The "Colour" Project Pt. 3: Prototyping the Motherboard December 07, 2012 17:39 20 Comments

Link and I received the first prototype PCBs from MyroPCB at the end of October, 2012. Since then, we've been testing and tweaking the circuit as detailed in Part 2: Going Modular. It works! Which is nice. But a lot of problems and opportunities for improvement have emerged during the prototyping process. Today, we'll discuss the biggest changes to the motherboard and the direction of the project.

Making the Color Modules Larger

Our primary aim in making the distortion stages modular is to make "Colour" into an open platform for design and experimentation. With this in mind, we realized that we should make the modules as large as possible for more complex circuits and bigger toys like large caps, transformers, and optical cells. In order to make the modules bigger, however, we need to make some room on the motherboard. Most of the following revisions will probably make their way onto the 0.2 PCB:
  • Using DIP8 rectifiers instead of discrete diodes
  • Changing mounting holes from #6 to #4 drill size
  • Removing some of the test points
  • Placing some low-profile components under the modules (.25" clearance)
  • Using 16-pin IDC connectors rather than MTA for the stereo link

Standardizing the Connectors

.1" header and pins

Because we are opening up the color modules to third-party developers, it's essential that we specify a standard, off-the-shelf connector for mating the modules with the motherboard. We settled on 6-pin, single row, 0.1" (2.54mm) pitch pins and headers.These are the same used to plug Arduino "shields" into the motherboard and are available cheaply from countless parts distributors.

Arduino shield

Arduino shield connected to motherboard

Rethinking the Interface

Since the concept phase, we've heard a lot of feedback, floated a lot of ideas, and even had some mildly heated debates about which controls to include on the front panel. The various controls we've at one point considered including are:
  • Input gain (master or for each module)
  • Output trim
  • Wet/dry mix for each module
  • Bypass (master or for each module)
The front panel layout for our current prototype consists of:
  • Master bypass
  • Three wet/dry mix controls (one for each module)
  • Master output trim
And, following a suggestion from GroupDIY member "kevinkace," we added input trim controls for each module on the PCB, not accessible from the front panel. In theory this setup seemed pretty close to ideal; in practice we weren't thrilled with it. The input trim pots proved to be redundant early on, as we found that we almost never wanted less than the full input signal going to each module. We could, however, envision a scenario in which we might want more signal to get some more love from the modules. So, for the next revision we're looking at integrating a master "Saturation" switch to select between unity, +3dB, and +6dB input drive. We also haven't found the wet/dry mix controls to be a very satisfying way to interact with unit. A common practice when adding an effect is to first hear it "completely wet" to evaluate the tone before mixing it in with the dry signal. This leads to a lot of sweeping back and forth with the wet/dry knobs: first to hear what each module is doing to the signal, then to hear how the modules blend together, then to get a nice mix with the dry signal, etc. Link proposed that a more elegant solution would be to switch to individual volume controls for each module and one for the dry signal. This way, one can tailor the harmonics with the three "color" controls, and then bring in the dry signal to taste. This brings the total front panel controls for the next revision to five knobs and one two switches:
  • Master bypass switch
  • "Saturation" Input drive switch
  • Three module volume controls
  • Dry signal volume
  • Output trim

Dropping the "U"?"

Our working title seems to have some staying power. Nothing else we've thought of communicates the intent and use of the unit quite as succinctly as "Colour." But I just can't get used to typing that extra vowel. And even Link, whose Canadian tendencies inspired my choice for the British spelling, prefers the "more efficient" American spelling. I'm currently leaning towards dropping the 'u', but let me know in the comments if you have a strong (or even mild) preference.

What's next?

The revisions we've looked at today all have to do with the motherboard. Next time we'll get into the actual "colors" with schematics and sound samples documenting our revision process for the modules. Thanks for following along while we slog through the gritty details!