Minimoog VCF

[Sean Costello]

tomg wrote:
> 
> > this one oscillates
> > reliably, though the amplitude of the oscillation rises with frequency. I
> > don't know if this is part of the minimoog sound or not? (Any opinions?)
> 
> hummmm......here I go again....the 3080 version doesn't do this. The
> amplitude stays fairly constant. You can play with the freq all you
> want and no real change in level is apparent. I would think this is
> they way you would want it to respond.

All depends on what your goal is.  For bass lines, the reduced resonance
at low frequencies is useful.  For FM using filter resonance, this would
be a disaster. Many people complain about the sterility of the
Micromoog/Multimoog filter, but some of the best sounds out of the
Multimoog come from using the oscillating filter for FM waveforms (this
is also greatly assisted by the fact that the filter and modulating
oscillator have their expo convertors in the same heated 3046).  Mind
you, the sterility of the Micromoog filter has been attributed to the
high number of components the feedback signal has to go through before
it gets back into the ladder.  Your design is nice and simple, with a
minimum of components in the feedback path, so it should sound great.

Just had an idea: To use the tomg filter design, but get that "phat"
Minimoog bass sound, just change the coupling of the ladder to the 3080
diff amp. Instead of using direct coupling, couple the ladder to the
diff amp with .22 uF caps, with 47k resistors going to ground between
the caps and the 3080.  Heck, you could have this as a switchable
option, for a constant resonance/Minimoog response option.  Seems easier
to build than the Minimoog filter. (If this is a bad idea, hey, it's
late at night, I've spent the last 18 hours modifying and compiling a 60
second composition that takes 15-20 minutes to compile, and I suck at
analog circuits.) 

Sean Costello