[sdiy] CA3280 VCA Design

[James Patchell]

Hmmm....I actually found quite the opposite...

I measured about .1% distortion using the Sims circuit...I used this for 
the gain control in the VCO I did (for compensating for temperature 
drift)...because it was very linear...and cheap....

At 06:55 AM 11/21/2005 -0800, Harry Bissell Jr wrote:
>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
>> >
>> > ***************************************************************
>> >

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

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