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  1. Turns out that it’s possible to use cheaply available (~2€ each) 7-pin SPI OLED displays with the magpie modular 8hp microbraids PCBs. A hardware and software modification are required, although as the ATMega is using software SPI it may be possible to work around the hardware modification with further software modifications.

    Please note that I would not recommend building the magpie modular micro braids module! Instead, try one of the newer adaptions (e.g. the one from Antumbra) which uses normal LED displays and have prettier panels. This guide is intended as reference for people who still have one of the old boards lying around and want to get it working without shelling out 20€ or more for a tiny OLED module.

    The seven pin SPI header on the cheap OLEDs I bought maps to the eight pin header of the original Adafruit OLED (and therefore the microbraids PCB) like this:

    CS DC RS MOSI CLK Vin GND
|    X      X      |        \
CS RS DC CLK MOSI Vin 3.3v GND

    I cut the relevant traces on the OLED board, scratched away some solder mask and used enamel-insulated wire to connect the traces to their new pins. Then, I installed a 7 pin female header on the braids PCB, and bridged the 3.3v and GND pins, which sounds like a terrible idea, but the board doesn’t make any use of the 3.3v pin, I’m just using it as a convenient way of connecting the OLED GND pin to the braids GND pin.

    If your OLED module has the same layout as mine, it should end up looking something like this:

    The display ends up being in a slightly different place to the original, but five minutes of filing the aluminium panel fixed that. It ended up looking a little bit messy, but the panel (from pusherman, not a magpie modular original) is extremely ugly anyway so it didn’t make much difference. I quickly cut out and filed a 3mm acrylic screen, wedged it in place and secured it with some high-viscoscity superglue.

    After reflowing and flashing both the ATMega and STM, everything was working perfectly, except the display was upside-down. Looking through the Adafruit graphics library led to a simple solution: adding the following line to the init function on line 39 of mbraidsv3.ino:

    display.setRotation(2); // Invert display for use with cheap Aliexpress 7-pin OLED

    After re-flashing the ATMega, everything worked perfectly.

    It was only after I made the hardware modification that I noticed the ATMega is using a software SPI library, with the pin definitions on lines 9-13 of mbraidsv3.ino. It should therefore be possible to achieve exactly the same result by leaving the OLED module unmodified, bridging the 3.3V and GND pins on the 8 pin OLED header on the PCB, and swapping the pin definitions around so they look like this:

    #define OLED_MOSI   10
#define OLED_CLK   9
#define OLED_DC    13
#define OLED_CS    12
#define OLED_RESET 11

    The display rotation mod will still be necessary, unless your display module is oriented differently. If anyone attempts the software modification please let me know, I’d be curious to know if it works!

  2. Augmented Hurdy Gurdy Experiments

    https://www.youtube.com/watch?v=hPGKeMdRU5I

    As I can’t currently commit to building a new series of gurdies due to my living situation, I’ve been keeping myself busy developing the MIDI system for my instruments, to develop new extended, augmented playing techniques.

    This video is the first demonstration of some hybrid electronic-acoustic experiments using the prototype MIDI system installed on my hurdy gurdy.

    0:22 Technique: Pitch-shifting Polyphony

    Gurdy MIDI and Audio → Apogee ONE → Macbook running a puredata patch

    Monophonic acoustic gurdy signal is pitch-shifted down in real time to play chords and harmonies. Chords and intervals on the keyboard can also be used to pitch-shift the trompette signal (0:55) or the drones. Inspired by an idea from Sébastien Tron.

    1:18 Technique: Expressive MIDI Controller

    Hurdy Gurdy MIDI → DIY Hybrid Poly Synth based off Mutable Instruments Ambika

    The keyboard and wheel sensors send MIDI note, expression and polyphonic aftertouch messages to a polyphonic synthesizer. In this case a split keyboard effect is used to play two sounds.

    1:36 Technique: Layered Acoustic and Electronic Sound

    Hurdy Gurdy Acoustic audio, Gurdy MIDI → DIY Hybrid Poly Synth based off Mutable Instruments Ambika

    1:36 The acoustic string plays a melody, the bottom half of the keyboard controls a synthesizer with a long release for subtle held chords

    2:08 Using trompette technique can send MIDI messages, used here to play synthesized percussion on an Ambika voice assigned to MIDI channel 10, whilst the keyboard plays chords.

    2:30 Acoustic trompette and melody string sound layered over subtle polyphonic synthesized chords

    Playing, Instrument and Software: Barnaby Walters https://gurdy.is https://waterpigs.co.uk
    Polyphonic Pitch-shifting idea: Sébastien Tron
    Filming, editing: Adriana Borger

  3. Some tips for building a Mutable Instruments Ambika, based on my experiences:

    Based on the part numbers in the BOM, suppliers will sometimes pick a CD4050 chip. These are not fast enough and will cause SD card access to fail, it’s necessary to use a 74HC4050.

    If you’re using a 74HC4050 and SD card access is still failing, check whether communication with all of the voicecards works. I had a lot of issues where failing or badly connected voicecards would interfere with the serial lines and prevent SD card access from working.

    On that note: shell out on good quality stacking headers for the voicecards. I picked cheap ones, and suffered a long series of hard-to-debug issues caused by them not reliably connecting. I ended up having to coat all of their legs in solder to make them thick enough to reliably connect. Good headers are worth the premium to avoid these headaches.

    The 3.3V regulator, DACs and all the MCUs can be obtained free from the Microchip samples service if you’re willing to do it over a few months in limited quantities of two part numbers at a time.

    My Pololu USB AVR Programmer wasn’t able to provide enough power whilst programming, so I had to power the boards for programming. For the voicecards, having both 6 pin headers connected interfered with programming because of data being sent to the voicecards over the serial lines. I ended up powering up the motherboard, placing a stackable connector in the power/audio socket voicecard socket and plugging the voicecard into that for flashing, so that it’s powered but the serial lines are disconnected.

    I programmed my Ambika with the YAM firmware to get those smooth sounding PolyBlep square and saw waves. I built the firmware myself, downloading CrossPack 2010-01-15 which provides avr-gcc 4.3.3, the correct version for compiling most old MI AVR firmwares.

    I would recommend buying higher quality pots and encoder with metal shafts rather than the flimsy plastic ones in the BOMs. There are so few on the Ambika that the added expense is only a few euros in total. An Ambika is such a large time and financial investment that there’s really no reason to use flimsy, wobbly plastic pots.

    I got my PCBs from the Pusherman group-buy, they‘re very cheap and work fine.

  4. I built an Ambika to join my family of Walnut Mutables!

    I messed up the LED holes in this one, but the laser engraved front panel graphics and text came out really well. The back panel is acrylic so I can admire my electronics handiwork and Emilie’s amazing design any time.

    I took the opportunity to give my Shruthi a knob upgrade, too.

  6. Put my Shruthi XT back together after the final missing piece for the 4PM filter board upgrade arrived, complete with its fancy new laser cut walnut panel! The whole case is now solid walnut.

  7. Has anybody tried using concentric pots (like this one) on a module to have the control and attenuverter for a parameter in the same place? i.e. the upper knob is the value/offset, and the lower knob would control CV input attenuvertion.

    Generally I prefer having a real knob for control and a trimmer for attenuvertion, but stacking them could save space for “micro” modules.

  8. Put my laser engraver to good use for the first time, engraving text and graphics for a Walnut front panel for my Shruthi XT synth. I’m really happy with the results!

    I ended up using Eazydraw to design the panel as usual, then after some searching discovered LaserWeb4, an amazing open source CAM tool designed for laser usage but also with routing capabilities.

    It took some time and experimentation to figure out how best to engrave the various text and graphics. In the end I found that Inside cuts along the text, on-path cuts on most of the graphics and some particularly thin text, and inside cuts+fills with very carefully set margins on the inverted filled sections produced good results.

    I’m definitely going to be looking into LaserWeb4 in more detail. I want to investigate it’s conventional routing CAM cabapilities, and potentially switch to this over using Easel, as LW4 is open source, allows for much more detailed control of the machine, and runs offline.

  9. ✅ improved gurdy MIDI system
    ✅ improved 3 channel gurdy preamplifier
    ✅ DIY hybrid digital+analogue synthesizer

    After a year of work on the hardware and software, finally all the pieces are in place to start composing!

  10. I built a Shruthi XT!

    The circuit boards and panel were group bought with the Pusherman facebook group, I ordered the components from Mouser, and built the case myself out of walnut left over from a dulcimer build.

    I used the BOM from the Shruthi XT build page, with Mouser’s BOM import tool. Generally everything worked fine with a couple of caveats: it auto-detected the wrong encoder (the horizontal mounting version of the same model) so I had to order another one. The MIDI sockets it found were also different, and had a metal spring on the outside which I had to remove in order to get them to fit the case.

    Watch out when soldering the board-to-board connectors! I put them on the wrong way round the first time and had to remove them, which was tedious.

    On my future MI builds I’m going to try using Bourns PTV09 potentiometers instead of the Alps ones on the BOM, as they cost significantly less and should be approximately the same quality.

    I built the SMR4 MkII filter board but am going to upgrade to the 4 Pole Mission as soon as I get the board and components for it (along with boards for an Ambika, and a normal size Shruthi to inherit the SMR4…)

    Overall I’m very impressed with the synth! It sounds great and is a lot of fun to make sounds with, although it’ll take me a little while longer to get to grips with all the features and wavetables.

    If you want to get into DIY synths, but skip past the “circuit which makes bleeping sounds” straight to “professionally usable synthesizer” I’d definitely recommend building a Shruthi.

    Future improvements planned: upgrade to a 4 Pole Mission filter board, make a laser-etched walnut front panel, more UI improvements in the software, maybe a built in battery and USB port for powering MIDI controllers.