[sdiy] some diy: ssm2040 filter

[ryan williams]

harry, I tried your suggestion, no luck. but a good idea, I never would 
have thought of that.

Scott,

Scott Stites wrote:

>The absence of the servo cap caused a problem with noise at certain cutoff frequencies, I do remember that.
>  
>
I tried without out this cap and the opamp oscillated so it deffinently 
needs to be there.  What I can't figure out is why I still see the pop 
on the output of that opamp in the servo. the voltage at the inverting 
input of the opamp stays at 0V (only since I have reduced the current 
limiting resistor in the servo from 1k to 470ohm).  the base voltage at 
the expo converter input is not jumping around.  I think that during 
this pop, a bunch of current must be groing through the servo cap. I'll 
have to think about this a little more.

>For popping, I recall that sweeping the input CV would cause a pop at a certain point, and I could observe it on the scope as a sudden spike in amplitude of the signal.
>
yea, it sounds like we had the same problem.  I used dmmt3904w, 
dmmt3906w supposedly matched pairs.  datasheet says to a max of 2%.  I 
figured that since the OTA circuit isn't very precise (input resistance 
is off on between the npns and the current mirror has error) that just a 
medium close match would be good enough.  I intended to measure how well 
they really were matched but these were just so small I couldn't deal 
with them very well (sot-363). 

-ryan