[sdiy] some diy: ssm2040 filter

[ryan williams]

hi scott,

Scott Stites wrote:

>I'd love to see a pic of your version of it - tiny filter, big sound!
>  
>
well, it is not exactly tiny.  the only smd parts are the transistors, 
and bypass caps (i find it makes it easier to get them really close to 
power pins this way).  even so, it could have been much smaller but I 
did this board layout in an afternoon and sent it off to be made the 
next day.  I had most all the through hole parts in my parts boxes 
though so I didn't see the point in spending the extra money there.  It 
met my size requirement which was length long enough for 4 pots with 
motm/oakley spacing. here is a pic taken before I starting trying to 
test/fix it.
http://www.sdiy.org/destrukto/temp/smd2040-populated.jpg
its been through hell now and doesn't look so good.  I'm going to go 
ahead and order a front panel. real pics farily soon.

>So - is it small enough to fit on a DIP socket? ;-)
>  
>
lol, no.  but I did do a tiny layout of just the discrete ota + 
darlington and it fit on an 8pin dip. I think they were going to be 
$0.50USD each for around 35 of them. I just decided to put them on the 
board for now since I didn't know how well they would work.

>I've been idly wondering if one couldn't take the gain cell sections and
>re-arrange them into other configurations (SVVCF for example) as could be
>done with the original SSM2040.  JH, Rene, ever tried this?
>  
>
you probably could,  about 8 months ago I took the discrete OTA cells 
and built the SVF.  unfortunately at that time I didn't realize that the 
darlington buffer was an important factor in the sound.  Instead I used 
inverting opamps because I was thinking that they would allow a larger 
signal input without clipping it.  that was correct but the sound was 
somewhat boring..  the dc offset voltage at the output was going all 
over the place with CV changes, and I even had trimmers there (that 
might be normal?).  It might be worth trying it with the darlington 
buffers. I wish I would have known to try that at the time.

-ryan williams