[sdiy] CA3280 VCA Design

[Harry Bissell Jr]

It was an EDN Design Idea by Mike Sims, iirc.
  You should be able to find it via Google.
  
  I tried it and found no noticible improvement...
  13700 halfs don't really match as well as you
  wish they did.
  
  I made two VCF, one with the original 13700, one
  with the Sims circuit.  Twice the parts and no
  noticible difference. :^(
  
  H^) harry

Grant Richter <grichter at asapnet.net> wrote:  Terry Michaels told me about a design he saw in a trade magazine. It  
uses a second section of OTA to predistort the input and fully  
compensate for the input non-linearity.
As I recall, you run the voltage straight in through a resistor, tie  
the first (predistortion) section back to it's own input and into the  
input of the gain control cell.
The first section will generate the compensation curve needed to  
produce a full level linear output after the gain control block,  
without input attenuation.
This allows you to run full +/- 10 volt signals through the OTA. The  
monolithic matching of the LM13700 sections should allow the  
compensation to be perfect.

Anybody else see that article?
Maybe someone said Jim had already posted this somewhere as a  
subcircuit in a drawing?

It seems like you could use a dual LM13700 like a single CA3280 and  
get maximum S/N from the LM13700 (get your 60 cents worth).

Otherwise the input diodes on the LM13700 allow you to input +/- 500  
mv at around the same THD as +/- 20 mv (10K/500 vs. 100K/220)

Also, who still sells CA3280s? I should buy some before they  
disappear altogether. Thank you.

On Nov 20, 2005, at 1:32 PM, James Patchell wrote:

> 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
> ***************************************************************
> http://www.oldcrows.net/~patchell
>
> ***************************************************************
>



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