Cgs synth

>I have a pro one synth (first synth bought, still going after 25
>years) and I like to modulate the vcf from other voltage sources. The
>vcf cv input looks like it wants 0-15v, based on the schematic.

I assume it's the Sequential Circuits Pro One. And you are referring to the
external filter cv input at J2.
The filter cuttoff pot is indeed tied to +/- 15 volts, but if you look at
the whole summing amp and resistors used, it's
being divided down a bit.

You _could_ try changing the summing resistor on the external VCF input to a
smaller value.

The 3310 ADSR chip puts out a 5-volt peak envelope (IIRC) and is going
through a 47K summing resistor. If you were to change
R1173 from 75K down to 47K, you'd get about the same effect out of a 5-volt
external control as the 3310. You could
reduce it further to say 33K and your 5-volt input would have the effect of
a 10volt input.

>Most of the voltages I have for sweeps and random modulations are 0 to
>5v. My question is will a simple opamp with a 100k input resistor and
>a 330k feedback resistor provide the nessary voltage gain to drive the
>vcf to its full range?

Yes. But probably a 220K feedback resistor and a 50K or 100K input
attenuator on it would work well enough.
You might want to put a 1K resistor on the output to protect the opamp from
smoking if you accidently ground the output.

Jeff (not the same one)






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