 
resonance starts...
it starts at the zero crossing instead of the peak of the waveform, like 
Moog ladders
and SVFs do.

I ~wonder~  why that is. It could make a big difference in the sound, 
especially in not
clipping later stages...

I'm not fond of the Sallen-Key VCF... it sound like a touch-tone 
telephone more than a
filter... almost the sound of a sync'd oscillator.

OK... it does sound better than a BBD... :^P

H^) harry


Aaron Lanterman wrote:
>
> On Tue, 2 May 2006, Ryan Williams wrote:
>
>> the filter. Incase you haven't done an analysis of it, this should 
>> give a good idea how the filter really works. I have considered 
>> putting in an OTA based amplifier to replace the bjt amp in the 
>> normal version. One important thing to remember is that the input 
>> impedance to the amp needs to be fairly
>
> Thanks to the many who have posted a lot on this topic. I think I 
> better see what's going on now. it had been a while since I read the 
> steiner paper. I see the sallen-keyish structure.
>
> My students have actually built Ken's board - I happened to have one 
> and though it could be handiest if it there were a set of instructions 
> on hacking Ken's board, what to cut, where to patch in, etc...
>
> Probably the easiest thing to do would be to patch in (looking at the 
> later Synthacon on Ken's page) right at the right terminal of the 1k 
> resistor in the lower feedback loop, just to the left of the 
> T-junction connecting the 1K resistor, the pot, and the collector of 
> the transmitter.
>
> Then, you can toss pretty much any VCA circuit you find there, an 
> OTA-based one will do fine. Just be careful to pick the gains 
> appropriately... remember Sallen-Keys will freak out if the feedback 
> gain is greater than 3, and they don't do anything interesting if the 
> gain is less than one. So you want you VC gain to go between 1 and 3 
> as you twizzle 0 to 5 volts into your control input (I think).
>
> So, essentially you'd have two amps, which is a bit redundant, but 
> that's OK. It's also nice in that the CV control will effect the Q but 
> not the overall gain, so you would get more flexibility.
>
> - Aaron
>
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