Cgs synth

On Dec 6, 2007, at 4:31 PM, Richard Brewster wrote:

> Jeff,
>
> I'm answering your questions inline below. Have you seen my CGS VCO
> project? I built a minimal VCO with a modified schematic.
>
> http://www.pugix.com/cgs-vco.htm
>
> -Richard Brewster
>

I have seen your site, great work.

>
> > I'm looking at a pair of the CGS48 VCOs as my first project. I
> want to
> > build it in Eurorack format (12V with an adapter power cable) using
> > the 12V mods listed on the project page. I've done quite a bit of
> DIY,
> > so I'm semi-confident I can handle this one. I have a few questions
> > about the project:
> >
> > - I don't see an initial pulse width on the schematic. Could I just
> > add a 100k lin pot with the wiper selecting between V+ and V- into a
> > 120k and merge it with the PWM CV in, as the electronotes article
> > shows? Or would I need to change resistor values?
> >
> If you compare the ASM-1 VCO schematic with the CGS VCO, you'll see
> that
> the ASM-1 has an op amp mixer for PWM. To mix an initial PW with a
> control voltage you would need to add an op amp for active mixing. In
> practice, I find that mostly what I need is an attenuator pot for the
> PWM level, and I leave initial pulse with at 50%. You can add a 100K
> panel pot as an attenuator with the wiper connected to the PWM input.
> That would work fine to attenuate the PWM CV. I have two MOTM-300 VCOs
> and I find I don't use the initial PW pot nearly as much as the PWM
> level pot.
>

So if no voltage is applied to the PWM input, the squarewave duty
cycle is 0% (non-functional), correct? Or is it 50%? If there's a way
to get a vanilla 50% square out of the thing without using up more
modules, that would be good enough.

Could I rig up a circuit with a switch for manual PWM/CV PWM? Then tap
the +VDC for a simple voltage through an attenuator for initial width
if I'm not using PWM?

>
> > - If I'd like to add an additional FM in, can they just sum
> together,
> > or would I need to change the resistors? Is there a danger of
> > overloading the opamp if both are sending 12V?
> >
> It's easy. See my schematic. I added a Fine Tune pot with a 4M7
> resistor to the summing node of the FM op amp. You can add more FM
> inputs at whatever sensitivity level you want. You won't damage the op
> amp or the VCO. 100K gives you a 1V/OCT response. Lower values give
> less than 1V/OCT, higher more than 1V/OCT.
>
> > - To add an FM Amount control, is this just a 100k lin attenuator
> pot?
> >
> Yes.
>
> > - What's the difference between the Processor input and the FM
> input?
> > Linear vs. Log scale?
> >
> The Linear FM goes into a different place in the circuit. I added a
> switch on my one FM input jack, so I could route it either way.
> > - If I'm not going to implement the suboscillator, just don't stuff
> > anything past staircase in on schematic? Or do I need to ground any
> > additional pins on LM3900?
> >
> If you're using any part of the waveshapers, I recommend stuffing the
> whole circuit, even if you don't bring all of it to your panel. It's
> only a few cents worth of parts.
>

Thanks for the above.

>
> > - If anyone has front panel layout ideas for Eurorack I'm all ears.
> >
> Check out the ModularSynthPanels Yahoo group.
>
> http://launch.groups.yahoo.com/group/ModularSynthPanels/
>

Wow, there's a group for everything. :)


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