SMR-4 Multimode voicecard

I am currently drafting a SMR4-based multimode filter, loosely based on the pole-mixing models of Shruthis 4-Pole-Mission. It works rather well in simulation so far, with LP,BP,HP and Notch filters.

I have not tested it in the real world yet, but before I gather components or even order PCBs, is there any other implementation of a SMR4 based multimode filter, or reason why there exists none of these?
There were some old topics in this forum with a filter by Pharmasonic, but the history of the multimode filter seems to be erased from the internet, maybe for a good reason.

If not so: How much intereset is there for such a filter board? I’d probably design it as SMD, but they are easily manufacturable by JLCPCB and I’d release it under the Ambika license anyways.


If anyone is interested on this, I have pushed the first revision of schematics to a github repo:

This sounds awesome. Please report back when you test the real thing. I’d definitely be interested since the 4P is my favorite Shruthi filter – it’d be nice to have that option for my Ambika too (I currently have it stuffed with SMR4).

Alright, I’ve tested it, and it works very well! Now I’ve got to build some patches which actually use the new 3 filter modes. The voicecard 100% compatible with the default firmware, although YAM unlocks the fourth notch filter option.

The only problem in the current version is that I accidentally reversed the order of the filter types, but as the current board layout is ugly anyways, I’ll post an updated version and manufacturing files as soon as I update the board layout.

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Second board revision works even better. Here’s a little (terrible) sound-demo running the default sequence through various filter-modes at different resonance levels. Headphone users be careful!

Also I had some fun with the boards silkscreen :slight_smile:

The design files are on GitHub should anyone be interested.


Waw that’s fantastic jhbruhn ,great job !
Would adding a multi-pole option be possible or does that require a major re-design? Just out of curiosity because having multi-mode on this beautiful sounding voicecard is great. And I guess it would be hard to select them anyway.

love the silkscreen btw ! :slight_smile:

Certainly! The pole-mixing section is the same as in the 4-Pole-Mission Shruthi Filter so one could also add a second analog multiplexer to implement all the modes and poles the 4-pole-mission has. The current design is using 100k resistors in parallel to avoid using esoteric resistor values, so the space required by the resistors is quite large, but if one would switch to 0402 for example, everything should fit onto the voicecard. It would require firmware modifications for both the voicecard and controller of course, but the space left should be enough. But yeah, comparibly large redesign of the mixing section would be required, although it is rather simple.

I might have to make a set of these! Multimode would be pretty amazing to have while still getting some SMR4 sounding filtering!

I’ll report back if I decide to make some PCB’s
Thanks for sharing!

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