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Modular Synth (Eurorack) - DIY Modules

DIY Modules

I decided to build a number of DIY modules: (As of 12/11/18, I have built 72 DIY modules).

Mutable Instruments CVPal (USB Midi to CV)

22/04/17 Based on cost and simplicity I built a Mutable Instruments CVpal. This has some limitations - only 5V CV and no MIDI clock. It also has latency issued under Windows but is fine on Mac and iOS. But it was cheap and will get me started!

Works really well and tracks well.

Three Frequency Central VCOs (Roland System 100 clone)

15/08/17 I built two FC VCOs with the heater boards

16/08/17 I also bought (dirt cheap) a non-functioning VCO from a forum that I will fix (it was cheaper than the faceplate on its own! One of the transistors was soldered in the wrong orientation. A quick swap and it worked fine.

Looking at the schematic it looks really similar to the Doepfer a110-1 (it sounds really similar).

Great sound. They track well over four octaves and seem stable on long sessions. They also go very low so double up as LFOs.

Frequency Central ADSR (Roland System 100 clone)


Worked straight away at power up. Great envelope generator - may have to build another one !

25/10/18 I did build another one ! (Very painless and easy build - no issues).

NLC Double LFO


Had a few issues with this build - one of the jacks was shorting a resistor (which was located underneath the jack !) One of the LFOs was giving a mix of both LFOs. Also the LED wouldnt light up - worked when I reversed it (although that meant inserting it on the PCB opposite to the polarity marked on the PCB !) All working now though.

Frequency Central Vogue Filter (Moog Rogue clone)


Really nice sound - very vintage. Love it !

Frequency Central Sample and Hold / Noise


Really impressive module - quite deep and you can do an awful lot with it.

Two NLC Sloths (chaotic slowly evolving modulation)

24/08/17 I decided to build two of these for modulating slowly evolving patches.

The x-y scope demonstrates the strange attractors. I tried using toner-transfer to make the front panels - needs a little practice!

Passive Multiple


Simple passive multiple. Bought the panel for 1.75 (!) from the pusherman group. I wired it as either 2x one-to-three or 1x one-to-seven (depending on whether the jack into the fifth socket is inserted or not).

MI Ripples Filter

10/02/18 Scary small pitch SMT (0603) - resistors the size of about 2 grains of sugar! I spent ages summoning the courage to start soldering SMT. I decided to get lots of magnification and try soldering with a normal iron. After trying different tips, I settled on the tip I usually use for through-hole as it got lots of heat to the soldering point in a very short space of time. Using really fine tips took too long to heat up the pads. I did use very fine solder though (0.4mm diameter). I used lots of flux which really helped.

I bought one board from and one from the pusherman facebook group (much cheaper !). I wanted to check if there were any quality differences before soldering more complex boards. They both seemed equally good.

The silver panel is a Greyscale panel (bought secondhand from the muffwiggler board) and the black panel is from pusherman.

The boards were surprisingly easy to build. Initially neither worked. The purple board had a cold solder joint on one of the opamps. A quick reflow and it was working perfectly. The black board was a pain to debug. It needed lots of probing with an oscilloscope and multimeter and hours of poring over the schematic ! In the end one of the resistors in the filter cascading circuit was open circuit ! (I hadn't heated it much at all so I have no idea why it was dead). After swapping it out the filter worked perfectly.

The following photos show the build process (the oscilloscope is showing the VCA working correctly).

O+C CV Utility Module

24/02/18 Ornament and Crime is a multipurpose module for generating CV. It only exists as a DIY module. It uses a more managable size SMT (0805). It is based on a Teensy 3.2 Arduino board and runs firmware inspired and adapted from multiple sources (quantiser, Turing Machine, various Mutable Instruments modules).

It was a fairly straight forward build and worked on power up. The image below shows the AD generator (Piques, adapted from the Peaks firmware) running in the oscilloscope trace.

Mutable Instruments Veils

01/03/18 PCB and panel bought from the facebook group, Pusherman. These were quite tricky to build - 0603 SMT with tiny opamp packages. The PCB was quite compact! Surprisingly the first one worked first time after a few false starts due to potentiometer pins not making a good contact (when only pushed in, not soldered in order to test it). The second one was much more of a pain: a few cold solder joints around the opamps were quickly sorted. The real difficulty was tracing a bridge to the ground plane from one of the pads of a resistor (that shouldnt have been in contact with ground !)

Passive Attenuators

08/03/18 Panel cut from aluminium blank and wired point-to-point.

Mutable Instruments Braids

21/03/18 First attempt at soldering STM32 microcontrollers (tiny pitch SMT). It all went pretty well. Only one hiccup on the build - I soldered a capacitor in at 90 degrees on both boards - spotted it straight away and removed them with hot air. The flashing was painless. On startup one board worked perfectly first time - all tests ok. On the other board the display was screwy - I had a cold solder joint on one leg of one of the shift registers - very quick fix.

The encoders I used send a reversed signal compared to the factory encoders (Bourns vs Panasonic), so I needed to modify the source code and recompile the firmware to enable the correct direction of the encoders.

Sounds amazing and surprisingly painless builds !

23/04/18 It's so good, I built another one! (Partly so I could set up drums easily).

Very straight dorward build. Power section had no problems. Flashed immediately within the Mutable dev environment. Fired up fine. The only issue was no audio. A quick trace with the oscilloscope showed signals going from the STM to the DAC but no onward audio. A quick look at the schematic and it showed pins 3 and 4 should be bridged. Tapping both with the scope probe produced audio. A closer inspection showed pin 4 to be slightly raised off the pcb and not making contact. A quick bend with the tweezers and a dab with the solder and it was up and running.

Ken Stone's CGS60 Stomp Box Adapter

02/04/18 Eurorack modular synths have a much higher voltage than typical guitar effects pedals so they need attenuation before feeding a signal into a pedal and then amplification of the resulting signal back up to modular levels. The CGS60 Stomp box adapter has 20x attenuation and then 20x amplification. I had enough spare parts for a stripboard build. I could fit two channels into 8hp. It worked perfectly straight away.

Mutable Instruments Peaks

12/04/18 PCB and panel bought x3. Initially I built two of them. I built and tested the power section first. One of them had a cold solder joint on the 2.5v voltage regulator. I then soldered the STM32s and flashed them. One wouldn't verify. On inspection there was a tiny bridge which was quickly sorted. It then flashed fine, using the Mutable Instruments dev environment on a Mac. The rest of the build was straight forward with no other errors.

I have another board and panel to build another one at some point in the future!

The only issues with the builds was setting the zero offset in firmware. This is a hidden mode entered by pressing one button on boot up. I just could no enter it at all. I set up the debug client and server and peeked around in gdb. It all looked good, just wasn't registering the button. I think it was the debounce code not registering it. In the end I just set a breakpoint after the check and poked the right bit so that the check passed. Continuing on with the code meant I could easily calibrate both modules fine.

28/05/18 A month later I built another one. I flashed this with a hacked version of Dead Man's Catch alternative firmware, modified to include the latest factory Peaks code, along with software calibration. I needed to use openocd again in order to calibrate the zero offset.

Mutable Instruments Branches

22/04/18 Built two MI Branches. (PCB and panel bought x3).

Very easy builds. No issues at all.

It's worth making a note for the future about flashing them. I used an Olimex AVR-ISP-MkII. I had a real pain trying to get it to work on a Mac with avrdude in the Mutable dev environment. I tried it on a PC using Atmel Studio and it worked fine. I know I had to change the firmware for it to work with avrdude so I used Atmel Flip to do that which was straight forward. However avrdude still wouldn't read it. I saw that some people had had problems with avrdude 6 (used in the mutable dev environment), so I downloaded the avrdude 5.9 sources, installed the dependencies (usblib, yacc, etc) and compiled it. Hooray, it worked. Unfortunately avrdude 5 config file didn't have the m88p config. So I copied it from the avrdude 6 config and pasted it into the v5 config file. After that, it all worked trivially easily.

Mutable Instruments Grids

04/05/18 Built two MI Grids.

This would have been a nice straight forward build if I hadn't decided to try to program one late at night after going to the pub ! I think I bricked one of the MCUs when programming it from Atmel studio (I decided it would be a good idea to use this software for some reason !) I think I must have set the wrong fuses. I tried reflowing the crystal (removed it to check for bridges/problems) and it was fine. In the end (probably a bad beer-influenced decision) I removed the MCU with hot air. This meant I needed to buy another one on my next Mouser order. This time there were no issues programming it from the Mutable Dev Environment. There were also no soldering issues and it worked first time.

Mutable Instruments Module Tester


Flashed and built the firmware from within the Mutable Dev Environment. Very painless and straight forward build.

Mutable Instruments Links

04/05/18 Built two MI Links.

Both were quite fiddly builds with all the passives and ICs quite close together. However, they both worked perfectly first time with no errors.

Mutable Instruments Tides

07/05/18 Built two MI Tides.

Above photos: panels and pcbs, large pile of components, power section complete and tested, STM32 MCU and associated decoupling caps installed and connecting the STLink2 programmer, flashing the MCU, jacks and pots inserted but not soldered, testing, completed.

Both worked perfectly first time, with no issues.

Mutable Instruments Elements

30/05/18 Built an MI Elements.

The above images show the: bare pcb and panel, the power section built, the microprocessor soldered, flashing the microprocessor, finished soldering the surface mount components, testing all the inputs and outputs before soldering the through hole components, and the completed module.

It was a surprisingly easy build with lots of space and it all worked first time on power up with no issues. It sounds very nice !

Mutable Instruments Shades

01/06/18 Built two MI Shades.

Very quick and straight forward builds. One worked perfectly straight away. On the other one the second and third channels weren't working initially. I had a couple of dry joints on two opamps that was a pretty much instant fix. (Initially the iron wasn't heating the joints well - the tip wasn't well-tinned and I think that was the problem with those op-amps).

Very useful modules - offset/attenuator/attenuverter/mixer.

Mutable Instruments Ears

05/06/18 Built three MI Ears.

I ordered an 0603 version of Ears with matching panels from (The factory Mutable Instruments version is 0402).

The images above show: the bare boards and panels, surface mount parts soldered, the completed module, testing the gate output with external audio, testing the contact mic and finally the three modules completed.

Straight forward builds. I substituted 1n5819 diodes for PMEG2010AEBs as I didn't have any. I had to stack two 0603 470n caps in parallel as I didn't have any 1uF caps of suitable voltage. Would have worked first time if I wasn't such a numpty and used 6v "special" lm324 opamps instead of general purpose ones !! (Who needs LEDs when ferrite beads glow so pretily). A quick hot air rework and they were swapped out. They then worked fine with no issues.

22/10/18 I thought I'd build one more so I can have one on each channel of a four channel polyphonic voice:

Passive Attenuator

05/06/18 Built two Pusherman Levels: 4-way, 4hp passive attenuator.

Not much to say. Trivial builds. Makes things quieter !

27/09/18 I built another one. This had an error on the pcb. One of the tracks from the input of channel 2 was shorted to the pot of channel 4. I broke the track and jumpered it with a cut off piece of a through hole passive.

Mutable Instruments Warps

15/06/18 Built an MI Warps.

Straight forward build with no issues. Images show: bare pcb and panel, power section built and tested, MCU and associated components soldered, flashing the MCU, module completed apart from control hardware, control hardware placed but not soldered, testing the module, the finished module.

Mutable Instruments Kinks

17/08/18 Built three MI Kinks.

Images show: bare pcbs, 0402 resistor size, populated boards, testing and then the completed builds.

These modules use teeny, tiny 0402 surface mount parts (see photo above of a resistor with my manky thumb for reference ! I was quite nervous before starting these as they are very closely populated boards with tiny parts - the diodes are really fiddly and very tricky to see the stripe which marks polarity. For these builds I still hand-soldered them with a soldering iron and solder. I did consider using solder paste and reflowing with either skillet or hot air but decided to stick with a soldering iron, which I'm glad I did.

Apart from the eye-strain, these were reasonably straight-forward builds in the end. All three had issues in testing: the tl074 opamps all had dry joints on some of the legs (strangely the opa4171s were all ok). These were pretty easy to sort out - just touching up the opamp legs. After that they worked fine.

Mutable Instruments Yarns

21/08/18 Built two MI Yarns.

Images show: bare pcbs, pcbs with the pile of parts, power section of the boards populated, MCU and associated parts installed - front, MCU installed - rear, flashing the MCU, completed build - front with no face plate, completed build - rear, completed build, calibration (more than 1/1000 cent accuracy in this image!).

Straight forward builds. Only one issue - on one of the modules the display didn't fire up initially (I had wondered if I had a questionable short on the MCU but it looked ok visually so I had left it - it was on the display data pins of the MCU so I quickly reflowed that pin and it worked perfectly straight afterwards. I had bought encoders that were reversed (compared to the factory modules) - intentionally as that was what was available. I needed to make some changes to the source code to change the direction of the encoders which was trivial (and indeed I had made those changes before startiung the build).

Mutable Instruments Streams

01/09/18 Built an MI Streams.

Images show: bare pcbs, power section populated, MCU and associated parts installed, flashing the MCU, completed build - all surface mount parts placed, completed build - all through hole parts placed as well as the led level metering sub-board, testing the module, completed module.

Fiddly but straight forward build. The main hassle was cutting down the surface mount socket mount to fit the sub-board assembly. The module worked first time with no remedial work needed.

Mutable Instruments Frames

08/09/18 Built an MI Frames.

Images show: bare pcbs, power section populated, MCU and associated parts installed, flashing the MCU, completed build - all surface mount parts placed, completed build - all through hole parts placed, testing the module, completed module.

Pretty straight forward build. I had two issues: I missed a bridge on the DAC (that was simple to fix) but there was an issue with the addition of the keyframes - the frame light stayed on a long time and seemed inconsistent. I worked out it was a dry joint on the ADC pin of the frame pot. It took at least an hour to work out if it was not working correctly and then what the problem was! After that it worked perfectly.

Minx - Buffered Multiple (Based on Mutable Instruments Links Buffered Multiple Circuit)

11/09/18 Built an own-design buffered multiple.

PCB and panel bought. However, looking at the schematic, there was an error. The output resistor is included in the feedback loop, leading to the possibility of opamp oscillation in the presence of a capacitive load (eg with long patch cables). For peace of mind, I decided not to build it and used the parts to build two Links modules instead.

I came back to this (28th August 2018) and spent a few hours laying out a circuit and pcb based directly on the buffered multiple part of Links (with normalled connections between groups). I sent the pcb off to (a Chinese pcb manufacturer) and am awaiting 5 test boards. I have enough parts to build three of them if I want.

On the first run of pcbs, I made a mistake in Eagle and didn't create vias to join the ground planes front and rear, so needed to join up three islands of ground. Works perfectly when that is done. (The image showing the module being tested with the oscilloscope has a 'Y' offset on the scope to see both traces clearly - it's not a voltage offset !)

02/10/18 Built another two own-design buffered multiple. I corrected the Eagle files and have bought some more test boards.

Modules work perfectly - very straight-forward builds. The files can be found on my github.

Mutable Instruments Clouds

22/09/18 Built two MI Clouds.

Images show: bare pcbs, power section populated, MCU and associated parts installed, flashing the MCU, completed build - all surface mount parts placed, completed build - all through hole parts placed, testing the module, completed module.

Both worked immediately with no issues.

Buranelectrix Dredrum

27/09/18 Built three Buranelectrix Dredrums.

After testing one of them, I built the other two:

Worked perfectly first time. To compile the firmware I needed to change the source code change as well as change to the avr-gcc flags to enable compatability with latest avr toolchain.

Mutable Instruments Rings

07/10/18 Built two MI Rings.

Images show: bare pcbs, power section populated, MCU and associated parts installed, flashing the MCU, completed build - all surface mount parts placed, completed build - all through hole parts placed, testing the module, completed module.

Both worked immediately with no issues.

Mutable Instruments Marbles

22/10/18 Built two MI Marbles. PCBs bought x6 from allpcb as well as the parts to build two of them. I also made my own panels for the first time!

On the newer modules (plaits, stages, marbles so far as of Oct 2018) MI ow use SWD to program the MCUs rather than jtag. I jumper the pins of the STLink V2 to the SWD pins on the board. With the STLinkV2 pin 1 (target voltage) needs to be jumpered to pin 19 (3.3v *from* the programmer. Here is how I wired the jumpers to flash marbles:

In order to calibrate the dac, edited the source code to output 1v and 3v. I then measured the 4 dac outputs for both 1v and 3v. From knowing the dac scale and offset as well as the calculated dac code to output a desired voltage I derived the scale and offset for each channel. I then hard-coded this into the firmware.

To calibrate the adc, long-press the rate and spread buttons. Feed 1v into rate input, press one of the two buttons, feed in 3v, press button, feed 1v into spread cv, press button and then feed 3v into spread cv, press button and the calibration is done.

No issues with the builds - both worked first time. It's an awesome module!

Mutable Instruments Shelves

28/10/18 Built an MI Shelves.

No issues with the build - worked first time. Wishing I had built one ages ago!

Mutable Instruments Plaits

02/11/18 PCB x4 and panel x3 bought as well as the parts to build three of them.

Straight-forward builds, no issues, worked immediately with no problems. I am still awaiting parts to complete the last of the four.

Frequency Central Amplifier (VCA)

06/11/18 Built a Frequency Central System X Amplifier (VCA) from a PCB and I made a panel from sheet aluminium as FC no longer sell the panels.

I built my own panel (and didn't do a very neat job ! Trying toner transfer technique - I think I need to try more glossy paper next time). Straight forward build. The only issue was me being a numpty and installed the two power rail filtering electrolytics in reverse - bang! First time I've installed electrolytics the wrong way and first time I've blown up electrolytic caps! Other than that, it worked well!

Mutable Instruments Stages

10/11/18 Built two MI Stages. (PCBs and panels bought x6 from

Straight-forward builds. (Quite long and tedious though). No issues. Worked straight away with no problems.

808 Kick Drum

12/11/18 Built an 808 Kick Drum.

I bought 10 pcbs from and also ordered 10 panels from

Boom!!! Simple build. Just needed to adjust the value of the pull-up resistor on the accent to allow a nice "thump". Now responds nicely to low (eurorack levels) trigger voltage and low voltage accent (with impedance of ~1k) as well as nice and loud when no accent plugged in. I used a tl072 in the end (just used a jrc4558d initially as I had no spare tl072s at the time. I'll probably build a couple more.

Mutable Instruments Blinds

PCB and panel bought x2 and the parts to build them.

Mutable Instruments Edges

PCB and panel bought and the parts to build one.

Mutable Instruments Marbles

I have another two PCBs and panels spare as well as most of the parts to build one more.

Mutable Instruments Ripples

I have another two PCBs and panels as well as the parts to build them.

Mutable Instruments Warps

I have another PCB and panel bought x1 as well as the parts to build it. I plan on running the parasites firmware with this module.

Mutable Instruments Branches

I have another PCB and panel bought x1 as well as the parts to build it.

Mutable Instruments Grids

I have another PCB and panel bought x1 as well as the parts to build it.

DSO150 Shell Oscilloscope

An oscilloscope kit from Banggood bought. I have also bought a conversion kit (panel and pcb) to mount in a eurorack case, as well as the required parts.

Antumbra 6Mix

PCB and panel bought.

Planned Modules

Temps Utile Clock Utility Module

I have a PCB and panel bought x1

O+C CV Utility Module

I have another PCB and panel bought x1

Mutable Instruments Shades

I have extra PCBs and panels bought x3

Mutable Instruments Frames

I have an extra PCB and panel bought.

Mutable Instruments Peaks

I have extra PCBs and panels bought x3

Mutable Instruments Tides

Another PCB and panel bought x1, along with a Sheep panel. The aim with this module will be to run the Sheep alternative firmware.

Ken Stone's Real Ring Mod

I have bought 4 transformers and have a number of 1n34a germanium diodes hanging around, so I'll build a couple of passive ring modulators on strip board.

Mutable Instruments Breadboard Friends

A complete set of pcbs bought x1

Mutable Instruments Ears

I still have two pcbs and panels left after giving away six sets that were surplus to my needs.

Passive Multiple

I have another panel, to buld another passive mult if I need one.