[sdiy] Lowest distortion neede for VCA - linearizing the LM13600/13700 & other dual VCAs

Neil Johnson neil.johnson71 at gmail.com
Tue Sep 17 11:04:06 CEST 2013


Hi,

Andrew Simper wrote:
> 1) Sims' design using two OTAs (13700) to keep within the linear
> region of the differential audio input:
> http://www.teaser.fr/~amajorel/sims/

> I've only had a brief look at the Sims' OTA linearisation, but can
> someone please help explain what is going on there? Is the first OTA
> with negative feedback generating a current to send to the second OTA
> to amplify while keeping the input voltage near 0?

I think it's doing two things:
- converting the input voltage to a current, minimising the
differential input to the second VCA.  Note that BOTH VCAs have almost
zero differential voltage -- as per the datasheet you want to keep the
differential input voltage to below a few mV for lowest distortion.
- pre-distorting the input signal, further improving the overall performance.

Note that Sims doesn't use the linearizing diodes in his scheme.
Given the differential input is almost zero the data sheet suggests
they won't help, but it might be worth experimenting just to see.

And if Sims' numbers are to be believed he's got the LM13600 (at
0.015%) behaving better than the 2164 (0.02% best case).  You'd have
to jump to THAT chips for any further improvement.

Cheers,
Neil
--
http://www.njohnson.co.uk



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