Michael Irwin Re: [sdiy] THAT 2181C VCA's

[Cynthia Webster]

> You can linearize a VCA response by putting a second VCA in feedback loop -
> EDN has a short article on doing this with the SSM2164 which also has
> exponential response. Tracking is very good, especially if you use two VCAs
> on a single chip, but of course you double the number of VCAs you need.
> 
> The article may be found here:
> 
> http://www.reed-electronics.com/ednmag/index.asp?layout=article&articleid=CA23
> 3728&rid=0&rme=0&cfd=1

Yes and if you look closely...

You might see that the article is credited to our own ~Michael Irwin~
Designer of the 
Modcan 30A Super Delay, 38A Flanger, and 38A Frequency Shifter,
among others, and an incredibly talented member of this here DIY list!

Not bad, eh?

So if you have any questions specific to the article,
you can ask the man himself via the list.

Cynthia

> 
> For VCFs and VCOs, having an exponential response might get rid of needing
> an external expo converter, but you'll still have the temperature
> coefficient to deal with (at least, that's what it looked like with the
> 2164.) I'm not sure how good the log conformance is, I haven't tried myself.
> 
> Roman Sowa's site has VCFs and VCOs built with the SSM2164 here:
> 
> http://www.sowa.synth.net
> 
> I would imagine these circuits could be adapted to other audio VCAs.
> 
> ...Craig
> 
> ----- Original Message -----
> From: <WeAreAs1 at aol.com>
> To: <snd_audio at yahoo.com>; <synth-diy at dropmix.xs4all.nl>
> Sent: Monday, November 03, 2003 8:41 PM
> Subject: Re: [sdiy] THAT 2181C VCA's
> 
> 
>> 
>> In a message dated 11/3/03 8:06:44 PM, snd_audio at yahoo.com writes:
>> 
>> << Hello,
>> Wondering if anyone here has worked with the THAT
>> 2181 VCA's. I have 200+ of these and would like to use
>> a couple of them for VCF's and VCO's as well as VCA's.
>> Any idea's, suggestions...? >>
>> 
>> Hi Steve,
>> 
>> Isn't the 2181 an exponential-only control response?  That makes it very
> easy
>> to use it in audio apps such as compressors and volume control, but makes
> it
>> a little complicated for use in filters, VCO's, or for attenuating control
>> signals such as envelopes and LFO's.  Yes, it's true that we want
> exponential
>> response on our VCF's and VCA's, but we need to have a lot of control over
> the
>> design parameters of the exponential amp, and if I'm not mistaken, the
> 2181
>> doesn't really give you much access to the expo amp section.  If you want
> to use
>> it a VCA for attenuating (DC) control signals, it's almost always better
> to
>> have a linear response VCA for that purpose.  There might be some way to
>> "linearize" the control input on those chips (by building some kind of
> external
>> anti-log amp), but that also seems like a lot of trouble, and I'm not sure
> you could
>> ever really get the linearity to exactly track.
>> 
>> Maybe you could build a nice analog mixer fader automation system with all
>> those chips!
>> 
>> Michael Bacich
>