"Nelson Pass: And so when using RCA connectors in parallel with XLR, you short pin 3 to 1, grounding the negative input. "
"XLR -> RCA
Pin 1 -> Lift
Pin 2 -> Hot or RCA center pin
Pin 3 -> RCA Ground
That would probably work. You would get the extra gain you mentioned. "
Actually the gain would remain unchanged from a balanced connection.
"MY Balanced to Unbalanced configuration:
XLR -> RCA
Pin 1 -> RCA Ground
Pin 2 -> Hot or RCA center pin
Pin 3 -> Lift"
This would cause a loss of 6dB from a balanced connection.
"Throw in a patchbay where you try to mix and match balanced and unbalanced, watch out."
I agree, your method for hooking Pin 1 -> RCA Ground cannot get rid of a ground loop. Electronically balanced inputs cannot get rid of a ground loop. Only a transformer can provide galvanic isolation. Or an optocoupler.
What gives the best bang for the buck is cross coupled electronically balanced outputs with transformer inputs.
"My first suggestion is the scheme that is standard practice for proffesional audio and video applications. "
I agree, 99% of those guys don't know what they are doing. Electronically balanced inputs using 1% resistors only have 40dB CMRR. Even if you trim the low frequency CMRR with a pot, and trim the high frequencies with a cap, the transformer will perform better. You will still have hum because of the input off-set current of the op-amps and the high end will sound like an electric shaver if you are in a high RF environment. If you really want it clean and quiet, use a transformer. The ground loop issue can be fixed with a transformer at either end, but the RF can only be fixed at the input.