repl pots with vc

[Martin Czech]

Some modules have pots that are not so easy to replace with voltage
control, e.g. Q in filters. No change in sound should occour. Over the
weekend I tryed to figure out, how this could be done.

1. Use a fixed load resistor and a LDR to ground as a voltage
divider. Works, but range is limited to 2 decades, which seems to
be enough for Q control. The scaling looks also god, faster in the
beginning, then flat. Exactly what  you need for Q, but maybe not so good
for other things.  Problem: the LDR gets very slow at the 1:1 divide
point, it takes a noticeable fraction of a second, even too slow for
hand adjust. But what's more negative is that a eg. sinuoidal modulation
will change the average value is the modulation gets faster, towards
more attenuation , ie. more Q. Undesireable, not intuitive
for the user. "Through" resistance changes with LDR.

2. Use two LDR in a divider scheme, with op amp circuitry to drive
the LEDs. Gives more attenuation range, but suffers also from
nonliearity in time behaviour. Level may stick at one end
for faster modulations. Also "through" resistance changes with LDR.

3. Use ota. Better use 13700. Has anybody tryed to give a state var filter
a ota resonance control ? I must try this.

4. JFET attenuator. Problems with large signal amplitudes, much
distortion one is able to see on the scope, distortion changes widely
with attenuation, ie. bias.


Any comments ?

m.c.