[sdiy] Synthancon VC-Resonance

[Aaron Lanterman]

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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