[sdiy] Matrix patch panels

[Michael Bechard]

Hi everybody, I'm trying to put together a semi-modular using a DIY matrix patch panel, much like the one described here: http://monopole.ph.qmw.ac.uk/~thomas/synthdiy/patchpanels.htm. There's not a whole lot of info on the web about doing this, so I figured I'd ping you guys.

Here's an overview of what I'm planning. My matrix will be a fairly large (31x28) and combine both audio and control signals, with configurable switching between unipolar and bipolar attenuation like this: http://www.cgs.synth.net/modules/cgs33_matrix_mixer_euro.html. However, instead of using pots for each connection (would be expensive!), I'm using jacks for switches, much like professor Thomas' design. Each output and each input will be separately attentuable (pre and post matrix processing) via knobs setup along each of the matrix's axis. I'm also thinking of using RCA jacks instead of 2.5mm, since I can get them fairly cheap @ $.25/per at bulk. The PCB-mounted versions seem to be easiest to use, since the entire jack housing is metal and forms one or the conductors. I figure I can solder 31 of them directly together to form one row, while using bare copper wire to connect the other conductors vertically to each other. Here are the RCA jacks I plan
 to use:
http://mouser.com/Search/ProductDetail.aspx?R=161-0370-Evirtualkey11180000virtualkey161-0370-E. My main concern here is the lack of grounding should I use these jacks, but I really like the idea of being able to solder an entire row directly together.

I should also mention I'm a complete newbie when it comes to building synths (or most any kind of electronics), so please forgive any obvious blunders I may include in this email and my plans. So far I've only put together a couple of my modules.

Anyway, I would greatly appreciate any pointers and feedback you guys could give me on my proposed design. I'm mostly concerned about crosstalk, grounding, and wire capacitance/inductance at this point. I noticed professor Thomas used bare copper wire to connect the leads of each jack. Does anyone know if this works well? How much crosstalk/noise problems should I expect? Should I plan instead on using grounded jacks, like 2.5mm stereo? Lastly, do you guys think this design scales well to a matrix the size of which I am planning?

I'm also wondering about buffering on the inputs and outputs. Since I will be following Ken Stone's design on the outputs (to get switchable uni/bipolar attentuation), I know they at least will be buffered. Do you think I need to buffer the inputs as well, like Ken has designed in the CGS33? Also, what effect do you guys think the extra current will have on the signal crosstalk, if any?

Wow, this is a long email, so thanks for reading. I appreciate any and all input.

Thanks,
Michael Bechard