HI Ted!
Well, the very simplest way would be to use a Lorlin 2 x 6 rotary switch if all the sources were unbalanced. Maplins do them. This could be fed into the balanced input, shorting one of the legs to ground to un-balance it. Obviously, there would be a drop in level compared to the output of your desk or protools for instance, but you could make a simple gain make-up stage with two op-amps( or a dual op-amp), anything from NE5532 or 2 x NE5534 to 2520, OA10, etc etc. You could use an output transformer to balance, or a dedicated balancing chip, or even just impedance balance the signal.
If this is just to be able to easily route the signal from cd, iPod etc, and not to "A-B" with protools and/or desk output at a comparable level, I'd just hack a Lorlin switch into a diecast box with phono and 3.5mm sockets in, and an un-balanced XLR output. If you need to trim all these different inputs to be same level, you will need much more jiggery-pokery - a pair of op-amps for each input, and a variable gain for each.
KIndest regards,
ANdyP
ps I've got limited technical electronics knowledge too, and keeping it simple is often the best way!
