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KELPIE: Synth Module for Keyboard Controllers

A portable polyphonic synthesizer designed for use in live settings with a keyboard controller

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The KELPIE is a portable polyphonic digital synth with a hardware interface inspired by the Moog Sirin that was released earlier this year. The device consists of 15 knobs and 4 LED buttons. IO consists of a standard MIDI input, 1/4" mono output, 1/8" stereo output, and a micro-USB input (for programming). After several experiments with the Teensy audio shield over the past year (and a lot of learning about synthesis in general) I wanted to design something that I could actually use in live performances, as I'm still not the most creative when it comes to synth programming. I've honestly had the most fun and learned the most about sound design by tinkering around with my keyboard while tweaking knobs, so I decided to design a portable synth-module for a MIDI keyboard with an emphasis on the real-time hardware-interface design. Project files and progress can all be tracked here:

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 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 bulk of the mechanical design, including the large MIDI and phone jacks, knobs,...

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  • Enclosure Prototype Progress

    Kenneth Marut7 hours ago 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 Maruta day ago 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!

  • Prototyping knobs for potentiometers

    Kenneth Marut4 days ago 0 comments

    I printed 16 potentiometer knobs on the resin printer and built a little jig to hold them in place while I clear coated them and let them dry. Q-tips to the rescue!

    I am going to attempt the crayon method to fill in the tiny notch on top of each knob with some yellow wax

  • Audio Design So Far

    Kenneth Marut6 days ago 0 comments

    Wanted to share the synth block diagram so far, it's getting pretty ridiculous! Still some changes coming but so far so good

  • Packaging Concept #1

    Kenneth Marut6 days ago 0 comments

    A fun "action figure" style packaging concept I designed (not complete, but pretty fun tangent from writing code)

  • Greeeen

    Kenneth Marut08/09/2019 at 19:33 0 comments

    Printed out a final prototype enclosure using some transparent green resin. I honestly think these are the best prints that have ever come out of my machine. There's only a tiny bit of warping which I'm hoping to flatten out with a 20lbs weight. Still need to do a bit of post processing to get a high transparency finish, and also looking into options for decal application

  • Initial graphical assets and knob layout

    Kenneth Marut08/07/2019 at 17:44 0 comments

    I got started on some very initial graphical assets for the control panel of the synthesizer. I'm realizing now that this would probably would pretty awesome if it was applied via screen printing. I don't have the capability to do that at home with my current tools, but I'll hopefully be able to fake it using some other methods for the time being

    I've also made a lot of progress on the firmware and have gotten 12 individual voices sounding in polyphony, each with their own amplifier envelope, which is totally awesome. Whenever a key is pressed, the code checks a 12 note buffer for the next available spot where

    envelope.isActive() == false

     it then finds the first available voice and activates that note. When the note finishes sounding, it is removed from the buffer. The next step is to add a filter envelope to each voice to get some more dynamic sounds!

    Stay tuned!

  • Prototype #1

    Kenneth Marut08/06/2019 at 00:49 0 comments

    I was able to print the two halves of my enclosure using my resin printer and was really happy with the results. The upper have barely fit in my printer so I had orient it at a pretty steep angle, which led to a lot of support removal during post processing. Here are some photos of the assembled prototype below. I used a transparent white resin (which is actually kind of yellowish) but I'm pretty happy with the effect. I especially like how the red LED diffuses on the enclosure. I'm also pretty satisfied with the diagonal-line texturing on the front and back.

    I've also been doing a lot of work on the firmware and have gotten MIDI input to work, and a very barebones monophonic synth working with 2 oscillators. My plan is to expand the device to 6-8 voices. So far I'm really excited how this is coming together and hope to get an MVP done before the 25th of August!

  • resin prototyping

    Kenneth Marut08/03/2019 at 04:06 0 comments

    A couple photos from some resin enclosure prototypes I cranked out this week. Still some tweaks to sort out, but pretty happy with the prints!

  • Getting started on firmware

    Kenneth Marut07/22/2019 at 21:59 0 comments

    I got started on writing the firmware using PlatformIO. I've been trying to be more organized with my folder structure and making a better effort at encapsulating all my code properly so I've been trying to contain a lot of the IO and setup functionality inside its own custom library. I just got the audio shield setup working and am nicely polling all my knobs and buttons (with debouncing). Not terribly exciting, but still happy with the results!

    I also got started on a pretty straightforward synth engine. 2 Oscillators, 1 noise generator, ADSR envelopes for filter and amp. Likely will change as I move further along, but it'll do for now.

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