[sdiy] CA3280 VCA Design

[James Patchell]

On the CA3280E, you don't have a choice...you have to use the current 
sources.  So...it would be difficult to compare.  The LM13700, you have 
choices, at the expense of adding more parts.  However, with the LM13700, 
you are starting out with an inferior part (oh...I shouldn't call it 
that...it is the last remaining DUAL OTA :-).....personally, a few dB isn't 
going to really make a whole lot of difference...although, soon, I guess I 
will be exploring this since I will be looking into ways to get the most of 
what I want out of those parts.

One of the main reasons you get better S/N with the linearizing diodes is 
because you can drive the input harder (more signal), and still get the 
same distortion level that you would get with a standard resistor 
attenuator on the inputs (which has to knock the signal down quite a bit, 
depending on how much distortion you are willing to live with).

At 11:10 AM 11/20/2005 -0800, Jay Schwichtenberg wrote:
>One question I have is about using the linearization diodes. Obviously
>you'll get some SNR improvement by using the diodes. But, I've gotten the
>impression (I can't remember if it was from the data sheet or the
>Electronotes discussion) that if you drive the diodes input with a current
>source vs. a resistor that you can get a few extra db on the SNR. I think
>that might be what Paul does with the MOTM VCAs.
>
>Any comments?
>
>Thanks.
>Jay S.
>
>
>-----Original Message-----
>From: owner-synth-diy at dropmix.xs4all.nl
>[mailto:owner-synth-diy at dropmix.xs4all.nl]On Behalf Of James Patchell
>Sent: Sunday, November 20, 2005 10:26 AM
>To: Rob Keeble; synth-diy at dropmix.xs4all.nl
>Subject: Re: [sdiy] CA3280 VCA Design
>
>
>You might try looking here:
>http://www.oldcrows.net/~patchell/archives/archives.html
>
>and here:
>
>http://www.oldcrows.net/~patchell/smb1/smb.html
>
>lots of other crap on my website as well....
>
>At 05:20 PM 11/20/2005 +0000, Rob Keeble wrote:
> >Hi,
> >I have a tube of CA3280's which I plan to turn into dual VCA's, however I
> >can't find a circuit design on the web...so I have cobbled one together -
> >which is here (37k GIF) - mainly using ideas from the SCI Prophet 5..
> >
> >www.emulatorarchive.com/assets/vca.gif
> >
> >Before I put this on the breadboard, any changes I should make?
> >Audio and CV ins are 0 to +10V standard. VCA is linear, unity gain, and
> >should't invert signals. Only one VCA shown...
> >
> >Many thanks
> >Rob
> >www.emulatorarchive.com
>
>          -Jim
>***************************************************************
>http://www.oldcrows.net/~patchell
>
>***************************************************************

         -Jim
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http://www.oldcrows.net/~patchell

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