[sdiy] How to avoid clicking gates

[David G. Dixon]

Michael,

(This is offline.)  Thanks for your great reply.  I already responded to
someone else with the same advice (use an EG between the gate and the VCA)
so this is a private response.

This is my first synth, so I am groping in the dark a little bit, just going
by what I read, etc.  I was gating (and triggering) an EG to control a
4-pole filter, but was just gating the VCA (a slightly modified Irwin-Gallo
SSM2164-based model, set in linear mode) directly with the gate.  This was
my mistake, I think.  I'll test it out later today to confirm.

To answer your questions, my system PSU is +/- 15V, and I'm using (a highly
modified version of) Ray Wilson's "Music From Outer Space" keyboard matrix
scanning circuit.  I haven't modified the gate and trigger generator parts
of the circuit, and they put out gates and triggers at the positive rail.  I
could easily change these to +5V by using the 78L05 output as the rail for
these transistors instead of the PSU positive rail.  Maybe I'll do this on
my next board (the first is a prototype which will require a couple other
minor changes, so no big deal).  I can eliminate the clicks by attenuating
the gate signal coming into the VCA anyway, so it's really no big deal.  I
was just worried that there might be a design flaw somewhere, but I don't
think there is.

Cheers,
Dave


David G. Dixon
Professor
Department of Materials Engineering
University of British Columbia
309-6350 Stores Road
Vancouver, B.C. V6T 1Z4
Canada
 
Tel 1-604-822-3679
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> -----Original Message-----
> From: Michael Zacherl. [mailto:sdiy-mz01 at blauwurf.info]
> Sent: Monday, August 10, 2009 3:57 AM
> To: David G. Dixon
> Subject: Re: [sdiy] How to avoid clicking gates
> 
> Hi David,
> 
> > Well, I've finally got my 12 x N matrix scanning circuit up and
> > running
> > tonight with one of my salvaged Farfisa keyboards, and everything
> > works
> > great!  My modular (all seven modules) sounds fantastic under keyboard
> > control.
> 
> nice!
> 
> > The only problem is that when the keyboard gates the VCA,
> 
> what VCA is it?
> 
> > it makes an
> > audible click unless I turn the CV attenuator well down.
> 
> It's most likely that your VCA doesn't need that high voltage to be
> fully open.
> And your Gate signal is pretty fast I suppose.
> 
> > The keyboard
> > circuit genetares a 15V gate signal.
> 
> What's your PSU voltages?
> 
> > I'm thinking this is too large.
> 
> AFAIK a typical VCA can't be overdriven by the CV - depending on the
> circuit of course.
> It's either fully open or not.  CV bleed through also comes into my
> mind (which I think is higher when the CV gets much higher than needed
> to drive the VCA fully open)
> 
> I think you'll also notice that the click noise probably also depends
> on the type of signal you feed through the VCA.
> E.g. sine vs. rect vs. noise etc.
> 
> > What
> > would a typical keyboard gate voltage be?
> 
> e.g. 0V = off, 5V = on
> But this shouldn't be an issue.
> I often drive modules which are traditionally indended to be driven
> with a Gate with other things like a VCO which are not witin this range.
> 
> > Also, what about "softening" the
> > gate rise a little bit with a capacitor?  Does anyone ever do that?
> 
> no! You want the Gate signal to be sharp with the lowest slew rate
> possible.
> A low slew rate gives transients - that's what you probably hear.
> 
> You _can_ use the Gate signal directly as a CV, usually an envelope
> generator (which is much slower than the Gate signal)  is the device
> you traditionally use to control filters, VCAs etc.
> Even at its fastest settings it's slower than a Gate.
> 
>    I'm sure you're aware of this things but maybe it helps
> nevertheless to get on track - Michael.
> 
> 
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