[sdiy] some diy: ssm2040 filter

[harrybissell]

Maybe your filter is starting to self-oscillate at ultrasonic frequency ?

I'd look at "Q enhancement" ... basically there may be too little
feedback at some high frequency because of circuit delays ?  This
is common in the SVF (not the 2040) but I ran across this problem
in a Moog ladder clone, and the problem was the same.

I'd try a small capacitor across the resonant feedback resistor

Hope this helps !?

H^) harry

ryan williams wrote:

> René Schmitz wrote:
>
> > ryan williams wrote:
> >
> >> first:  once the cutoff CV gets to a certain point (almost full CV)
> >> there is a glitch and an audible pop is heard.  looking at the
> >> scope.  the ouput waveform starts to decrease in amplitude just
> >> before the pop. when the pop happens the amplitude returns to the
> >> level it should be.  it makes no difference how quickly I change the CV.
> >
> > Try adding a 1n (or thereabout) cap from the inverting input to the
> > output of U2B. Maybe that expo is starting to oscillate at some point.
> > (It's missing on my schemo too, sorry!)
>
> I checked the output of that opamp, it is not oscillating but the
> voltage drop across the 2.2k current limiting resistor was so much that
> the opamp output was maxed out at the -12.5V or so that a TL072 can do.
> I did some quick calculation and it seems that it was taking in about
> 12mA at full CV.  I reduced that 2.2k to 470 ohm and the opamp output no
> longer jumps to -12.5V.  but the pop is still there and I am closer to
> the problem. Now I can see a pop at the opamps output. that is the first
> place in the signal that I can see the pop on my scope.  If I follow it
> up through the expo converter it is also visible at the bases and
> emiters of the OTA npn transistors.   I also noticed that I can see a
> very small (a few millivolts) version of the audio input signal at the
> opamp output, but only after the CV input is greater than the voltage
> that causes this pop.  I'm out of ideas.
>
> -ryan