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OT: voltage processing

2006-05-09 by Jeff Foster

hi folks,
This is OT from CGS, although related being that the modules are
used :-) I have a basic fundamental question which will reflect my
less than ideal foundation in electroinc design.

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.

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? I found several voltagr processing DIY circuits
on the web, but none seem to amplife the voltage, only add an off set
amount or mix voltages. Could I add a pot in an output buffer of one
of thess designs to get overall voltage gain?

thanks for any suggestions.
Jeff

Re: OT: voltage processing

2006-05-09 by jeff brown

>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)

The CGS Modular Synth home page: http://www.cgs.synth.net/

Yahoo! Groups Links

Re: OT: voltage processing

2006-05-09 by Richard Brewster

To answer the question, you would need a DUAL op amp. You described an
inverting buffer with a gain of 3.3. So if you put +5V into it, you
would get -16.5V out theoretically. But you can't ever get more that
about 14.5V positive or negative from an op amp with +/-15V power
supplies. I would recommend a 100K/270K ratio. The second op amp would
reinvert this at unity gain 100K/100K.

If you look at the schematic for my Mixer/Comparator, you get a better
picture. That module can multiply an input x2 if you put the same
signal into both MIX 1 and MIX 2 inputs.

http://www.pugix.com/mixer-comparator.htm

Make sure you get the 2006 version schematic.

-Richard Brewster

Jeff Foster wrote:

Show quoted textHide quoted text

>hi folks,
>This is OT from CGS, although related being that the modules are
>used :-) I have a basic fundamental question which will reflect my
>less than ideal foundation in electroinc design.
>
>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.
>
>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? I found several voltagr processing DIY circuits
>on the web, but none seem to amplife the voltage, only add an off set
>amount or mix voltages. Could I add a pot in an output buffer of one
>of thess designs to get overall voltage gain?
>
>thanks for any suggestions.
>Jeff
>
>
>
>
>
>
>The CGS Modular Synth home page: http://www.cgs.synth.net/
>
>Yahoo! Groups Links
>
>
>
>
>
>
>
>
>
>
>

Re: OT: voltage processing

2006-05-12 by Jeff Foster

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:

Show quoted textHide quoted text

>
> >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)
>
>
>
>
>
>
> The CGS Modular Synth home page: http://www.cgs.synth.net/
>
> Yahoo! Groups Links
>

Cgs synth

OT: voltage processing

OT: voltage processing

Re: OT: voltage processing

Re: OT: voltage processing

Re: OT: voltage processing

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