Just finished some mods to SMR-4, FM, bandpass and more

Hey guys,

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!

>>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.

i’m not sure what you mean with the polarity of the envelopes. if you mean inverted envelopes (with filter opening in release) it can be done in the modulation matrix.
anyways, very interesting
looking forward for your results!

Would you be willing to make a quick drawing of your concept for a bandpass-style filter and HF hash eliminator? These sound like excellent mods and I would love to try them!

OOh, nice liking the sound of this, bandpass! =)

Do you just run these mods from the points mentioned to ground, with a switch in between. For example, from “one of the unused pads for C25 (side connecting to IC8D pin 14)” to a SPST to ground?

Has any one tried theese mods.
This is very interesting


I would love a bandpass. How do you adjust the frequency offset between the HPF and LPF?

Anyway, if anyone tries this, please let us know the results!