Pre-distortion Diodes for Moog Ladder Filter

[Magnus Danielson]

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

 MBI> Hi,
 MBI> Has anyone thought about or tried adding linearizing diodes to the audio
 MBI> input of a Moog VCF ? This would be similar to the predistortion circuit
 MBI> used in the CA3280 and LM13600. Presumably this could be used to change
 MBI> the filter performance from the traditional warm-sounding gentle
 MBI> distortion ( with plenty of background hiss ) to a cleaner, higher
 MBI> signal-noise ratio mode ( more like other VCF's). It would be neat to be
 MBI> able to switch between these two modes.

Well, I have considered these diodes, only to figure out that they
would probably not really help all that much. They would help to
compensate for the tanh distorsion of the input transistor pair.

However, this is not the same as compensating the distorsion of the
transistors higher up in the ladder. In Dr. Moog's filterpatent it is
clearly stated that the signal level is kept so that the signal
current is much lower than the modulation current in order to keep
distorsion down. What happends is that the signal will not only
experience direct distorsion, but also modulate the poles in the
filter. This is a non-linear filter responce, but it may be nice to
the ear.

I am not sure that one should spend time to get the Moog ladder
approximate other filters and why should it? You could build that
other filter directly instead. There is surely stuff to do with the
Moog ladder, but is it really the things to move it into the line
sonically with the others?

I have been spending some time in figuring out how to set the zeros to
various points, now I know how to do that. I have also been thinking
of what can be done with various pole placements, but there is more
homework to do on that. This is only part of the small-signal models,
I haven't spent time doing large-signal models.

There are however some people experimenting alot with various ladders
that has had great success in there experiments and I expect more to
come out of that work.

Cheers,
Magnus