[amcananey]

I am building a balanced M^3 with 4 inputs:
 
2 unbalanced (RCA - with two cables for left and right signals for each of the two unbalanced sources) and
2 balanced (3-pin XLRs - two cables for the left and right signals for each of the two balanced sources).

First off, am I correct that in an XLR input cable, only two of the pins are actually used to carry signal in each cable? The answer to this will affect my next question.

Second, I am trying to figure out what the specifications for the rotary switch need to be. Am I right about the following values?

# of positions: 4 (because I will have 4 sources I am selecting among)

# of poles: 2 (for the + and - signals for each balanced input cable, or the + and GND signals for each RCA cable) In theory, I assume this could also be 4 (so that I could connect L+ and L-, R+ and R-), but I haven't seen any switches that have 4 poles and more than 3 positions, so I think I need to go with 2 poles, and add another deck. Note also that this assumes that for the XLR inputs, I only need to worry about L+ and L- / R+ and R-. If I need to worry about the GND pin on the XLR cables, then I would need 3 poles.

# of decks: 2 (So that I can connect L+ and L- to one deck, and R+ and R- to the second deck, for each of the 4 positions)

Is that right? I find switches surprisingly confusing... Thanks in advance for your help!

Best regards,
Adam

 

[KimLaroux]

The beauty of XLR cables is that the shield is nothing more than an extension of the audio gear's enclosure. It's nothing more than a shield. It's not used as a reference voltage for the signal. All you do is connect the shield to your enclosure (taken for granted you have a metal enclosure), and forget about it.
 
So you just need a 2 pole switch. One pole is used for XLR V+ and the RCA's signal. The other pole is used for XLR V- and the RCA's ground. Of course you'll need a deck per channel, and as many positions as you have inputs.
 
I think you'll need to isolate the RCA's sockets from the enclosure. Else you may get dirty ground loops, and mess up the differential topology of your amplifier.