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

[David G Dixon]

The Sims linearized 2164 VCA is the heart of most VCAs being produced today.
Few manufacturers are still using OTAs.  The Sims circuit is also at the
heart of very many of my own designs for all kinds of stuff which bears the
Intellijel trademark.  The day I discovered that circuit was one of the best
days of my life.



> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl 
> [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of 
> Robin Whittle
> Sent: Tuesday, September 17, 2013 3:58 AM
> To: synth-diy at dropmix.xs4all.nl
> Subject: Re: [sdiy] Lowest distortion neede for VCA - 
> linearizing the LM13600/13700 & other dual VCAs
> 
> I have not tried this technique, but it looks good to me:
> 
>    Low-cost audio VCA has high performance
>    Mike Sims, Lectrosonics Inc, Rio Rancho, NM
>    EDN 1995-01-19
> 
> The original source of this no longer produces a proper article:
> 
> 
> http://www.edn.com/design/other/4352601/EDN-Access--01-19-95-L
> ow-cost-audio-VCA-has-high-performance
> 
> The page is archived at André Majorel's excellent site of Synth-DIY
> resources: VCAs, transistor arrays etc.:
> 
>   http://www.teaser.fr/~amajorel/sourcing/
>   http://www.teaser.fr/~amajorel/sims/     << the article
> 
> I don't know anywhere else which archives this page.  I have 
> kept a copy for myself in case André's site disappears.  I 
> recall seeing another article which involved the same system, 
> but I can't find whatever copy I made of it.
> 
> It works by using a feedback loop driving the signal into 
> both the main
> (2nd) and a first VCA, with both VCAs being closely matched 
> due to being on the same chip.  The first VCA has a fixed 
> control current and the feedback look drives whatever small 
> voltage is required into both VCAs to make the first one 
> produce an exact replica of the audio input signal.
> 
> This is achieved by the first one's output current going back 
> to the input pins.  The audio input voltage is turned into a 
> current by 30.1k
> R1 (of course any suitable resistor value will do here) and 
> most of that current goes through the output of the first 
> VCA.  This point remains close to ground voltage compared to 
> the input voltage.  Therefore, it is best to use a relatively 
> high input voltage range so R1's voltage and therefore 
> current is hardly affected by the small voltage changes at 
> its right end, due to the voltage required at the input of 
> the VCAs to produce the output current which brings this 
> point close to ground.
> 
> In doing so, the feedback loop overcomes whatever distortion 
> mechanisms are inherent to both VCAs.  The first VCA is used 
> otherwise normally.  I think this means that the VCA's input 
> pins can be driven harder than normal, since ordinarily 
> driving them hard would produce distortion in the form of the 
> output current of the VCA, for a given control current, 
> falling off at high input voltages.  The feedback network 
> takes care of this and would result in higher than normal 
> drive voltages to the input pins as required to get the 
> desired output current.
> 
> Therefore, the second VCA, which is the one we are using for 
> audio out, is being driven hard, without distortion, which 
> reduces (I guess, depending on the mechanisms) the noise 
> level at the output.
> 
> This would work for the LM13600 as described in the article 
> and for the LM13700s which are now still widely available.  
> See André's main page for all the low-down on VCA 
> availability, history etc.
> 
> The article reports that the technique results in:
> 
>    The circuit's maximum input voltage is +20 dBu (dBu=dB referred to
>    775 mV rms). THD measures less than 0.015%; noise, -70 dBu; and
>    control-voltage feedthrough, -70 dB.
> 
>   - Robin
> 
> On 2013-09-17 3:44 AM, Harald wrote:
> > Yes  but i want to see what is possible with an LM13700 in 
> the best case.
> > I am down to 0.04% THD. Have to look at the noise though.
> > 
> > Am Montag, 16. September 2013, 18:22:33 schrieb Colin f:
> >>> Pardon me for stating the obvious but... If you're 
> looking for high 
> >>> fidelity, wouldn't it make more sense to use one of the 
> following ?
> >>> - AD SSM2164 / Coolaudio V2164
> >>> - THAT 2162 / Coolaudio V2162
> >>> - THAT 2180
> >>> - THAT 2181 / Coolaudio V2181
> >>
> >> Seconded.
> >> Having tried the THAT chips, I would only use a 13700 as a 
> VCA for a 
> >> noise generator.
> >>
> >> Cheers,
> >> Colin f
> 
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