Cgs synth

thanks Jeff,
I check your suggestion against my schematic and see if i understand.
chnaging R1173 sound right...more control.

thanks again,
Jeff ( Foster)



--- In cgs_synth@yahoogroups.com, "jeff brown" <guitaricon@...>
wrote:

>
> >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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