DIY wiring of RCA-to-XLR umbilical cable

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rafal123 - have you considered using an active S/P-DIF to AES converter, with an impedance-matched balanced cable?  Maybe that might work better over the distance you need.  Something like the Mutec MC-1.1+ might do the job...possibly a bit of over-engineering, but the cost is not too high (compared with "audiophile" cables, anyway).  Might be worth having a chat with Mutec and asking them what they think; I've found them very responsive and keen to help.

ETA: I am of course assuming that the signal is using standard protocols...