[sdiy] Mood Ladder Filter: Many Questions!

[D A F]

Hey gang,

I'm thinking of building a Moog (or ARP) style transistor ladder VCF.  I had
some thoughts about possible variations on the basic design.

Can this style filter be made into a multi-mode filter, where you can have
high-pass or low-pass options?  I think that's usually done with a
state-variable filter, no?  But I am guessing you could somehow include the
ladder inside a feedback loop to get a high-pass response instead.  Or what
if you used AC - coupled inductors in place of the caps in the ladder?

Could you "tap" the filter at various points to obtain different numbers of
poles?  I mean to say that whereas the stock circuit has the differential
buffers at the top of the ladder and thus yields a 4-pole filter, could you
move down the "rungs" of the ladder to get 3 pole or 2 pole or 1 pole
responses?

Is there any reason why you couldn't add more "rungs" to the ladder to get
even more poles?

Also I'm wondering if there is some consensus on how to get the 'best' sound
from this style filter, by 'best' I mean the "moogiest" sound.  I have
schematics for the Moog circuit and also the ARP 4034 which use CA3086's
"selected" (for matching, I presume).  I have both 3046 and 3086 chips so
was going to use one of those.  I think I remember hearing someone say
something about using different capacitor values for the different rungs of
the ladder - was this ever done stock or was this a mod someone came up
with?

I've never made one of these before, so I'm just doing some thought
experiments before I decide on the final circuit.  Sorry for all the
questions... any insights cheerfully welcomed!


Dave