EMS VCF Failure/Happy Accident

[A.S.P.]

Last night I finished an EMS VCF clone, built it pretty much identical to 
the design in the book (Sorry, don't know the title off hand), except 
that I added a non inverting buffer/summer at the audio input.  I pump in 
a 2Khz squarewave, turn on power and look at the output on my scope - out 
of control!!!  I don't know how to describe what's happening, other than 
the sound is closer to two VCOs crossmodulating than a VCF.  Adjusting 
the cutoff and resonance changes the sound from buzzy to raspy to 
earsplitting buzzsaw, not the soft mellow filtering effect I'd expect.  I 
guess I've accidentally created a timbre modulator of sorts.  
  
At this point I'm tempted to leave it as is and build another one that 
does what it's supposed to.

Romeo