If these are connected directly, maybe it sounds originally anyway. But on 2 balanced cables, a part of another channel signal appears. from having a common GND ( http://everycircuit.com/circuit/5787205903056896 ).
This may be a solution these interfaces. To compare voltages, GND leves is connected, but it is not used other purpose. On this circuit, a center level of power supply is created just dividing it. At balanced connection, no need big current on GND. When the balance is kept, the current on GND is disappear. So, no need to prepare + and -, 2 powers.
This application is so useful to confirm solutions like this, I think :-).
