Re: [sdiy] Crossfade circuit for CVs?

[jhaible at debitel.net]

>Hi all. I need a simple circuit that can crossfade between two different
>CVs, using a third CV.
>It doesn't have to be particularly linear or anything. But it needs to be
>simple, as I'm going to use a whole bunch of them, in a planned module.
>I have a crossfade VCA, that I could use, but it uses two 3080 and five
>op-amps. I'm trying to find something with lower parts-count.

If it doesn't need to be fast, build a "pot" from two Vactrols.

I'm doing this all over my PS-3200 Clone. Example:
http://www.oldcrows.net/~jhaible/polykorg/jh_3200_signal_3of4.gif
See the "pot" built from LDRs (A) and (B) in th etop left corner.
(Of course you don't need any opamps if you have low source
impedance and high input impedance of a following module.)
The control circuit for this is shown here:
http://www.oldcrows.net/~jhaible/polykorg/jh_3200_signal_2of4.gif
(Look for (A) and (B) LEDs in top left quadrant.
Only two opamps needed (and a 3rd one if you want high input
impedance). 
There is a single trimpot to set the "LDR pot" to exactly 50%
for a certain CV. (@ 5V from a 0 ... 10V range in my circuit)
For the end positions, you need no trimming or matching of
Vactrols at all: regardless if you have 100Ohm : 10MegOhm
or 130Ohm : 34.8MegOhm, it's always "100%".
Tolerance of the Vactrols will shape the courve in between
to some degree, but with a 3rd point nailed at 50% with that
trimmer, it's fine for me.

(Please note that this solution is very different from the
original Korg PS-3200. They also used Vactrols, but
in a very different circuit, with more trimmers, and more
opamps in the signal path.)

JH.