[sdiy] some diy: ssm2040 filter

[ryan williams]

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