Finally! (LP2+Delay filter board)
  • This has been on the back burner for quite some time.

    Doing a MS-20 like LPF is easy. Doing a PT2399 delay is easy. But making the delay controllable from a Shruthi-1 filter board and/or getting the whole thing to fit in the Shruthi-1 filter board format was much harder than I thought.

    The Shruthi-1 emits 2 CVs. This is not enough for controlling a delay for which we usually want at least 3 parameters (time, feedback, level). My first try with just a time + feedback control (output signal taken out of the feedback loop) kept the length of the delay “tail” coupled with its level…

    So I had to find a way of cramming more controllable parameters out of the Shruthi-1 system. My very first intuition was to do shitty PWM (a few bits of resolution, and a carrier frequency of 200 Hz) on the digital expansion port pins. This required super big filter caps to get reasonably smooth CVs, or an even bigger 2 or 4-pole active filter. The efforts that followed – and I was stuck there with no improvement over 2 or 3 months – focused on using the same approach as on the other filter boards (74hc595 receiving data through the expansion port), and toggling resistor networks through 4051s or 4053 in the feedback path – with CV1 controlling delay time and CV2 controlling a VCA on the delay output. The original design gave 8 unique feedback values, and another 4053 provided 4 different feedback routes (with fixed LP / HP filters)... not too bad but still it did not fit on the board :(

    At some point I considered doing the whole filter board in SMT so that everything would fit there. Another option I considered was to use good old pots, and make a dedicated bigger case for this version. with an extra column of controls on the side for the delay.

    Finally I had two epiphanies. The first one was that at the exception of the “main” VCA which has to be dead silent, a tiny bit of bleeding was allowed for whatever other VCA was used in the circuit (resonance control, delay mixing, delay feedback) ; and that some amount of compensation could be done software-side. So I could use very very simple, non-ideal V to I converters for those VCA (a single transistor), trading an op-amp for a bit of software-side calibration. The second epiphany was that I could use the TX> pin, normally used for serial communication with the digital filter board, as an extra GPIO to do proper SPI communication to a SPI device with the expansion port. This is a bit strange to do because the expansion port is really designed for chaining 595s, so everything we want to do with it has to go through the LEDs 595 and has to be done in sync with it, but I successfully managed to talk to a SPI slave through the LEDs 595 and the spare TX> pin. Yeah! At the same time I found about Microchip’s dual digital pots chips, and this was exactly what I was looking for!

    The 4251s were delivered on friday morning, I got a proof of concept in the afternoon, and finished the whole thing over the week-end.

    The last hurdle was that even with a minimal circuit for the delay, the board was too small. I managed to make it fit by putting to work the LM13700 Darlingtons as buffers / mixers and getting rid of 2 TL072.

    The final feature list is:

    • OTA MS20 LPF (not the original Korg35 design, the later version)
    • PT2399 delay with adjustable delay time (from a few ms to 550ms), delay level, feedback level, and feedback tone control (the echoes can get darker and darker à la cave dweller or brighter and brighter as often heard in dub…)

    Preliminary audio sample here

    There’s a lot of noise but there’s a very good chance it’s just due to the messiness of the proto and poor grounding of breadboards…

  • Nice! Is this an add-on or a new Shruthi-1 board?

  • Awesome.

  • This is a new filter board. There wouldn’t be enough space for this + another filter board + a control board!

  • Wow! Huge work, and very good result, love it’s sound!

  • this is pretty awesome!

    If I may give a bit of advice, think about trying your design with several pt2399 chips. Those are not always very consistant, especially regarding the minimum delay time, causing them to stop working. Some have used a current source to overcome this issue and allow the pt2399 to work as some chorus effet, using the minimum time allowed by every pt2399 chip.

    Oh, and from what I understand, since you’ve used the TX> pin to handle both feedback and level of the delay, is there another CV output left? Or maybe you used it for the echo tone control.

    It’s been some months that I’ve been considering about using digital pots. What are the references you used?

    edit : did you try using the pt2399 as a chorus? With no feedback, minimum time delay, maximum level, and modulating the delay time with some LFO : are the digital pots precise enough for this kind of stuff?

  • I actually have a double shruthi in a partially premanufactured case that has some spare room… was actually thinking of cramming a pt2399 based delay in there – they are CEM3379 filters though so you haven’t completely made my project irrelevant :)

    chance of me not building this new filterboard – 0

  • Wowowow, that sounds excellent! It’ so buttery and OTA-ey. I am a huge fan of Korg filters. I’ve been waiting a while for this one, and even just the preliminary stuff is making it totally worth the wait!

    I guess I just would have wished that it had the additional bandpass of the MS 20 to get vocal sounds, and other weirdnesses.

  • @MicMicMan: the CV1 and CV2 pins are respectively controlling the amount of feedback in the low-end and in the high-end. This is done with simple passive RC filters at the input of two VCAs (yuck). The UI presents this as a level control and a tone control, and this gets translated into the right pair of voltages (or rather PWM ratio) for those. Now, and this is a bit tricky, if you route a modulation source to CV1 / CV2 on the Shruthi-1 mod matrix, it won’t modulate those ; it will modulate instead the delay time and the delay level – I think this is more useful…

    I’ve observed a PT2399 stalling during developed, so I plan to do a software adjustment to avoid the minimum delay time to be reached. That or an extra resistor as often seen. Too bad because I love the super short delay times, especially with heavy feedback in the low-end.

    The digital pots I used are MCP4251-503 (dual, 50k, 257 values, SPI interface, no persistence of settings). The biggest caveat with those is that there’s a digital glitch in the output everytime the values are refreshed (CS pin transitioning from low to high), whether the actual value changes or not. This means they would sound horrible with fast modulations. I had to write code that monitors successive values and skip the refresh if the values do not change to reduce the glitching. The RockIt uses 42050s – I don’t know if they are any better in terms of glitches, if not, using this as a fast gain control element is an audio circuit is totally insane. These are certainly not the same kind of animals as DACs.

    The resolution of the digital pot is not good enough to allow a convincing chorus. If I try modulating the delay time with a LFO in the mod matrix. With a modulation amount of 1 or 2 it gets very steppy of course (since the delay time goes only through 4 values), with a higher modulation amount it gets all sproingy. I think this needs to be done in the analog domain directly by sourcing a very small triangle current into the VCO pin of the PT2399…

    @n8bit: a full MS20 clone would have required a full board, and no delay :(

  • Congratulations !

  • awesome. i have not even put my cave dweller in an enclosure and now this :)

    i thought, after the 4-Pole i already own the perfect shruthi and i could skip the next filter-board because it would not be a “must have”
    i was wrong. i need another one….

  • ... if you have not done so already oliviere, it might be time to talk to papernoise about a nice PCB and Enclosure design/logo.

  • Yay, another one! Gotta collect em all!!


  • I can’t believe it a Shruthitron Delay Machine! Sounds awesome!

  • YARR!

    I hope that we get Dr Korg or some funky kamon on the case!
    Can I book one using my soul? Partials? I may have over-stretched and promised it away a few times already due to all those unread EULAs…

    Needless to say, it’s awesome!

  • Wooooo!!!!! oooo!!!! ooo!!! oo!! o! o


    Can’t wait now.

  • @pichenettes
    “The resolution of the digital pot is not good enough to allow a convincing chorus. If I try modulating the delay time with a LFO in the mod matrix. With a modulation amount of 1 or 2 it gets very steppy of course (since the delay time goes only through 4 values), with a higher modulation amount it gets all sproingy. I think this needs to be done in the analog domain directly by sourcing a very small triangle current into the VCO pin of the PT2399…”

    yay, this option is worth considering. That’s what I thought about the precision of digital pots.
    In that case, that would be über cool to have a mod-pad© on pin 2 of the pt2399, since this seems to be the better option for getting chorusish/vibish feel out of the pt2399

    Anyway, you’ve been talking about this board for a loooong time. I remember that since i came to lurk this board i’ve always read you talking about this ms-20_with_delay_filter_board project. This is quite an achievement, Congratulations :)

  • I love delay. I love MS-20. Gimme.

  • @MicMicMan: I have broken out ground + VCO control pin — primarily for people who want to use this board without a Shruthi and thus without the digital control ; they’ll stick a pot there — but this could be used as well for the mod you describe.

  • dope. Cant wait. I’ve been looking for a delay/filter deal for a while now and this looks like it hits that right on the head

  • Awesome! That demo nails the Severed Heads sound for me-they used an arpeggiated MS20 with delay-can’t wait!!!

  • @pichenettes, how the delay time is defined? Absolute value in ms or relative to BPM (that will be very great…)

  • It’s a awesome work.

  • @Schrab: at the moment, it’s just a number between 0 and 127 :) But this is just software, not a big deal… My concern is that there might be some variability from chip to chip and/or a dependency to temperature (from the VCO generating the clock) that makes it a bit difficult to get reproducible absolute delay times.

  • Wow, excellent! I’ve been waiting for this one for ages- thought you’d forgotten about it, pichenettes.
    Sounds great. Excellent work! I’ll definitely be getting one of these when they appear in the MI shop. Now, what colour enclosure should I get or it (it’s all about the colour of the enclosures)...

  • @pichenettes on the delay-time issue, would it be impractical to have the the option to synch delay time to MIDI clock or internal tempo, given the potential variability issue you mention above?


  • I’ll investigate more, but I’d rather say “at the moment no” than promise you something.

  • Maybe you could put a trimmer in to scale the cv for the delay time. This would allow users to tailor the discrete values to their requirements and/or work around any variability in the chips/circuits.

  • @pichenettes fair enough. It was an idle thought, really.


  • Sounds like a winner to me !

  • Really nice!

  • Few ideas about things that may be possible to do with the help of these digital pots! (even if their glitches production is a shame for parameters sweep)

    If we manage to use the spi port (AVRISP on the digital control board) to control 16 digital pots (with 4 free Pic outputs driving a 4-16 decoder) we can for example:

    - Increase the pole mixing possibilities on the 4pm style filterboard. I’m not a filter designer expert! Will we gain a lot in “sonic possibilities” having an “infinite” number of filter response shapes ?

    - i also thinked about putting tree different shruthis (with 3 different filterboard) in one box. And make a different routing. See schematics bellow.

    I see that people are polychaning shruthis (with 3 identical filter boards) to have polyphony! but if we polychain 3 control boards and send the sum of the signals to one single filter (instead of 3) we get the same result! Am i wrong?

    One alternative to these digital pots can be to use SPI DACs +VCAs! (“maybe” less glitches but for sure much more expensive )

    Sorry for going a little bit out from the subject ;)

  • Digital pots were helpful in this particular case because of the board space constraint. Sorry if I made it sound like they would cure cancer :)

    > – Increase the pole mixing possibilities on the 4pm style filterboard. I’m not a filter designer expert! Will we gain a lot in “sonic possibilities” having an “infinite” number of filter response shapes ?

    The 4PM board is already packed, we can’t do more. If you remove the filterboard size constraint, why using digital pots instead of proper VCAs?

    Also, note that in this “infinite” set of filter response shapes, most of them are crap. That’s why I didn’t get excited by the Schippman filter thing (worst thing is that in all the fancy responses it can do, some of the obvious ones are missing).

    > I see that people are polychaning shruthis (with 3 identical filter boards) to have polyphony! but if we polychain 3 control boards and send the sum of the signals to one single filter (instead of 3) we get the same result! Am i wrong?

    The difference between 3 voices with an individual filter for each of them ; and 3 voices sent through the same filter/VCA is huge. Imagine what happens when you play a chord note after note, with a sound with an envelope on the filter. If all voices are sent through the same filter/VCA, there is only one single filter envelope, which will be retriggered at every note, so when you play the second note of a chord, the first note that is still playing will be “re-enveloped”. This kind of half-assed polyphony (such synths are called “paraphonic”) is not really my cup of tea.

    > One alternative to these digital pots can be to use SPI DACs +VCAs! (“maybe” less glitches but for sure much more expensive )

    Except in some special circumstances, and the delay board was one of those (super tight board space), I’d stay away of digital pots. The clear advantage of VCAs is that you can smooth the control signal to your whim, to reduce digital glitches. Digital pots not only have stepping, but short glitches whenver their value is modified.

  • About the one filter with more voices, this was one of the big drawbacks of the otherwise quite nice sounding Korg Poly800, which is an analogue polysynth

  • A Polyphonic Synth with just one filter is a bit like a Car with just one Wheel…..

  • @rumpelfilter the Poly-800s single filter wasn’t necessarily a drawback. It certainly gave it a special character. I always loved the sound of the filter opening up on low notes when you hit a much higher note- you’d get a really nice kind of sample-and-hold effect. Having said that, I’m not a keyboard player, so I imagine the limitation might be more frustrating for someone more musically able than myself.

    Incidentally, pichenettes, the P800 had per-note amplitude envelopes (two of them, in fact, in dual mode), but obviously only one envelope for the filter. There was the option to have the filter envelope retrigger on each new note, or only when all previous notes had been released.


  • Anyway, a shruthi filter board is one filter and one single VCA, so chaining several control boards while using only one single filter board would actually be a worst solution than the Poly800 as a polyphonic synthesizer.

  • @toneburst I mentioned it because many people seemed to have a problem with it. But of course this is always a subjective matter and often the technical limitations of an instrument become its distinguishing characteristics.

  • @rumpelfilter very true.


  • Ok, guys Thanks for the replies!

    All these are just ideas for eventually future projects or mods. Olivier electronics is already really nice as its is! That is why i bought a 4PM shruthi + some filter boards!

    I’m quite newbie in the synthesis domain! That’s why I totally forgot that a chord can also be played note after note (each one triggering a VCF parameter modulation(if needed) and a VCA)! Stupid mistake!

    I never had the occasion to listen to a shipman filter a bit to expensive for me ;-)

    I agree that using digital pots is crappy for heavy knob manipulators! But if the parameters controlled are changed only during a “sound design session” and than you save the patch for playing it latter ?!?
    The idea was that with this atmega we have a limited numbers of PWM outputs (so a limited number of voltages that can control VCAs).. And using the spi port +4 (or 5) bits to to have 16 (or 32,...) lower frequency “ analog outpouts” (a pots value) updated one by one by scanning trough all of them seemed to me (at first) to be a interesting and cheap idea!

    But i agree that increasing sound possibilities is not good if it increases also bad sound possibilities.

    So forget about that and continue your great job Olivier! We are waiting for the new synth (with big screen and sd card)!

  • @joma the problem with digital pots is that they are..well, digital.. Apparently the 4PM ones glitch every time you change the value, this isn’t a problem if you haven’t learnt about LFO’s yet, but if you want to make the squishy goodness of “dubstep” bass’s, pads that evolve over time etc, you really, really want something solid that does not bounce around randomly :(
    So, digital pots are not the optimal solution for filter boards, unless you only produce sounds without LFO’s, which will probably become boring quite fast :/

  • Hmm, there seems to be a lot of confusions here. I assume you refer to the inability of the 4PM to progressively crossfade from one mode to the other through LFOs.

    • The 4PM uses digital switches, not digital pots. So far there is no available Mutable Instruments using digital pots :)
    • Digital switches are not inherently glitchy, the commonly found 4053/4051 used here do a short pop when switching ; but there are some higher quality variants (say their DG variants from Vishay) which do not glitch when switching.
    • Still, no matter how glitchy the switch is, it’s still a binary control, that will not interpolate between its two states, so you can’t do a smooth LFO modulation from one state to the other ; and there’s a good chance that if you’re commuting audio, even with the most ideal switch, there’ll be a discontinuity in the waveform that will turn into a glitch. It’s not a bug, it’s a feature. Switching from one thing to an entirely different thing causes a step pin the audio that sounds like a click. That’s how things are supposed to sound… So using higher quality switches won’t solve your problem.

    If you want every on/off setting to accept intermediate setting for progressive LFO modulation, you’re going to need VCAs everywhere. A VCA crossfading between two values takes at least two chips (it can be done with a SSM2164 and a quad op-amp ; or a LM13700, 2 transistors and a quad op-amp)... Shruthi-1 filter boards can’t have more than 10 chips. I personally think that asking this kind of interpolation for stuff like mode selection or extra effects is being way too much picky for a DIY project… If you could pay 800€ for a dedicated eurorack filter module doing that and could find 100 other guys wanting so, maybe I could sell you one.

    I know people love to modulate everything with everything, but there are already enough modulation possibilities on the Shruthi I think (besides modulars, do you know many hardware analog or hybrid synths in which things like osc balance, noise level, sub level, resonance can be controlled by LFOs?)...

  • I’m with pichenettes on that- that kind of thing makes sense if the entire synth is realised in software, but if it takes loads of extra hardware to implement, it’s beyond the scope of a DIY project like the Shruthi-1.


  • Maybe you want this, though unsurprisingly, there’s a note about some transitions sounding glitchy :)

  • Hmm, i really should learn not to post when tired >_>
    Googlecached version of pichenettes link for those that cannot reach doepfers main site for some reason (like me) :
    About those switches – yeah, I’ve become quite good at recognizing the “click” sound digital switches(relays?) make, there is one in my soundcard that makes a incredibly loud (like can be heard across the room kinda loud) click/pop noise if you start changing parameters :(

    Anyways, you keep doing your thing, and i.. well, i will go to bed, where i belong.. :)

  • Oops the doepfer module I was referring to is A107, not A106.

  • No Eurorack allowed here. It’s blasphemy! :) Long live Mutable Instruments!!!

  • I just got a Soviet A-101 dekatron. Related?

  • While searching info about dekatrons, I found the schematic of a nice ADC (made with tubes) which maybe we can feet in a shruthi box!

    page 112:

    The Power consumption is only 270W :-)

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