[sdiy] Sanity check: equal power panner and VCA law

Rutger Vlek rutgervlek at gmail.com
Wed Jan 2 12:54:16 CET 2019

Hi list,

I'm working on an equal power (or -4.5-dB BBC law) panning and cross-fader circuit, and have been looking at this implementation using THAT2180 VCAs http://www.thatcorp.com/datashts/dn120.pdf. I've adapted the principle of operation from this circuit to work with the SSI2164, and in Spice that works relatively well (my breadboard version is still buggy). However, the error from the ideal equal-power curve is a bit higher than I was expecting, about twice the error the document reports. It made me think about possible improvements, and then I realized.....why use an exponential response VCA (THAT2180 or SSI2164) while a linear response is so much closer to the equal power curve? Out of curiosity I simulated a similar circuit with a linear VCA, using the same four diodes circuitry to obtain an approximated equal power curve and..... the result is a much better fit!

This makes me wonder who invented the approach with the diodes, and weather it was originally intended to be used with a linear VCA and slavishly copied by THAT corp disregarding the gain law. Does anyone know? Or do I miss something, and is there a good reasons for using this with an expo VCA, even though the fit to the ideal curve is worse (perhaps tolerance issues with circuit imply it's difficult to fully close linear VCAs)?


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