[sdiy] Filter strangeness.

[Pat Kammerer]

Hello all,

Back again to pick some brains  : )

Well, I've been losing lots of sleep (good thing) and making good progress
(very good thing) on that optical synth thingy, ever since I got the power
supply working.  I've even added on to the power supply, so I've got +9, +5,
and -5V now. I made a really sweet signal generator as OSC 1, which makes
square and triangle, both of which are pretty darn pure from 20Hz to
probably 25Khz.

Tonight I've built a supposed low pass filter, based on a 4th order
sallen-key filter.  Some of the inspiration can be found here;
http://www.crossovers.com/figure_2.html
I was focusing on the second schematic on the page. I am using R2 as my
cutoff control (maybe my problem?)

It seems to do it's filter thing, and under certain circumstances it sounds
very cool and acid.  BUT something is not right.  For one thing the output
of the filter is half or even less of the amplitude of the oscillator
itself.  Also the filter seems to change how the oscillator acts more than I
expected, like I can change the frequency on the OSC with the variable
resistor on the filter, especially when the OSC is running at very low
frequencies.  Sometimes it even seems to makes the pitch variation function
in "steps" instead of a smooth transition... weird...

It seems to be something like a ground problem, because I can raise the
amplitude by touching the variable resistor.  I do love the sound of the
filter though, I just hope I can work out the bugs.  It self oscillates
really nice, and I can stand or even appreciate the quirky interaction
between the two circuits, but the low amplitude is a bit bother some.  I
will be having an amp in series with it, but it seems like I could improve
the S/N ratio first and get a better result, especially since the OSC is
very quiet.

Do you think I need a buffer between the two?  They are running at different
voltages (OSC +-5V, filter +9V).  Or would you look more at my component
choices, not shown of course so I just went with my instict.  I know that th
e input of the filter should have grount sent parallel with the signal, but
since they are on seperate voltages these seems to not work quite right.
I'm supposing that it could also be the single supply filter dealing with
the dual polarity OSC.  Or so you think it could be the wrong value input
resistor.

If you wanted a photo resistor to vary cutoff, how would you setup the input
stage?

Also what would you do to add variable resonance to the circuit?  I was
experimenting and it seemed like a small value pot from the ouput of opamp 1
to the inverting input of opamp 2 did something like this, but it wasn't
quite what I am looking for I think.

All in all a good project and a learning experience, as I obviously need to
learn before I go at my modular.  : )

Thanks in advance for the help!
Pat