[sdiy] Sanity check: equal power panner and VCA law
steve at bendentech.co.uk
Wed Jan 2 15:09:39 CET 2019
On 02/01/2019 11:54, Rutger Vlek wrote:
> 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)?
The good reason may simply be that the THAT Corp VCAs have genuine
pro-audio levels of performance, but happen to be exponential, whereas
most of the linear OTA/VCAs still available are, performance-wise, poo.
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