Pre-distortion Diodes for Moog Ladder Filter

[Magnus Danielson]

>>>>> "MBI" == Michael B Irwin <mirwin1 at istar.ca> writes:

 MBI> Magnus,
 MBI> Thanks for your helpful advice.

Well, anytime. You can also see it as I do my part to keep the
confusion up (or was it down, I could never tell) ;)

 MBI> Is the major source of distortion in the Moog ladder (under
 MBI> normal operating conditions) distortion from the input
 MBI> differential amp stage or distortion due to instantaneous 
 MBI> modulation of the filter cutoff frequency by the audio signal?

Well, I haven't done any real measurements or simulations, but if you
ask for my gut-feeling guess I'd say that the non-linear modulation of
the poles migth in itself not be that important, but it is certainly
there.

 MBI> If the signal-induced modulation is the dominant effect, it would be
 MBI> interesting to take a 24 dB/oct OTA VCF and feed some of the audio
 MBI> output back into a control voltage input in an attempt to produce a
 MBI> controllable amount of warm-sounding distortion in a low-noise VCF.

There is just one way to learn, try it!

Personally I don't think this is the most important non-linearity, I
would say that the tanh/ln distorsion in combination with the linear
filtering effect accounts for much of what is going on. See it as my
current theory, nothing more. There are people that has done much more
homework than I have, I leave this to them.

Cheers,
Magnus