[sdiy] Mood Ladder Filter: Many Questions!

[harrybissell]

MOOD ladder...  ?

I've heard of mood ring  :^P

iirc Osamu Hoshuyama has a scheme to do highpass with the ladder
structure.  The idea is not his, but he added a really clever implementation
where he uses capacitors to form the voltage divider at the top of the ladder.

This gives about an 18dB hipass.  I think you can get bandpass as well by
injecting
the input to the top and bottom of the ladder at the same time.

The Oberheim multi-mode filter is 'possible' with the moog ladder, but I would
NOT recommend that.  You need to buffer each stage of the ladder, have inverted
outputs, AND be able to kill one stage.  If you were willing to forego some of
the
outputs, you need not kill the one stage (hardest to do imho).  All told the
amount
of circuity would be incredible.  Use some four pole chip like the SSM2040,
2044,
CEM 3320 and start from there for this method.

You can add extra poles... until you run out of power supply voltage. Someone
made
a five pole version.  As you add poles, the cutoff becomes more abrupt (or
should... if they track well).  Might not be as musical as you might like.

H^) harry

D A F wrote:

> 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