[sdiy] Moog filter self-FM questions

[Aaron Lanterman]

On Mar 27, 2010, at 7:27 PM, cheater cheater wrote:

> 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, 

Minor nitpick, but they're not really butterworth. A butterworth has poles that form a circle. 

The raw 4 stages of a Moog ladder (or Prophet 5 style OTA-C cascade, or whatever) each has a pole in the same spot along the real axis - they pile on top of each other. When you do the feedback then they spread out in an "X" pattern.

The rest of your writeup looks correct to me.

- Aaron