Moog ladder filter question

[Magnus Danielson]

From: Jaakko Lofstrom <jlofstro at pcu.helsinki.fi>
Subject: Moog ladder filter question
Date: Tue, 1 Feb 2000 13:37:19 +0200 (EET)

> Hi all,
> 
> I know this question is a bit lame but one has to start somewhere, eh?
> I've got this mini-Moog ladder filter schematic ( from synth fool, if I
> remember correctly ) that I've been trying to understand for a while.
> I certainly am not asking anyone to give me a complete lecture on it's
> design, but I would greatly apperciate if somebody could give a couple of
> hints that would help me to understand how the damn thing works.

It's all in Moog's patent, but the short story is that standing current in
the ladder will change the operating point for the transistors (or diodes in
the EMS diode ladder) and by this will the transconductance change and by that
will the filter be tuned diffrently. If you view each transistor as a variable
resistor will the transistor ladder be a bunch of serially connected RC
filters. Now, you feedback the output and by varying the feedback you can
control the Q factor.

It is worth noting that the transconductance is a small-signal property, put
if you run larger signals through it you get distorsion at each step and on
top of that things gets more or less filtered.

I hope it gave you a hint on what goes on... then there are these guys here
that can give pretty extensive talks... I'd love to hear some again...

Cheers,
Magnus