[sdiy] Moog filter cutoff modulation with drive
colin at colinfraser.com
Wed Jun 1 23:08:46 CEST 2011
> I've only had a quick think about this, but I would guess
> this effect is something to do with the 'tanh' response of
> the differential pair at the bottom of the ladder - when
> being driven hard, outside of its 'comfortable'
> linear zone, I think it will tend to 'choke off' the current
> in the ladder, which would be tantamount to lowering the
> cut-off frequency. The closeness of the plots of your model
> and the SIMetrix simulation output suggests that your model
> includes the tanh nonlinearities in some way - is this the case?
Isn't it simply that the audio signal is added to the static control current
in the ladder in order to filter it, and thereby has a direct effect on the
dynamic resistance of the transistor junctions which are the R's in the four
stages of RC filter ?
I avoid maths like the plague, but that's how I understand it.
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