[sdiy] Moog filter self-FM questions

[Magnus Danielson]

Aaron Lanterman wrote:
> 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.
>   
If they are a little miss-tuned, they will be a little spread out at 
first, but the first initial feedback will force them together before 
they spread out as Aaron explained. Only two of the poles will be near 
the jw axis to make any fun effect, the other two will just support the 
additional nulls for the slope thing.

Cheers,
Magnus