Category: music

I’ve not been working on my Touring Machine Eurorack modular synth module, but I have been hacking a bit in that space. Back in 2019 John Park and I saw the Waveform Magazine DIY PT2399 delay module article. I got the parts and half-assembled mine, but the build guide was pretty vague in some areas so I just left it. This week I picked it up again and got it working!

The PT2399 chip is a weird little beast but is very cheap for a delay unit. The ebay listing for a fully-built PCB with preamps and jacks is <$9 (and forms the basis for the Waveform module).

Some quick hacks on it and it turns into a usable Eurorack delay module that’s also noisy and distorty, which can be fun. Here’s a little demo video of it in action:

Also to make it more usable with Eurorack-level audio signals, I added a 10k trimpot to the input jack, before the “mic2” input, kind of like the pic below. It provides you with a much more usable range on the “wet” knob before it distorts.

When designing my new Touring Machine synth module, two things worry me: the analog I/O section (it must be pitch-accurate) and the user interface. The UI feels like a harder problem because I’m not sure what I want. For the analog part, I can lean on others (like Émilie and Thea) and that puts me at ease.

So to address the UI as its own thing, I made a little “knobtester” PCB that is the scale and layout of the upper third of the module. It contains all the things you put your fingers on: a rotary encoder w/ switch, two buttons, and two potentiometers. Just from the outset there are questions: Should the encoder have detents? What’s the spacing of the two pots so your fingers can grasp them? Should the pots have knobs or be trimmers? How tall do the tact switches need to be? Making the board physical answered some of these questions immediately (“yes”, “as much as possible”, “trimmers, maybe with micro knobs“, and “13mm”)

Here’s the board fresh off the Othermill.

I’ve been playing more with algorithmic melody generation by working on my own module I’ve been calling the “Trinket Touring Machine“. Like my previously-mentioned “Trinket Trigger“, this one uses a Trinket M0 running CircuitPython. Initial experiments were in Arduino using the Mozzi audio synthesis library, which totally worked, but CircuitPython is much faster to develop in, especially when you’re trying to discover what the UI should be.

My original intent for this Touring Machine module was to make a CircuitPython-based version of Music Thing Modular’s wonderful “Turing Machine”, which is essentially a clever shift-register circuit hooked up to a DAC (no microcontroller!), with an adjustable noise generator for randomness. But my module has gone a bit more melodic, having the concept of scales and root notes. It’s pretty fun, but I know now the two knobs and one button UI is too limiting for what I’m envisioning. So time for Touring Machine Mk2!