[sdiy] Formant 24dB filter

[marmot1 at magma.ca]

Check the archives for phase-reversal stuff. One technique that seems to 
work well is to use the op amp as an integrator instead of a simple 
follower. The (+) input is grounded, and it is much less likely that 
common-mode problems will occur. The integrator inverts however, so the 
OTA inputs will need to be reversed to compensate for the inversion. 
Another possibility is to use a FET or FET + bipolar buffer intead of 
the op amp follower.
Mike