2 Filters 1 Shruthi

Hi guys. I would really like to use 2 4PM filters in series… Currently I think I have 2 options:

a) 2 full Shruthi’s with 4PM filter boards. And simply have the output of the 1st going into the 2nd, set up the 2nd with no oscs and open VCA etc. But then there’s no shared env’s or lfo’s…
b) 1 Shruthi and 2 4PM boards. Using the Shruthi in the 2xSVF filter mode and wiring CV 1 and 2 to the 2nd filter’s cutoff and rez. And of course the output of the 1st filter to the input of the 2nd.

Is the b option feasible?

A bit like this

You’ll need to check whether the digital signals for the digital switch output by the SVF mode make sense for the 4PM filter, otherwise you’ll need to hack the code a bit to create a custom filter mode.

Beautiful, yes! I will learn the ancient art of stacking and chaining 2 filterboards from your great posts. Thank you!

It really depends on how much you think your time is worth!

If you go with the b) option, you’ll need to create some circuitry to generate the digital data for the shift registers and muxes that control the mode selection on the 4PM boards. You can’t just get this from a digital control board in SVF mode! This will mean a big bunch of toggle switches, some firmware hacking, or a separte arduino-controlled device. All of these are fun hacking projects which might take several hours to get right. Are these several hours of hacking worth it compared to building a second control board to get a second Shruthi? It really depends on how much time you want to spend on this project, what’s your experience with electronics (analog and digital), and how much you’re willing to learn…

Ah good point, as I would definitely want to change filter modes of course!

Firmware hacking + circuitry to translate 2nd filter control signals to 4PM friendly signals sounds like a good option. I should be right with the firmware hacking, but the circuitry might be difficult (but interesting) to work out. Thanks for the suggestion!

I should mention that I subsequently disassembled my three-storey Shruthi and built another controller board to use with the second SMR4mk2 filter.

On further thought I’m leaning towards this option now too. If I have 2 filters I would prefer to be able to tweak them both at the same time. With some firmware hacking I could move both cutoff’s and rez’s to the first filter page. But the reason for this is easy realtime simultaneous control, and if I’m going to do that then I would also want stuff like LFO amount, rate and envelope stuff for both filters too. My aim is for an MS-20-a-like filter section. The liquid 4PM modes are a great bonus too!

So, what might be better is having 2 full 4PM Shruthi’s and a MIDI knob box controller with CC’s assigned on different channels for each filter parameter. And common things (like LFO rate and envelope ADSR values) assigned to single knobs outputting on both channels. I think this is possible with a BCR for example… (as much time as I’m making for Shruthi building, I don’t have time to plan and build a MIDIbox, though it would suit this custom application better probably)

Hang on - can the XT board provide pots for extra parameters in the non-SMR filter boards?


Ah damn, then I’m back to the dual Shruthi setup. But that’s not so bad as it will be loads easier for me to build!

>Hang on – can the XT board provide pots for extra parameters in the non-SMR filter boards?

Well sort of since almost all the other parameters have knobs you can use the 4 ‘standard’ edit knobs almost exclusively for this.

And you can polychain dual Shruthi: