4x4pole: I dream of wobbly resonance.

2013 started brilliantly, let there be no doubt. By April, my newly found obsession with DIY synthesisers reached a peak, and a small pile of worthy project PCBs was collected. Then work took over. For a year.

There were times during that winter of DIY-lessness that I looked forlornly, hopelessly, at the cardboard boxes of fresh components, and the half-built units soaking-up dust on the shelf.

Whilst unable to actually build, dreams of what was possible prevented me from going mad, and I spent my spare moments planning, plotting, scheming for the day I would wield the soldering iron again. 

Now that that day has come I can reveal the outcomes of my lengthy pause for cogitation:

The original plan  for the 4x4Pole Mission was to house the 4 voices together with a scavenged Fatar keybed. Many panel designs were drawn-up, with this being the final iteration in that direction (click to view).

The problems with this setup soon became obvious:

  • Originally I was to cut the 79cm panel in 5mm thick Acrylic, however the panel would likely still flex during use. Not ideal.
  • The positioning of the screens and controls was too dispersed. If possible, I wanted to be able to see all screens at once, and I tried grouping them together, but still without satisfactory results.
  • Since I was going to all this trouble to mount a keybed, why should I restrict it’s use to just one synth?

In the end, I went for a compromise solution – but one I am very pleased with:

  • The keyboard/MKE and UC16 now become a master controller, with in-built MIDIpal, a 3mm anodized aluminium faceplate, and joysticks!
  • The 4×4 Pole is racked on a 4U 19″ Aluminium Panel, and with (wait for it…) more joysticks!

The redesign went much faster now that I had a clear idea of the endpoints. The alumium faceplates were quickly designed in FPD and sent to be cut by SchaefferAG.

I was very pleased with the results – with one minor niggle: I only used 2mm Aluminium for the Rack faceplate, meaning that there is some flex. This is fixable, but I should have used 3mm aluminium, similar to the keyboard panel, which has no flex at all. Oh well, you live and you learn.

Must say though, the anodized panels turned-out way sexy-looking. Victory candy included!

No excuses now….and no time to waste….

In my last post – a full 18 months ago! – I ravaged my poor old Evolution UC16 MIDI controller to prepare it for the installation of panel-mounted pots. In the intervening period the assaulted PCB eyelets gave-up hope and fell off, leaving me with a considerable amount of microsurgery to get the board working again.


Not pretty, but perfectly functional. Liberal amounts of hot glue and some PCB varnish should give it a few years of extended life.

Once that was done it was a matter of mounting the boards and wiring-up. Time-consuming but largely trouble-free.


Colourful though….


So, the keyboard components have been tested and all work together. MIDI is internally wired from UC16 > MKE > MIDpal > out. Initial tests show this to be a stunning combo. The MIDIpal rules!

All I need now are the side and back panels. I know what I want, I just can’t afford the additional cost right now (about 70 euro). However, this is no problem because the 4x4pole is quickly coming together. As of last night, all boards were working as expected, the filters have been tuned, and the sound of awesomeness is in the air.

Just the wiring to go…..


in 3 or 4 days I should be there.

4x4Pole: Controller decisions

The build is proceeding nicely…

Firstly, the boards have been populated with resistors and most capacitors. The digital control boards are complete save for the OLEDs, buttons and pots, which will be panel-mounted. Tragically, and despite my heroic efforts at military planning/sourcing, I didn’t order enough 100n capacitors for the filter boards. I now diligently plan my next components order.

Quite some time has already been spent working on the case design and front panel. Using Inkscape, I’ve already ordered and received some test cuttings in 3mm acrylic glass. I’ve firmed-up on some nice ideas that I will expand upon in another post. Still deciding whether to invest in wooden cheeks, but then again, they can come later.

I’ve flip-flopped daily over whether to implement the Shruthi XT programmer, or stick with generic midi control. I’ve now firmly decided to not use the Programmer this time – though I did grab 2 PCBs in the latest bulk order, and I’ve already gathered the parts. The reasons are:

  • Using the programmer would mean direct control over only one Shruthi, which would be fine in Poly mode, but not ideal for other configurations (e.g. 4 independently-controllable monosynths).
  • By using the programmer I would lose CV control, and thus the ability to indulge my current joystick fetish.
  • I’ve got other cool ideas for the programmer. Foremost of which will be a keyboard version of the new Ladder filter. 😎

So, MIDI it is, and by chance I have an old beat-up Evolution UC-16 which I will be cannibalizing.


Quite easy to take apart….


I thought there might be an extra MIDI port hiding in there, but alas…


Now, these pots are a bit worn so they need replacing. I suspected they were 10k linear pots. A quick test confirmed this. Problem is that these pots are so welded to the PCB that the cleanest route is to just cut them off…


So now I can add my own pots and arrange then as I wish on the front panel. By also adding the keypad I will have access to 25 banks of 16 MIDI CCs, each knob being freely assignable to any CC on any channel. That’s a lot of control!


Further, this can take MIDI input directly over USB, so I can sequence the whole caboodle from a DAW if I wish. But where’s the fun in that?

And why stop there? I’m also integrating my as-yet unbuilt MIDIpal to make this a super-charged master keyboard. Better than the plastic shit available these days, and fully customised. Cheaper too.

MIDIpal has 8 CV inputs = Joystick pron! hehe

The bonus is that, with 4 Shruthis I will also have a routable 4-in 4-out MIDI interface.

4×4Pole: Doepfer MKE vs Novation – Revenge of the Wheels

I’ve had some time to return to and investigate the wheel connectors on the MKE.

Remember that, when I tested the Novation wheels the range was minuscule. It turns out that the wheel receivers are calibrated to Doepfer’s own standard. It also accounts for the narrow range wheels have normally, due to the spring on the pitch wheel and the large diameters.

A closer look at the Novation wheels revealed 2 x 5K linear pots like so:


The Doepfers are 10K so I decided to use a trusty joystick, this time with the spring intact, to see if it could be used as a mini pitch-wheel.


The result was disappointing. At rest pitch was always at maximum, and moving the stick to one side again swept through the range an a narrow band. No, this won’t do.

To see if I could exactly define this range, I attached a standard 10k linear pot. Again, same result. Hmmm.

So, how could I limit the range of a pot?

Thankfully the answer came quickly and it’s such a simple fix it’s almost embarrassing…

…how about if I just twist the Novation pots in their housing? This way I could tune the range by hand. So, after cleaning the solder off the pins, I plugged the Novation wheels back-in, loosened the pots and re-orientated them.

First, the potentiometer tabs needed to be clipped-off. (I now know what they’re for – holding the pot in place!)


Then plugged-in (after removing the solder I left on the pins 😛


And, by Jove, it works like a charm!

The main caveat is when re-orienting the pitch-wheel pot – you need to account for the spring, so best to have MIDIox or other such monitor open to see when you’re at zero before tightening the nut. The pots are d-shaft, so in this case I could easily find zero.


Here they are with their pots newly-oriented and wheels re-fitted.


Everything is now up-and-running with the MKE. Superb!

Buoyed by this success, I’ve decided to adopt a modular approach to the 4×4 pole enclusure, wherein the synth section will be detachable from the Keyboard.

Because, who knows, I might want to build a 4xLadder Filter at some stage…


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.