Nonlinear Feedback Filter Update

[Scott Gravenhorst]

Current schematic at:
http://members.xoom.com/chordman/NonlinearFeedbackFilter.html

I did the first test with inverse polarity parallel diodes (1N4148).
Injecting a sine wave and playing with resonance and cutoff makes for
obvious added harmonics where there were none without the
nonlinearity.  I noticed that the resonance was effected by the output
setting from the nonlinearity driver opamp.  I will add another opamp
to buffer the output of the nonlinearity circuit later this week.

Right now, I'm just using a non-VC test oscillator (it's the only one
I have that does sine, square, saw, triangle and pulse).  I can
already tell that one of the more interesting ways to apply this is by
allowing the filter to track a VCO.  Perhaps more interesting when the
slopes are not precisely the same (if that were even possible with
this design).

The circuit is also sensitive to the input level, in that the
harmonics produced vary with amplitude.  Because of this, it may be
useful as a device placed after a VCA.

It needs work, but it looks good so far.

-- Scott Gravenhorst
-- FatMan Site: www.teklab.com/~chordman