[sdiy] some diy: ssm2040 filter

[ryan williams]

hi all,

I've been building an ssm2040 filter based on JH and René Schmitz' 
schematics. basically it is the René Schmitz version but with VC-Res 
circuit from JH's design.  the VC-Res circuit was modified for a 5V 
instead of ground just like the OTAs were.  The transistors I used are 
the dmmt3904w and dmmt3906w pairs from diodes.com (these were a pain to 
solder).   Everything seems to be working properly except two problems 
I'm having.

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.

second:  the DC offset at each stage's output changes about 3.5V as the 
OTA bias current is increased.  Actually, this change is only happening 
at stages 1 and 3.  stages 2 and 4 have a small change, something like 
.2V.  I expect this is because of the asymetrical distortion of each 
stage. I'm measuing this with 0V input signal.  Is this normal? I think 
that is way too much to be normal.

any ideas? 

I put up a schematic in case anyone want to look at it.  obviously it 
still needs work so don't go building it like this yet.  There were a 
few other problems that I already fixed. first is the 5V regulator used 
for the 5V in the filter wasn't regulating because the minimum cururent 
wasn't being achieved. I added a resistor to ground to fix that. and the 
input/output gain resistors need to be changed, right now only very 
small input signals are not distorted:
http://www.sdiy.org/destrukto/schematics/smd2040a.pdf

-ryan williams