[sdiy] 4-Quadrant VCA?

David G Dixon dixon at mail.ubc.ca
Wed Sep 13 23:05:20 CEST 2017


I personally wouldn't give 5 cents for half the circuits in the 13600/13700
datasheet.  It's pretty straightforward to realize a 4QM with 2164, and it
works like a dream.
 

> -----Original Message-----
> From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On 
> Behalf Of Guy McCusker
> Sent: Wednesday, September 13, 2017 1:20 PM
> To: synth-diy at synth-diy org
> Subject: Re: [sdiy] 4-Quadrant VCA?
> 
> There's a design for an OTA-based 4QM in Thomas Henry's book 
> on the 3080. He says it started life in an app note for the 
> LM13600 and was written up in Electronotes 107. I don't have 
> my copy of that to hand but Tim Stinchcombe's index suggests 
> that EN107 has an article called "A (Gulp) Simple Balanced 
> Four-Quadrant Multiplier With An OTA" -- looks promising!
> 
> The circuit in the book uses just one OTA. One signal is the 
> input, the other is the gain, and with a bit of offsetting of 
> the gain input
> 4 quadrant behaviour is realised somehow.
> 
> Will the version with rectifiers and switches run the risk of 
> being glitchy around the zero-crossings?
> 
> On Wed, Sep 13, 2017 at 9:06 PM, Tim Ressel 
> <timr at circuitabbey.com> wrote:
> > Ah, so that is what they are doing in the LM13700 datasheet 
> when they 
> > have a single 13700 doing a 4-Q multiplier!
> >
> > --Timbo
> >
> >
> >
> > On 9/13/2017 12:54 PM, Olivier Gillet wrote:
> >>>
> >>> I know I could use AD633s but they are really pricey. Any other 
> >>> clever ways of pulling this off?
> >>
> >> Add enough DC offset to one of the signals to make it unipolar.
> >> Subtract the other signal from the result. Choose wisely 
> which branch 
> >> of your circuit is labelled as the "control" path and which one is 
> >> the "audio" path.
> >>
> >> 
> https://forum.mutable-instruments.net/t/blinds-how-does-it-work/8287
> >> 
> https://mutable-instruments.net/modules/blinds/downloads/Blinds-v60.p
> >> df
> >>
> >> Pros: no distortion, no odd behaviour at zero-crossings, 
> doesn't cost 
> >> more than a normal VCA circuit.
> >> Cons: need a couple of precision resistors, non-ideal behaviour at 
> >> high frequencies.
> >> _______________________________________________
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> >>
> >
> > --
> > --Tim Ressel
> > Circuit Abbey
> > timr at circuitabbey.com
> >
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