[sdiy] Formant 24dB filter

[jhaible at t-online.de]

> Sure, it has non-inverting buffer on the output, but the input is
> inverted. What was you hinting at? Incorrect feedback-sign or what?
> Please speak up on the full story...

Phase reversal. Once your signal is strong enough to go out of
the allowed common mode range, negative feedback turns positive.
As somebody else pointed out, use opamps without phase reversal,
and the problem should be cured.

DC offset has nothing to do with it. Causes a little CV
feedthru, but won't latch the circuit up.

JH.