[sdiy] some diy: ssm2040 filter

[Scott Stites]

Hi Ryan,

Sorry, I'm not close to my filter at the time, but some of the stuff you describe sounds rather like the problems I ran into when I had the circuit on breadboard (IE, the 'pops').  Wish I could remember exactly at what point the popping went away, but, as luck would have it, the 2040 filter was the fastest breadboard-to-module project I've ever done (I like it so much) so I didn't spend a whole lot of time playing with it before planting it in my modular.

The absence of the servo cap caused a problem with noise at certain cutoff frequencies, I do remember that.

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.

Throughout it all, I do know that I matched the transistors in the gain cells - can't remember if that's what got rid of the 'pop' or not.  I used the Moog trannie matching procedure that's so ubiquitous on the net.  If you haven't tried that, you might give it a go.  What type of transistor did you end up using, BTW? (I used 2N3904's/2N3906's).  I've got a passle of low noise Japanese transistors left over from the Dim C project - I'd like to try them to see if there's any variation in the sound.

Hope you get it working - it's a heckuva nice filter.  It can even do a pretty fair Australian Frog Fart (AFF).

Cheers,
Scott


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