[sdiy] 4-Quadrant VCA?

S Ridley spridley1 at gmail.com
Wed Sep 13 23:37:02 CEST 2017


Serge modular used a 3080 based 4QM from around 1973.  It's on Ken Stone's
site.

I've seen the zero-crossing trick used in a high-end analog mixer from mid
1980s.  It was on a voltage controlled EQ card.  They used exponential VCAs
(dBx) and by switching input polarity at zero (centre on the pot) they got
an exponential (dB) response either side of zero.  Avoiding clicks around
zero takes care.  For a linear response it's not worth the effort.

Steve

On 13 September 2017 at 21:19, Guy McCusker <guy.mccusker at gmail.com> wrote:

> 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.pdf
> >>
> >> 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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