[sdiy] 4-Quadrant VCA?

Bernard Arthur Hutchins Jr bah13 at cornell.edu
Thu Sep 14 05:57:13 CEST 2017

Please do NOT call a 4 Quad Multiplier a VCA.   A VCA is and should be only two quadrants.  The reason for this is that if you have 4 quadrants, the only way for the VCA to turn OFF is to have an envelope go very precisely to zero.  If the envelope remains slightly positive, the signal leaks through, and if it goes slightly negative, the signal leaks trough (inverted).  Keep in mind the extremely wide  dynamic range of the ear.  What you want is to be able to "bury" the tail of the envelope ever so slightly negative so that it shuts down (a 2QM).  In addition to the difficulty of trying to get exactly to zero, it is contended that burying the tail sounds more natural.   Apparently, most physical musical instruments shut down (friction?) ahead of a theoretical exponential decay.

So, the OTA is rather inherently a 2Q multiplier (a VCA).  As originally presented in National's app note on the LM13600, it can be made to work as a 4QM.  This we can suggest as an adequate "ring modulator".  The RM effect need not be exact - ARP once used a left-over logic gate for what they called a RM!  The OTA  4QM is not suggested for something more precise such as a frequency shifter.

The OTA 4QM is in fact analyzed in Electronotes EN#107 - pages 13-15.  Unfortunately, I can't at present post this - my (old) Lexmark scanner is not supported by my (new) Windows 10.   If someone can scan these three pages and attach them to me, I will add them to the requested section of the EN site.


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