Unbalanced outputs carry a single copy of the signal and the ground connection. This means that unbalanced cabling over longer distances can be prone to interference, which can be heard as noise on the signal.
Balanced outputs carry two copies of the signal, that when combined at a balanced input cancel out noise and interference that has been picked up.
Balanced outputs should be used for longer cable runs to speakers / devices, as they have lower noise and are better for rejecting unwanted interference.
To interface an unbalanced piece of third party equipment to any SSL balanced input use the following wiring scheme:
Positive to Positive
Screen to Negative
No Connection to Screen
The image below is showing the difference between and balanced and unbalances signal.
This image shows an unbalanced and balanced 1/4-inch Jacks.