[sdiy] Oakley Moog filter

[Mike Granger]

Paul,

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.

Mike Granger

pfperry at melbpc.org.au wrote:

>Just built up the VCF-1 issue 2Oakley ladder filter.
>Very nice board (& I expect the new product that obsoleted it is even better!).
>What I wonder is, can it oscilate below 200Hz? 
>As I run the osc freq down, it falls off pretty sharply around 200Hz & won't
>osc. But, I seeem to remember that the Moog ladder doesn't osc at low freq anyway.. is this true?
>I wanted to just sledgehammer it by putting a bit of a LP filter in the feedback loop, but the EE I was with turned white & said something about
>"The poles! the poles!" so, if anyone has an idea about this, let's hear!
>
>(Incidentally, increasing the caps in the ladder shifted the freq range down,
>but had suprisingly little effect on the lowest osc freq, which seems 
>counterintuitive..)
>
>paul perry (Melbourne Australia)
>