A couple of (DIY) questions

[Don Tillman]

   Date: Thu, 19 Sep 1996 10:03:25 +0100 (CET)
   From: "Erik Schuijers" <E.G.P.Schuijers at stud.tue.nl>

   First of all, about Rick Jansen's Moog VCF. Can someone confirm whether it
   uses positive feedback (so Q gets bigger as the pot get smaller?)!

Negative feedback, BUT the main filtering section shifts the phase of
the signal over frequency, so it's 0 degrees at DC, easing to -90
degrees as you approach the tuned frequency, through -180 degrees at
the tuned frequency (-12 dB down), through -270 degrees above the
tuned frequency, and hitting -360 degrees at really high frequencies.

So the feedback at low and high frequencies is negative (lowering the
gain) and the feedback near the tuned frequency is positive (making
the resonance).

  -- Don