[sdiy] Moog filter self-FM questions

[cheater cheater]

Hi guys,
I have a question about the control current on the transistors in the
moog ladder filter.

First I'll explain how I see the filter, this is how Richard Atkinson
explained this to me (thanks Richard!) - I'm not that great with
electronics so I might be screwing up something he told me.

I understand the moog ladder is (basically) 2x butterworth filter,
connected in parallel in differential mode, with the resistors
replaced with transistors; the transistors change resistance based on
the DC current across them; that same current contains an AC signal
that is the audio. Because the whole thing is differential, once the
signals are mixed together, the DC current which is common mode gets
rejected whereas the audio which is flipped on the left side gets
amplified (via normal differential operation).

The key thing is that on each side of the ladder the signal and the
control current are the same signal. In this case the audio modulates
the cutoff to some extent too; on one side of the ladder it modulates
it positively, on the other negatively. The question is to what extent
this happens.

I understand that this is different depending on the implementation
(again pointed out by Richard); so when talking about this let's
mention what synth we mean and if it's modular or custom-made then
also what specific transistor model and manufacturer.

Questions: 1. what is the swing of the (common-mode) DC control
current? What is the maximum and minimum?
2. what is the swing of the (differential-mode) current that
represents audio input?
3. if audio is patchable into the cutoff control, what's the swing of
the current control that can be created this way?
4. are the ladder sides biased by some DC that's always present?
5. what's the DC injected at the minimum and maximum cutoff setting?


BTW, when you mix down the filter sides they will actually be
(somewhat) different, as I understand it; to the extent to which the
audio is actually FMing the filter.

Thanks a lot
D.