[sdiy] Irwin dual VCA--any interest?

[synth at charlielamm.com]

What I found empirically:

You build an ADSR using the same cap n 4066 arrangement a lot of
designs use (there are 2 on my site that do, for instance).  Since the
voltages at attack and release rise in a linear fashion over time, I
consider these to be "linear" devices.  Someone should correct me if I am
using this terminology wrong.

You set the decay to be 5 seconds, from the end of the gate, to when you
want, say -100db (ie., silence).

For argument's sake, lets say that 5V CV equals 0db (no boost or cut in
amplitude) when plugged into the VCA's modulation in.

In 2.5 seconds, you have 2.5V at the ADSR output, and you want the VCA to
be -6db down.  In my mind, this means you have a "linear" ADSR, and so
you want a "linear" VCA.

As I understand it, and I may be wrong and get cut to shreds by the group,
an "exponential"  VCA won't do this, since it's set to some fixed V per
DB, it will probably be way more than 6db down after 2.5 seconds; for
practical purposes, you won't hear anything after a much shorter time than
this.

Hence a lot of folks say expo VCA's are "good for percussion" but I think
this assumes the sort of cap n' 4016 ADSR I just described.

The Irwin VCA, by my tests, works that way.  With a REF IN of -5V, if you
send in 2.5V positive CV, the output is about -6db relative to where it
was when the CV was 5V.

The important thing: When you hook it up to a standard, cap and 4066 type
ADSR, it sounds "right".  The release fades out in a satifying, slow
manner.

Does this make sense?

On Mon, 12 Jan 2004, Tim Parkhurst wrote:

> > -----Original Message-----
> > From: synth at charlielamm.com [mailto:synth at charlielamm.com]
> >
> > I got my first 4 "dual linear VCA a la Michael Irwin/EDN magazine"  PCB's
> > back from APCIRCUITS.  I put a dual VCA together last night, fired it up,
> > tested it, and it worked great--no trace cutting or last second fixes
> > necessary--at least not that I have discovered yet--thank goodness!
> >
> > The PCB is based on this article:
> >
> > http://www.reed-electronics.com/ednmag/contents/images/72502di.pdf  scroll
> > to the bottom of this PDF--that's the one I mean.
> >
> > Mr. Irwin's design idea has a lot of things I like: it's simple, low parts
> > count (less to go wrong), sounds good (_truly_ linear, so it sounds smooth
> > and satisfying when responding to an AR or ADSR's attack and release), and
> > is flexilbe (I esp. like being able to "program" the reference control
> > voltage with a single DC voltage below ground....)
> >
>
> So I'm thinking, where do we want a linear VCA, and where do we want an expo
> VCA? Do we want linear if the VCA is being used to control/attenuate a CV?
> I'm also assuming (at least for audio uses) that a linear VCA sounds best
> when driven by an expo source, and a expo VCA sounds best when driven by a
> linear source. Right? (and by "sounds best" I mean smooth, natural sounding
> changes in volume without abrupt or uneven changes)
>
> Tim Servo
>
>
>
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