[sdiy] Mood Ladder Filter: Many Questions!

[Antti Huovilainen]

On Sun, 27 Feb 2005, D A F wrote:

> 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
> ...
> 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

Yes and yes. You can use the same trick Oberheim XPander used to provide 
16 different modes. You need to tap each section and combine them with 
different strengths (depending on mode). The downside is that you need 4 
differential amplifiers and a whole bunch of 1% resistors, so parts count 
will be large.

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

No. I have doubts whether more than 4 poles would do much good though. 
Because resonance is realized with negative feedback, self oscillation 
frequency will move downwards as more poles are added.

Antti

"No boom today. Boom tomorrow. There's always a boom tomorrow"
   -- Lt. Cmdr. Ivanova