[sdiy] Moog filter cutoff modulation with drive

Tim Stinchcombe tim102 at tstinchcombe.freeserve.co.uk
Wed Jun 1 22:57:53 CEST 2011

> Well I finally got the simetrix model to work and have 
> confirmed that my model behaves the same way, so there is 
> considerable modulation of cutoff frequency when you drive 
> the circuit without an input clipper.

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?

This would also mean that in a real circuit this effect and the clipping
would go hand in hand.

Tim Stinchcombe 

Cheltenham, Glos, UK
email: tim102 at tstinchcombe.freeserve.co.uk

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