Wich low-cost filter for my Sk-1 can I use?

I want to add a lowpass filter to my casio sk1, but nothing too complex

any ideas ?

Smr4?

The SMR4 is certainly a great filter, and should be relatively easy to attach to an SK-1, you’ll probably need to separate the power supplies, but the good thing is the SM4 board already has the PSU on it. You just need to figure out how to wire the pots to it.
Another option is what I have used: Shane Williams’ Synthacon VCF, very easy build, more overdriven sound-wise, but certainly inferior to MI’s SMR4…

Since the SK-1 has 4 voices you could of course build 4 filters and have each voice run through a separate one, then attach the SK-1’s internal CVs to the filters… this is what they had done with the Phat Filter Mod (or how it was called). I’m not sure it’s worth it though.
I just wired the main output to the above mentioned Synthacon, and it works fine.

How can I access to the filter param like cutoff etc… without the shruthi’s digital section ?

With the “F” - cutoff ? and the “Q” - resonance ? : (top left)
http://mutable-instruments.net/static/img/smr4mkII_test_points.jpg

Yes… Take a 10k (or 50k, 100k…) linear pot, connect one end of the pot to ground, the other to 5V (there are ground and +5V headers on the filter board) and the wiper to the “F” point to get a cutoff control. Same for the resonance control.

thx !