[sdiy] [OT] Analog synths with 2 pole filters

[cheater cheater]

The fix, then, seems to be to add a little of filter output to filter
input, as is usually done.

Cheers,
D.

On Fri, Feb 22, 2013 at 3:20 PM, Andrew Simper <andy at cytomic.com> wrote:
> On 22 February 2013 22:00, <rburnett at richieburnett.co.uk> wrote:
>> Op-amps operated as finite-gain blocks will exhibit more phase-lag at high frequencies, so I would expect state-variable filters
>> to be more inclined to self-oscillate at the high end of their cutoff frequency range than the low end.  Given that they are theoretically
>> critically stable when there's no damping feedback, this effect should be quite noticeable.  Can anyone who's built one confirm or dispute this?
>
> I can confirm that this is the case with my friend's Oberheim SEM,
> which is I think one of the synths that prompted him to think that
> "most" 2 pole filters didn't self oscillate. I'm not sure if this is
> an original SEM or the recent re-make version.
>
> Andy
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