[sdiy] Moog filter cutoff modulation with drive

[Colin f]

> 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.

Cheers,
Colin f