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

Andrew Simper andy at cytomic.com
Tue Sep 17 09:26:26 CEST 2013


David: are you talking about the Mike Irwin's linearized 2164 VCA design?

There are two linearizations going on here and I think people are
getting confused.

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

2) Mike Irwin's design using two 2164 VCAs to lineararise the
exponential control of the 2164:
http://www.sdiy.org/philgallo/mgbvca.html and
http://m.bareille.free.fr/modular1/quadvca/quadvca.htm

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?

Andy


On 17 September 2013 13:16, David G Dixon <dixon at mail.ubc.ca> wrote:
> 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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