Pole mixing VCF as stand-alone?

I don’t have Shruthi but have interest in building the white board 4-pole mission Filter as a module for my MU modular.

I see that power is already rectified/onboard, and the schematic shows clearly, the cutoff & resonance, vca envelope and pre-filtered raw OSC audio… I’m no expert in reading schematics (just learning) so could somebody give me a few hints on how the Filter mode is decoded (I assume this comes in through the 74HC595 shift register). Beyond this, could the mode be selected by faking (via panel mounted switches and some encoder/decoders) the desired mode for filter type and resonance flavor?

Otherwise, just wanted to thank everybody for this ‘local’ community. I’ve found and learned so much just from reading across the past month and checking out all of the schematics, pictures, and build descriptions. I’m inspired by all of this!

Thank you.

the 74HC595 just output bits (+5v or 0v signals) to the 4053 and the 4051, that actually do the switching. The 595 awaits a serial digital input, which is not very practical to use unless you use a micro controller. So, if you want to use the 4PM as a standalone unit without any programmation and control board, you better not put in the 74hc595, and use switches and/or roto-contacters (is it an english word?) wired to the outputs of the 595. Don’t know if it’s really easy to use, but you could probably just wire a switch to every of its outputs, and write down the usable configurations on a sheet of paper…

Beware that the inputs are not following a v/oct scale though, and even worse, that depending on the filter resonance modes, a same voltage won’t yield a comparable result : those scales are ‘corrected’ by the shruthi’s software (example : a “soft” resonance in the wobbly mode could result in a very harsh, self-oscillating resonance in the ms-20)

  • For mode selection, the best solution is to use the same trick as on the Papareil Extrapole : use an ADC with parallel output, and wire the 4 highest bits to lines A, B, C, and SLP. Feed the ADC with a 0 to +5V voltage, so you can both select the filter mode from a standard pot and from a CV.
  • Wire a SPDT switch with central wire to SMS, and the two other pins to 0 and +5V to enable MS-style waveshapping.
  • Wire a SPDT switch with central wire to SFM, and the two other pins to 0 and +5V to enable FM feedback.

Keep in mind that the Shruthi-1 filter circuits are designed for low voltage / battery operation, so they work with weak signal levels (2 or 3V pp) ; and with a 0.46875V/Oct standard. You might have to correct all the input summing resistors and output feedback resistors to get higher levels.

Roto-contacters - I’d translate that as a rotary switch/encoder. For instance the Grayhill 25LB22-H used in the x0xb0x which outputs 4 bits, or if want to deluxe it: you can use a cheap A/D that reads a pot plus mix it’s input signal with an analog CV if you want real-time modulation of filter mode. Then, you can add LEDs or some other readout in parallel with the output bits. Olivier beat me to it above.

You could get an easy to program Microcontroller to convert any CV to a suitable set of lines that substitute the 595…

Thanks for all of the feedback. I think I’m sold on this and am going to order the board set this week.

I read though a bunch of old threads, one which discussed that some of the SSM chips are difficult to find; Can somebody give me the latest info on how difficult the 2144 or any of the ICs are to obtain?

Otherwise, I read a bunch about the philosophy behind Shruthi and it’s power and simplicity view very few controls and a relatively small display; So I aim to do both; have it wired up behind an MU panel with insert jacks to break the connection between control and filter board. Ambitious (for me) yes but I think it will be great even if I can only get 1/2 of my objective satisfied. Will keep this forum informed of progress. Thanks.

The thread you refer to were probably about the old SSM2044 used in 80s synths. The SSM2164 is a “modern” chip and is still widely produced by CoolAudio (V2164), hence cheap and easy to find (SmallBear, Mammoth Electronics)