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 last year. I suspect it’s because I never really understood what the difference was between a synthesizer and an electric keyboard with a bunch of preset sounds. Additionally, it’s pretty hard to find a synthesizer out there that can serve as a stepping stone for those of us who are keyboard players and want to start off with the basics in a friendly and approachable way. Not to mention, as a keyboard player, a new keyboard or electric piano is a huge space commitment, it would be nice if you could just reuse your existing MIDI keyboard and buy a cheap, portable sound module.

The Problem:

The KELPIE, like other toy synths, is fun and approachable. But unlike other toy synths, it’s completely designed to be interfaced with a keyboard (or another MIDI device). There is no hardware touch-keyboard, or push-button keyboard, saving a bunch 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, simple, but also incredibly useful for beginners to advanced players. 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?

Coming up with a name for anything is a tough task, whether it’s for your rock band, your dog, or your synthesizer. The easiest place to start is to look at existing product names out there and try to find inspiration. 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...

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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

  • 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.

  • Revisions in process!

    Kenneth Marut08/29/2019 at 15:24 0 comments

    Been working on a couple revisions to the PCB/Enclosure that I want to have done before making another prototype. I decided to go with all thru-hole components for the upper PCB, since this board will have to be hand assembled anyway, might as well stay consistent. Additionally, I swapped out the bulky 30-pin connector to a flex-cable connector to save space, and cost. I also notched out a section of the top board to make some clearance for the MIDI connector, this allows me to lose about 1/4" of thickness off the enclosure.

  • Prototype Assembled!

    Kenneth Marut08/22/2019 at 16:07 0 comments

    I just finished assembling the final prototype. Hit a couple snags in my enclosure design where I ended up needing to dremel some bits of plastic out, but that's to be expected. I took some photos along the way. Really happy with how they turned out. The look of the yellow and red LEDs is really cool, and I love how you can see the electronics through the plastic! 

    I have a couple bugs to work out in my firmware, but I'm really close to finishing. Once everything is in place, I'll try to prep a demo video.

  • Decals!

    Kenneth Marut08/21/2019 at 21:30 0 comments

    Totally geekin' out with how cool these decals turned out. I've never used this method before, but it was incredibly easily and straightforward. Super excited!

  • Enclosure Prototype Progress

    Kenneth Marut08/20/2019 at 20:26 0 comments

    Wanted to share a couple photos of the prototype enclosure so far. I'm really happy with the results of my printer and the attempts at trying to get this thing as glossy as possible. I should be receiving my custom dry-transfer decals in the mail tomorrow so I'll hopefully be able to assemble the entire thing very soon!

  • Pretty nifty wax-melting technique

    Kenneth Marut08/19/2019 at 15:53 0 comments

    Really impressed with how the Crayon technique worked for adding some color to these knobs! Used my hot air station to melt the crayon directly on top of the knobs, then I took each knob to my dremel w/ brush attachment to clean them up. I will definitely be using this technique in the future!

View all 21 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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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

Adam Lane 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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