KELPIE: Subtractive Synth for Keyboard Controllers

A portable, cheap, and easy to use, monophonic + polyphonic digital synthesizer for use with a MIDI controller

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The KELPIE is a portable monophonic & polyphonic digital synthesizer. The device consists of 16 knobs and 4 LED buttons, a standard MIDI input, 1/4" mono output, 1/8" stereo output, and a micro-USB input (for programming). The goal of this project was to create a budget digital synthesizer for all experience levels that could be easily interfaced with a MIDI keyboard and used in live performances. This is a no-frills synthesizer, with no presets, arpeggiator, or storage. This was also an exercise in designing a "product" from scratch, including electronics, industrial design, mechanical engineering, prototyping, and software.

The Motivation:

Over the past couple of years, there has been a movement for music instrument companies to release “toy synths”, such as the Teenage Engineering OP-Z and Korg Volca Series. These toy synths are generally much smaller, cheaper, and more approachable than their full-size counterparts, and do not skimp when it comes to the feature set. I’ve had the chance to play around with a lot of these synths over the last year, and have had a lot of fun with them all. Unfortunately, even these toy synths range in the $200 - $1500 range, so unless I want to burn through my savings, it’s not really realistic to collect them all. 

Though I’ve been a lifelong musician and technology nerd, I really only became interested in synthesis later on. I suspect it’s because I never really understood what the difference was between a synthesizer and an electric keyboard with a bunch of presets. In my path to learning about synthesizers, I found it quite difficult to find a starter synth that was approachable and small, but had a reliable feature set. 

The Problem:

The KELPIE, like other toy synths, is fun and approachable. But unlike others, it requires an external MIDI input (such as a keyboard or another MIDI device). There is no hardware touch-keyboard, or push-button keyboard, saving a good amount of space. It’s polyphonic, so any player who’s used to playing the piano will feel very comfortable, but it also has a monophonic option. There are no preset sounds, banks, sequencers, or digital menus on the device, keeping it straightforward and simple, but also incredibly useful for beginners to advanced users. As the owner of a super-nifty 12-voice analog synthesizer, I can honestly say that I never touch the preset sounds and spend most of my time twiddling knobs and messing around the modules. The KELPIE is completely plug-and-play, requiring only a power supply and MIDI connection. It also features a stereo ⅛” headphone output, as well as a stereo 1⁄4” line output for direct connection to an amplifier.

What’s up with the name?

One trend I’ve noticed with synthesizers is to name them off of mythological creatures (Sirin, Minotaur, Osiris, etc.). The KELPIE follows this trend: “a shape-shifting water spirit”. Since synthesizers are essentially wave-shaping devices, I thought this particular name was appropriate. It also was the inspiration for the greenish color scheme of the device.

The Design:

Earlier this year I had a chance to visit the Moog Factory in Asheville, VA. I went on a tour and got to mess around with some of their newest products including the Moog One and the Sirin. Both instruments were super cool, but I really felt attached to the Sirin. The hardware interface was easy to wrap my head around, it was portable and colorful. The device had no built-in keyboard, but only a couple outputs and a MIDI in. The Sirin is an analog synth and handmade in Asheville, which probably explains the $600 price tag. Nonetheless, I came home feeling inspired and decided I wanted to make my own version of the Sirin.

I started off knowing that I wanted to use the Teensy Audio platform as the core since I had so much fun with it last year, and I felt like there was still a lot of exploring to do. I selected some basic electronic components that would define the layout of the mechanical design, including the large MIDI and phone jacks, knobs, and LED buttons. I fired up a Fusion 360 + Eagle joint project and started the initial layout of the PCB. I was very diligent about adding 3D files to all my Eagle library components because I didn’t want any surprises down the road during prototyping.

Once I was confident in my mechanical layout, I spent a lot of time interpreting and translating the Teensy 3.2 reference board and Audio schematic to create my own custom controller board. When I was ready, I had some boards manufactured and started on the industrial...

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All mechanical, electrical, and software build files from the Github Repo as of August 24, 2019

Zip Archive - 25.06 MB - 08/24/2019 at 22:46


  • 1 × Please see github for most up to date components

  • Some updates...

    Kenneth Marut02/25/2020 at 01:07 0 comments

    I've recently been exploring the idea of kit-ifying this synth and designing an FDM "print-your-own" enclosure. Here's a photo of some recent design iterations that have a lemon-y feel to them. I've also printed out a nice stencil jig for soldering which has really helped with assembly :)

  • An FDM enclosure

    Kenneth Marut02/08/2020 at 02:36 0 comments

    Just printed the first "official" FDM version of the enclosure. I think it turned out as well as I could've expected. I had to mess around with SImplify3D's "thin wall" settings quite a bit to make sure the ribs of the enclosure were properly filled in. There's a pretty stark difference in appearance compared to the resin prints, but this is a much less intensive process, and requires zero post processing and no support structures, so it definitely would scale better for small quantities if I'm manufacturing these at home (or if anyone is interested in printing their own enclosure). I may try to experiment with different top/bottom colors to keep things interesting. Either way, I'm pretty satisfied with the overall print, and definitely impressed with the infill. 

    More updates coming soon!

  • Another round of synthassembly

    Kenneth Marut01/26/2020 at 19:23 0 comments

    Gradually getting better (and more organized) at this...

  • Out in the wild...

    Kenneth Marut11/11/2019 at 23:34 2 comments

    Sending out the first unit for some user feedback! Hope it makes it in one piece :)

  • Finally had some time...

    Kenneth Marut11/03/2019 at 20:27 0 comments

    It's been a while since I've worked on the Kelpie as I've been so busy starting a new job. I finally had some time over the weekend to get things working again and attempting to tackle some bugs. I built a debugging/test fixture and tried to hook up the J-Link debugger with PlatformIO, but couldn't get it to wor.k I haven't found much support online for debugging the Teensy 3.2 platform, so I decided to move on and just stick with good ol' Serial.prints.

    I fixed a lot of issues, uncovered some new ones, but I'm happy with the progress and I'm planning to mail one of my units to a friend to get some initial feedback. I added some cool new functionality, including a MIDI LED that flashes whenever a key is pressed, as well as routing the LFO to the master gain, and support for MIDI CC messages, so the knobs can be over written by MIDI messages (which is super cool). I was worried that I'd have issues with the built-in knobs fighting with the MIDI CC messages, but luckily I had written the knob functionality with that in mind (accidentally? I can't remember...)

    I've been avoiding doing the glide functionality because I just don't have the mental capacity right now

  • 3 Complete Prototypes Assembled

    Kenneth Marut09/17/2019 at 19:10 0 comments

    I just wrapped up assembling the third prototype. It was a great experience to build 3 completely separate units, I learned a good amount from each build, and made tweaks along the way. I uploaded my current firmware and so far everything seems to be working as expected.

    Some of the major changes I included in the newer prototypes include:

    • Using shorter potentiometers and redesigning the indicator knobs
    • notching the upper PCB to make clearance for the MIDI connector
    • Swapping out any SMD components on the upper board for thru-hole components
    • Removing the SD card slot and RAM chip
    • Upgrading the 30-pin connector to a flex connector
    • Adjusting the bosses and ribs on the enclosure to be more robust 
    • Adjusting the 2 pots to the right of the left-most buttons (the original design was slightly misaligned)
    • Optimizing my resin 3D printing orientation to reduce the number of supports and make post-processing easier
    • Discovering a more efficient way of post-processing the 3D prints to get a glossy finish (220 grit -> wet 2000 grit -> clear acrylic does the trick)

    Anyway, here are some photos below. You can see the difference between the original design and the updated one, I managed to lose about 0.5" of height, which is great. Next steps are to resolve some firmware bugs and then I plan to send them out to friends to get some feedback!

  • Prototype #2 almost finished

    Kenneth Marut09/12/2019 at 18:34 0 comments

    Finished assembling all electronics and the enclosure (except for knobs). So far everything's working as expected! I'm working on making another prototype which I should hopefully be able to knock out in the next week and then I'll likely go back to the firmware as there are a good amount of bugs that need to be sorted out. Anyway, here are some photos of the build. I'm really happy with the new 30 pin flex connector.

  • Some more photos!

    Kenneth Marut09/09/2019 at 22:04 0 comments

    I've assembled most of the second revision prototype, including the PCBs, but unfortunately made a tiny mistake on the BOM so I'm missing a bunch of 2.2 uF capacitors (just ordered some more). I decided to leave off the SD card and additional RAM chip since I don't think I'll end up needing them, and they're definitely some of the more expensive components. Here are some photos, as well as a side-by-side with the original prototype.

  • New PCBs (and second prototype progress)

    Kenneth Marut09/06/2019 at 23:43 0 comments

    New PCBs just arrived today, went with Seeed's 'yellow' option, which actually turned out way cooler than I expected. Almost looks like there's no silkscreen, which I really like. Snagged a photo during golden hour :)

    Currently post processing the enclosure bottom and printing out another in green

  • some more prototyping, and new colors!

    Kenneth Marut09/04/2019 at 18:32 0 comments

    Been pretty busy, but have been making some progress here and there on a second revision of the hardware/enclosure. I just printed a new top-half for the enclosure which is about 1/4" thinner than the previous version. Decided to go with some transparent red resin this time for fun! I've also been trying out some new approaches to post processing that will hopefully save me some time, including more intentional support structures, and less tedious alcohol cleanup.

View all 26 project logs

  • 1

    The enclosure top and bottom STL files can be found here and can be 3D printed on any printer large enough. If using an FDM printer, face the flat side on the bottom. If using a resin printer, orient parts at an angle so that the majority of supports are facing the flat face

  • 2

    The Eagle PCB schematic and board files can be found here. Gerber files can be easily exported using Eagle. Note* I still have a couple of important revisions that I plan to implement soon!

  • 3
    Electronic Components

    To get the most up-to-date electronics BOM, run the Eagle ULP BOM generator script. I have made sure to include all links and MPNs for components inside the Eagle Library

View all 4 instructions

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Gauthier Legrand wrote 12/01/2019 at 09:15 point

Very nice project ! 

I just made a multiFX box with my Teensy 4 and audioshield, this plateform is amazing :)

I would be very interested to buy a Kelpie. The form factor looks a little tiny for my big fingers but I can probably handle it ^^

Any plans to sell a kit or something ?

  Are you sure? yes | no

David Strange wrote 08/27/2019 at 12:47 point

Well done that man! You've designed a really brilliant synth there. I too am very keen on securing one for myself!

  Are you sure? yes | no

Kenneth Marut wrote 08/27/2019 at 13:22 point

Thanks, really appreciate it!

  Are you sure? yes | no

David Strange wrote 08/27/2019 at 13:28 point

The fact that it's polyphonic is really good. I have a Meeblip, Modal Craft 2.0 and Uno Synth and they all make great sounds, but they are monophonic. Your magisterial synth is polyphonic and that makes it much more useful. Seriously, I'd love one and cannot wait until you find a way to producing them in numbers that you can sell. Really good job on this!

  Are you sure? yes | no

hackaday wrote 08/27/2019 at 12:04 point
Nice implementation. I did a knob-less (MIDI CC only) 4 voice synth years back with the Teensy Audio library. I was frustrated there wasn't a better resonant lowpass filter so I never did much with it. Have you found something better? Lowpass which behaves nicely at self-oscillation and also doesn't sound "boring" below that is seemingly closely held info I have not been able to find online. If you crave an affordable analog polysynth I recommend the Behringer Deepmind 12. It's not "cheap", but it's waaaaay cheaper than its Dave Smith counterparts and sounds just as good.

  Are you sure? yes | no

Kenneth Marut wrote 08/27/2019 at 13:25 point

Haven't dug too deep into the quality of the LP, but it's serving its purpose for this low budget synth pretty nicely. It's possible that the quality has been improved over the past couple of years, but I'll mess around with it some more. Oh, and also I actually do own a Deepmind 12, and I love it!

  Are you sure? yes | no

hackaday wrote 09/01/2019 at 14:06 point

BTW I wanted the new Behringer string synth but didn't have the money so I wrote what really mattered, which is the ensemble chorus:

It's beta (I need to implement stereo) but it works and turns basic pulse sounds into a very nice string sound. I'm pretty confident I can nail everything the Behringer does, including the beautiful 'vox humana' and of course the ensemble + phaser for the Jarre 'Oxygene 6' sound.

  Are you sure? yes | no wrote 08/27/2019 at 02:04 point

Hey, so how can I buy a Kelpie? 

  Are you sure? yes | no

Kenneth Marut wrote 08/27/2019 at 13:22 point

I have a bit more to finish up, and some changes I'd like to implement but hopefully I'll be able post a kit on Tindie sometime soon!

  Are you sure? yes | no

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