[sdiy] Signal bleed on moog ladder

Andrew Simper andy at cytomic.com
Tue May 31 18:17:30 CEST 2011

Has anyone that has a moog ladder style filter noticed much bleed
through of the audio signal to cutoff frequency producing filter FM
when the input is driven?

More specifically I've found through doing a model of the circuit that
the cutoff is reduced when a large input signal flows through the
ladder. I got a friend to test out his little phatty, but there is a
huge amount of clipping of the input signal before the level can
increase to really show the effect I've found. Can anyone that has a
circuit without as severe an input limiting stage confirm this please?
Here is the plot I generated using my model in self oscillation with
an exponentially decaying sin input:


The easiest way I've found to hear it is to have a sin(ish) input at
around 20-100 hz, and then put the cutoff in self osc at around 2000
hz and then increase the level of the input till you hear fm of the

I had a full circuit simulation ready in simetrix (trial) but my hd
decided to die so I couldn't test it out, so I really appreciate some
help groundtruthing this whole thing as there is nada I can find on
the internet about it.

All the best,

cytomic - sound music software
skype: andrewsimper

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