My first post here, hello!
I just spent a few hours trying to clear up some things I didn’t like about the SMR-4. The sound of this filter is great but the internal levels aren’t perfect IMO.
I installed a very tiny value capacitor across the feedback amp (IC4B) to reduce the HF hash that destroys the signal at really high cutoff and resonance. I also put in some signal diodes to clip the waveform a little and keep it close to sine wave, still not happy but it is better. Zener ones would be good here but I only have red LEDS and some diodes from an old ham radio, the diodes worked much better. I was hoping to get the nice resonance distortion that the ms-20 gets from it’s clipping LED but the levels in the SMR-4 just aren’t high enough it seems.
I had intended to mount the 2-4 pole jumper as a switch but I never really liked 2 pole filters. Instead I used the output of the first filter stage to provide a rough bandpass output to compliment the 4-pole LP. It’s not a great band pass but it opens up a whole new range of sounds. To access this output run a wire to one of the unused pads for C25 (you want the side connecting to IC8D pin 14) and connect to you favored switch etc. You could wire up a 3 way switch and have selectable 2-pole, 4-pole and bandpass. High pass would be possible but requires some hacking of the board and it’s not worth it if you cant adjust the polarity of the envelopes to suit.
The last thing I looked at was filter FM, it’s simple to just wire up a pot from the output to the spare CV inputs on the board and achieve FM. I haven’t settled on the ranges or resistor values but you can tap the signal either from the output of the filter or the output of the VCA, both will give different results. So far it sounds great but the useful modulation ranges are so small with FM it might as well just be on a switch rather than a pot. Once I spend more time on it I’ll draw up my preferred values and wiring scheme.
Am really enjoying the Shruthi so far!