24 db/oct filter questions

[macdonald at evenfall.com]

I have been experimenting with a 24db/oct state variable filter for a
while and have a few questions.  It is built like two 12db filters in
series (basically similar to National's LM13600 datasheet design - page
11 - but using the LM13700) with feedback from the final output back to
the initial input.  Very standard stuff, done by many others I'm sure (I
think I saw someone post a similar circuit recently too, except with
3080s I believe?).

Anyway, while swapping components around on the breadboard I noticed
that if I made the first and third filter caps 330pf and the second and
fourth 100pf, the character of the filter seemed to change a bit.  The
resonant sound seemed "thicker" somehow.  Looking at a saw wave on the
scope I noticed I could get a larger amplitude of harmonics at the
highest Q setting before self-oscillation.  I realize that using unequal
cap values in the filter ruins it's correctness (gives unequal corner
frequencies for the stages, correct?).  My question is, am I just
fooling myself here or is there a logical reason why I might like this
sound better than the "proper" way?  Perhaps the different cutoff
frequencies "fatten" or widen the resonant peak?

Second question.  The stages of the LM13600 datasheet state variable
filter design look  like simple passive RC filters to me, with the OTA
replacing the fixed resistor.  Is that an even approximately accurate
way to look at the design?  So the design in the datasheet is two
first-order filters in series, with some clever feedback paths to
produce band pass and low pass outputs?

Lastly, like many others, I too had to abandon the idea of using the
buffers included on the LM13700.  I found they caused a nasty clipping
of the output signal at very low cutoff frequencies (even at fairly low
amplitudes) and caused a very large DC offset as the cutoff is closed.
Replacing them with op amps did wonders!  I guess FETs work here as
well?  I wonder why they didn't opt for a better quality buffer in the
chip.  Maybe cost or technology restrictions at the time the chip was
designed.  Oh well.

I will post my schematic as soon as I have a chance to digitize it in
case anyone wants to have a look.


Thanks,

-Chris MacDonald