_MG_7377_thumbStringTheory is a lo-fi, 4-note, paraphonic string synthesizer, based on the Solina String Synth emulation designed and coded by Jan Ostman.

This extremely easy-to-build module delivers a complete, classic poly string synth voice in just 6HP. Based around an Amtel ATMega328 chip this module includes a built-in MIDI input and interface, gate out, audio out and controls for envelope, ensemble and modulation.

The full kit includes a ready-to-use MIDI adapter, 20cm 16-way power/bus connector and 2 x M3/8mm screws for racking (these are also included with the assembled version).


Width 6HP
Depth 30mm inc. power cable (35mm if using rear MIDI In header)
+12V 0
-12V 0
+5V 29mA max
Inputs Front panel stereo 3.5mm MIDI In (Type ‘B’)
MIDI In 2-pin header at rear
Outputs Front panel mono 3.5mm Gate Out (approx 5V)
Front panel mono 3.5mm Audio Out
Controls Envelope (combined AR)
CV None
Other MIDI Learn switch under front panel (accessible via “paperclip port”) sets MIDI Channel and MIDI Note Offset
Gate indicator LED
Gate-to-bus (switchable via rear jumper)

Audio Demos

First, some raw sounds straight out of the module. Starts with all controls low, then Ensemble and Modulation are gradually introduced.

Using the Gate output to fire an ADSR envelope which in turn shapes a filter on the audio output.

Some notes from lower down the keyboard, again with filter.

Full disclosure… why this module might not be for you…

This module won’t be for everyone. If you want an instantly-gratifying, simple string pad that takes up very little space in your modular then you’ll probably like it, but even so – it might not be quite what you need.

In the interests of having happy customers (we like happy customers), here’s a list of the pros and cons of this module so that you can make an informed decision about whether or not it’ll work for you in your system.

Yes, it’s MIDI-in ONLY

Implementing polyphonic CV would have been a big job, and would have left a lot of people without poly CV sources in their modulars scratching their heads in dismay. We therefore took the decision to implement this first release of StringTheory as MIDI-only in a “it’s-shortest-distance-between-two-points” kinda way. This won’t suit you guys who work with you modular as a self-contained universe; conversely, there of lots of musicians who routinely hook a MIDI keyboard or controller to their Eurorack gear who this will sit with just fine. At the moment it handles MIDI Note-on and Note-off messages only (this will be expanded in a future firmware release).

Note range

You’ll find that the processor starts to max out at higher frequencies, introducing some audio glitching; dialing back the Ensemble and Mod usually tames this.

No CV control

We tried providing CV control for the Ensemble and Modulation parameters; we found it to be of very limited value and would have provided little advantage at the cost of a couple of extra HP.

It’s not going to be as loud as your raw analog sources

StringTheory outputs at a  couple of dB above line level. This will be too quiet for some, but it’s not as if this is impossible to fix downstream. Again, we considered adding output amplification but we felt that this is a job that is already done (probably much better) by a whole bunch of other modules.

“It’s not a ‘proper’ Eurorack module”

We’ve seen a handful of comments like this in our pre-release market test; we feel it’s up to the individual to decide where they’d like this to live in their setup. Some people like the convenience of racking all their gear together and that’s fine; some prefer to keep their modular “pure” and that’s also fine (though we’d encourage you guys to take a look a our stand-alone version of StringTheory, too). We’re not here to tell you what to buy or how to organise your system;  we’re just here to offer you a really cheap little synth voice in a couple of (we think) useful formats.

If you’re still excited at this point, that’s great, welcome aboard. If not – check back in with us every once-in-a-while, as we’re aiming to address some of the above issues in future versions of this module. Either way – take a listen. We think it’s pretty lush!

Build instructions

The StringTheory build instructions provide a clear guide to making this project.


The full parts list can be found in the build instructions. All of the parts used in this project are easily and widely obtained from any electronics supplier.

Our parts kits contain all of the electronic parts that you’ll need to do the job; the only things that we don’t supply are:

  • Tools
  • Solder


Item Price

PCB and panel

A PCB and matching 6HP Eurorack panel. Includes 2 x M3/8mm screws for mounting the panel into your rack.


PCB, panel & chipset

A PCB, 6HP Eurorack panel and an ATMega328P-PU microprocessor, pre-programmed with bootloader and StringTheory software. Includes 2 x M3/8mm screws for mounting the panel into your rack.


Full kit

A PCB, panel, pre-programmed microprocessor and all of the additional parts you need to build this project. Includes 2 x M3/8mm screws for mounting the panel into your rack, a 20cm 16-way power cable and a Type B DIN-to-3.5mm MIDI adaptor.


Fully assembled & tested

A fully-assembled StringTheory module, ready to use. Includes 2 x M3/8mm screws for mounting the panel into your rack, a 20cm 16-way power cable and a Type B DIN-to-3.5mm MIDI adaptor. Please note that lead times will vary – contact us or see the shop for more details.


Software update

We intend to continue developing this product and will be releasing software upgrades from time-to-time. For those of you without the kit or skills to update the software yourselves, we’ll release each new version on a new microprocessor – simply swap out the new chip for the old. If you mail us your old chip for re-use, we’ll refund you £2.00

£3.00 – £5.00

Prices include packing & standard (untracked) shipping. All prices are given in GBP, your local price after currency conversion may vary.

All of our products are available from our Etsy shop. Although we make efforts to keep our stock levels as constant as possible, demand might occasionally mean that an item is out of stock. Stock replenishment typically takes less than one week.