[sdiy] 1v/oct expo converter front end for SSM2164 quad pole vcf..

[David G Dixon]

> Here is a cv summer suitable for a 2164 filter. Based on 10V 
> cv range, it will provide 0-2V to the control pins of the 
> 2164, with 2V being minimum filter freq.
> 
> http://www.blacet.com/2164cvsum.jpg

Yeah, that looks about right.  If you want tempco, you can put a voltage
follower after this with a divider on the non-inverting input, and put the
tempco resistor on the grounded leg of the divider.  I've got a spreadsheet
model for predicting the correct resistor values for the various tempco
options to minimize tempco errors based on size and tempco coefficient (it
isn't perfect tempco, but it comes close) if you're interested.  In that
case, I actually put a fixed feedback resistor on the CV summer and do the
tuning with a small trimmer on the upper leg of the follower's input
divider.

I don't use tempco in my own filter builds, but I leave the option on the
board for others.