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 :frowning:

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…

@Titus Raindrops They have no right to happiness. They exist to serve us.


Lol. I thought I read somewhere that the dekatrons want 350V to be happy. I hope I am wrong about that.

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 :slight_smile:

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

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

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

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

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 :frowning:

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

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.


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 :slight_smile:
  • 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?)…

@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 :frowning:
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 :confused:

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 :wink:

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)!

@rumpelfilter very true.


@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.

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.

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.


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 :slight_smile:

> - 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.