Physical Module Design

A project log for PolyMod 1.2 Modular Digital Sythesizer

My take on Matt Bradshaw's original PolyMod synth.

Roger LabbettRoger Labbett 02/22/2019 at 19:290 Comments

In the PolyMod design, Matt had used a module faceplate size of 153mm x 38mm.  I suspect that this size was arrived at primarily as a consequence of the use of 100mm wide stripboard for building the module circuit boards.  I initially wanted to use a slightly taller module size to allow for a bit more space around the pot knobs, but failed to find a source of wider stripboard at a sensible price.  So, I decided to use perfboard instead and found some 120mm wide boards that fitted the bill.  This would give me modules 170mm high.  However, having now built my first module I am thinking I need more faceplate space, so plan to switch to 150mm wide perf boards to give me 200mm high modules.  My standard module size will be 42mm wide, but I will also have double-width modules which are 76mm wide and other sizes are possible.

PolyMod has plug-in modules which is a nice feature but limits you to a predefined module pitch.  I wanted full flexibility in module size and arrangement, so will be using ribbon cables to connect my modules. PolyMod used plywood for the module faceplates, but I will be using aluminium angle – one leg will provide the faceplate and the other will support the module circuit board(s).  This will give me a lot more robust modules. 

On the PolyMod modules, Matt provided support for 8 digital circuits for jack sockets, plus 8 analogue circuits for potentiometers.  From Matt's comments about ideas he has for PolyMod 2 it was clear that more circuits would be required for more advanced modules.  In an effort to try and future-proof my design I have added 4 extra digital circuits for switches, buttons, etc (with scope for adding 4 more), plus a shift register bus to support multiple LEDs or an LCD display.  I have also added a status LED to each module to help in trouble shooting if I wire up the patch cables incorrectly.  The result of all these changes is that I will have a 20 channel backplane rather than the 12 channels of PolyMod.

It was clear looking at the modules Matt had made for Polymod that many used less than half the available I/O circuits.  It occurred to me that I may be able to create new combined modules that implement 2 logical modules in a single physical module.  Thus I may be able to build a single double-width physical module with, say, 2 oscillators or LFOs onboard.  That way I could extend the logical capacity of the synth beyond the basic 16 modules.