I find that the open and DIY aspects of Mutable Instruments is a never ending source of inspiration and learning. For me there’s always new things to explore and discover, even for the, now quite old, pre-eurorack designs.
For the Ambika SMR4 voice card I have been thinking about increasing the resonance compensation capacitor to make the Juno/Jupiter like attenuation of lower frequencies at high resonance less pronounced. I’ve looked at the Shruthi-1 SDE modifications, where this capacitor was completely bypassed for a full bandwitdth resonance compensation, and was also intrigued by the possibilties of multiple filter slopes. The problem was that for some sounds I really like the shiny Juno/Jupiter quality, while for other bass like sounds I’d like the full bass end, so being able to select this programmatically would be important.
When Pharmasonic published the excellent SMR-4Pi design modifications I had the epiphany that if the filter slope selection was limited to “only” 4P and 2P low pass, it should be possible to use the other half of the 4052 to select between resonance compensation with, and without, the capacitor. While I was at it I also incorporated the filter tuning possibilities from Andres SMR4-Plus design.
The resulting untested works-in-progress is available at https://github.com/bjoeri/ambika_smr4x .
My main concern is that this is my very first attempt at any kind of circuit design and/or modification, so it’s highly likely that I’ve made some major rookie mistakes. For example, I wonder if the routing is ok with regards to noise etc. As you can see some analog traces pass, or are close to, digital signals. Compared to the Pharmasonic design I added decoupling capacitors for the 4052 but I have no idea how important this is. Any kind of feedback is highly appreciated before I go ahead and try these out in real life.