[sdiy] 4-Quadrant VCA?

[Guy McCusker]

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.
>> _______________________________________________
>> Synth-diy mailing list
>> Synth-diy at synth-diy.org
>> http://synth-diy.org/mailman/listinfo/synth-diy
>>
>
> --
> --Tim Ressel
> Circuit Abbey
> timr at circuitabbey.com
>
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at synth-diy.org
> http://synth-diy.org/mailman/listinfo/synth-diy