Shruthi-1 Modding Session

I’ve been a bad boy, skiving-off to further molest my Shruthi-1.

These mods have been brewing for a while, as I fiddled and took notes from the Mutable Instruments forum and the Mutable Wiki. They are surprisingly easy to implement, once you gather the right components, and they are providing good learning points for the 4×4 build.

KK, on with the details…


There are 3 filter switches…

  • 1. Select 2 or 4 pole filter.
  • 2. Choose either of 2 bandpass modes. I haven’t quite got this one working yet. I think I need a new switch.
  • 3. A switch to allow the audio input to be used as FM modulator on the filter. An input level knob is also added to provide more control. Initial tests of this are very promising!

And two new knobs…

  • 1. A separate FM control knob allows a degree of filter feedback. It’s quite subtle, but adds lovely harmonics to the low-end. I’m going to see if I can beef this up.
    EDIT: Thanks to the helpful guys on Mutable forum, I removed the 47k resistor and used a C10K reverse audio pot to give a nice transition from subtle to a harsher FM grit. Lovely.
  • 2. Then there is the drive knob. This one is great! It overdrives to distortion very nicely. An output volume control is necessary to attenuate, because this can get loud. Shruthi size belies and delights yet again.

I’ve also put a LED on CV1 out. This can be hooked-up in the mod-matrix to provide visual feedback of modulation parameters, including tempo-synced LFOs. It works brilliantly, and the brightness is also adjustable from the mod matrix. CV2 is also available and will be used for the same purpose. This is going to look great on the 4x4Pole!

And of course, the 2 joysticks. I am loving these. In a moment of madness I imagined an interface dominated by an army of these fellas. It surely will happen 😛

Now, I could squeeze all these entrails into the standard enclosure, but I feel that would be unfair to poor Shruthi. I’ve really gutted this guy, so it’s time to buy some new clothes, with enough space to include these and any future mods. I already partially atoned by replacing the filter caps with fancy styroflex thingies. That’s a new OLED too which makes a big difference. Finally, I took the chance to re-align the boards, and now everything is where it should be.

I’ve already designed and ordered the case: the Shruthi NoisyLitteBugger edition should be ready next week 😉

4×4Pole: Doepfer MKE vs Novation KS Keybed

Amongst several boxes of goodies to arrive today was the Doepfer MKE. I took a bit of a punt on this, as I wasn’t certain that my scavenged Novation KS4 keybed would work. There’s a surprising dearth of information about the Novation keybeds. But all the clues pointed to it being a Fatar keybed (Diode matrix, Aftertouch strip, 16 Micromatch connectors), and thus fully compatible with the MKE.


I thought it would be an opportune time to pick-up a Doepfer DIY synth for the next project – the slide into Modular with Little Dieter. For now, let’s focus on the MKE….


It came with a 9v power supply. Overall it feels pretty well-built and sturdy. The LCD is adequate, but those buttons have gotta go. Yuck.


I wanted a quick test so I hooked-up the wheels, with MKE spitting data out to MIDOX.


Modwheel and Pitchwheel are recognised, however the range is extremely narrow. The MKE manual makes mention of these inputs being tailored towards Doepfer’s own ‘accessories’:

‘…the voltage range ~ 0 … 1.6 Volt corresponds to the Midi data range 0 … 127. The reason for this limited voltage range is the rotating angle of the wheels we offer as spare parts. An output voltage range of ~ 0…1.6V was measured for these wheels if they are connected to GND and +5V as they do not cover the complete rotating angle because of the end stoppers.’

This is going to need some research, so another day. But what about the keyboard?

First I needed to make cables using 16-strand flat ribbon and 2×8 pole male Micromatch connectors, and thus saving myself a small fortune.


With both keyboard cables connected, I could immediately see the notes being received by MIDIOX, but the zones were reversed. A quick swap of the cables remedied that.

Keyboard working! And with a smooth velocity response 😎


And so to Aftertouch. I was most uncertain about this one working. The Fatar cable has four pins, but the MKE only accepts three. After a while of random and fruitless jumper-switching I hit upon the brilliant idea of reading the MKE manual (doh!). There it was, in the appendix, the key to success. Only two connections are needed, necessitating a little hack using header pins and a dexterous disposition …



My scepticism came crashing down as the pressure signals appear on screen. A quick change of settings from the crappy-but-adequate interface and aftertouch working perfectly. What’s more, the aftertouch curve seems just fine, so no messing with switching resistors. Yeeeehaaaw…



Right, I am feeling pretty good now. Just mod- and pitch-wheels to calibrate and I’ve got myself a very nice, expressive master keyboard.

Is niiiice, I like-ah.


4x4Pole: Preface

‘I am a man of projects’

Being into something of a building binge at the moment, I’ve decided to extend myself for the next outing and do something quite special.

The ideas came to me in a rabid haze and before I knew it the Postman had delivered 4 sets of PCBs from Mutable Instruments. Specifically, 4x4 Pole Missions, with the notion of turning them into a 4-voice Pole-mixing polysynth, but with flexible MIDI routing allowing splits and other combinations.


It must have lots of knobs, either via a MIDI CC controller, or by using the Shruthi Programmer. To keep my options open I also bought 2 of the latest run PCBs. Thanks for the sweets Frank 🙂


It also demands lots of joysticks for real-time control. If I use the programmer I lose the CV inputs on one of the Shruthi’s, but that still leaves room for 6 joysticks on the other three units. Oh yeah.

Further, I wanted to scavenge my neglected Novation KS-4 for it’s key bed and Pitch/modulation wheels. After some uncertainty I’ve found that I can probably re-use them via a Doepfer MKE MIDI board. It’s been ordered…


The components for the 4 Shruthis have also been ordered. A lot of soldering and testing of the individual units should keep me occupied for a few weeks.

Another idea I’m looking into is incorporating a small mixer with pan controls and possibly a Send.Master FX. As this is a bit beyond me at the moment, I feel I should leave plenty of room in the case for expansion and further modification.

Finally (for now) I’m thinking about integrating my unbuilt MIDIpal to make it a control powerhouse. Perhaps to keep it modular – separate the keyboard/wheels and leave room to slot-in the 4×4, or whatever I choose to build in future.


Adding Gamepad Joysticks to Shruthi-1


I built my first shruthi last year and the poor little bugger has been poked and prodded since. Not only is it a great synthesizer, it is also a fun playground for a beginner audio noodler like myself.

One of the standard Shruthi mods is to attach controllers to the 4 CV inputs. These can be mapped to multiple parameters in the mod matrix, providing oodles of modulation possibilities.This could be controlled by knobs or touchpads or the like.

The most accessible method is to attach a 10k linear pot to the points on the control board (left of the LCD).

Looking at the pot, the legs should be connected thus:
1: +5V
2: CV (1-4)
3: GND

On the Mutable Instruments forums I read that PS3 gamepad joysticks are basically 2 10K linear pots assigned to X and Y coordinates. The only problem is that the spring can’t be removed without destroying the mechanism. I want the joystick to stay where I put it.

After a lot of searching, it was apparent that few joystick met this criterion, and they tended to be expensive; 45 euro for the Doepfer version, and an extra tenner for the shaft. Bugger that.

Then I came across this excellent blog post detailing how to remove the springs from an Xbox controller.

Not having an Xbox controller to hand, I instead ripped-open an old, broken Logitech PC gamepad I had thankfully kept – along with many other boxes of junk. The hoarder vindicated 😀

EDIT: You can also buy the joystick components separately if you wish. Here’s a good source: Uk-Electronics.

Click pictures for full size:


Removing the PCBs is easy…


..and there are our little beauties..


Noticing that the joysticks were similar to the Xbox ones, I proceeded to follow instructions in removing them from the PCB and then their springs.

The joystick housing need to be carefully de-soldered from the gamepad PCB…in this case there were 15 points attached to the board. Make sure they are all free before prising the whole thing free. Solder wick is essential.


The spring and button are house in the plastic base. Bend the metal legs to release it. It just pops out.


With base removed (and thus spring and button)…


Once the spring is out the mechanism is unsupported and will probably fall apart. But it’s easy to re-assemble – it’s just 2 pieces. The potentiometers can be snapped-off and reattached also. Very elegant design. To keep the shaft in place permanently, I soldered small wire supports like so:


For testing, I plonked it onto a piece of polystyrene…


Then I wired-up both sets of pins…


..and connected them to the Shruthi CV points…


At first I thought it wasn’t working, however I had wired the wrong CV pads! Switching to the correct ones in the Mod matrix brought a huge smile to my face. The joystick works perfectly, goes where you want it, and STAYS there.


Initially I kept the wires long for a reason; I wanted to test how sensitive the CV inputs would be to interference. At this length I noticed no adverse effects.

Now, how to house it?

Conveniently, it turns out that all the joystick legs will fit snugly into the honeycomb side panels, but they need affixing. However the pins are close together, so some insulation is in order. I used some heat-shrink tubing. To keep things tidy should I wish to re-wire, I attached all GRN and 5V wires to pins (discarded resistor legs), leaving just single pins to be soldered to the control board.


Notice that the wiring must be consistent if you want both joysticks to behave in the same way.


There’s plenty of room for the wires to fit snugly in the case. Note that the additional switch is not needed – it for 2/4 pole filter modes (another easy mod).


For today, to keep everything steady. I’m using some ‘blu-tack’. I’m waiting for extra components for more mods, so I’ll be taking it apart again soon. When satisfied I will probably glue them on. I also found two tiny covers to put on the shafts. Cute.


And there you have it: 2 cheap (free) modulation joysticks, without springs, for your Shruthi, or any other purpose you might be dreaming. I know I am….



Mutable Instruments: Building Anushri


Timelapse of my Anushri build over 3 sessions.
Audio is recording of the first-noodle. All sounds from Anushri in a single pass, only light reverb used.
Great fun to build and play. Listen to that filter!

Kit from, and big thanks to, Mutable Instruments