[sdiy] Oakley Moog filter

[Paul Perry]

At 12:59 PM 17/10/01 -0400, Mike Granger wrote:
>The problem is not in the ladder caps, but in the low frequency rolloff 
>of the rest of the circuit. The feedback that sustains the oscillation 
>decreases in amplitude as the frequency drops because less of it makes 
>it through the coupling capacitors. If you increase the coupling caps 
>value, the oscillation should remain at somewhat lower frequencies.

And, indeed, by replacing the two caps with 200mF ones, and tripling the 
ladder caps, the filter now resonates happily at 16Hz ;D
Thanks Mike & everyone! As for deranging the poles, I figure it can't be 
worse seeing it already has the 2 caps in the feedback loop. In all fairness,
as the designer pointed out, this particular one wasn't intended as a bass
oscillator anyway.

BTW it was pretty funny watching my EE friend telling me that changing the 
caps wouldn't make a difference. Even while the CRO was hooked up & you could 
see the freq. "OK the lowest resonance freq moved down after you changed the 
caps, but it couldn't be because of that."

-paul perry (sucking & seeing in Melbourne Australia)