Balanced Audio Connections

Someone once told me that a balanced audio connection works because of polarity. I wish I had a rolled up newspaper so I could swat him with it on the nose. Balanced systems are used to keep noise and interference out of systems. It is a common myth that balancing a system involves polarity. It does not. Polarity plays a part in keeping interference out, but the real reason balanced systems work has to do with impedance.


This type of connection is known as an unbalanced system. There is only one connection leaving the Op-Amp in device A. The second connection is ground. In device B the signal is brought in on one leg of the Op-Amp and the the second leg is a replica signal sourced to ground (or reference). In other words the signal is the same except opposite (polarity). There is absolutely nothing preventing noise and interference from entering this system.


This is a balanced input. Notice how the input connector has 3 connections instead of 2.


This a balanced output. Notice the 2 Op-Amps and 3 connections. The balanced system has both connections equally referenced to ground. How this prevents interference is an idea called Common-Mode Rejection (CMR). Because interference hits all three wires in a cable at once, they will all have an equal level of extra noise. It is voltage essentially. When the signal enters the Op-Amp at the input it looks at the ground line and sees what’s on it. It then compares what it sees on the two signal lines. It kicks out what all three have in common. Hence Common-Mode Rejection.

This is theory though. Not all inputs are perfect, and because all cables have something called cable capacitance, voltages differ minutely on each wire within the cable and the rejection doesn’t work as well as the theory states it should. But it still works pretty darn well. There is a whole science devoted to developing a standard for getting better CMR. One of my favorite resources is Jensen Transformers’ Bill Whitlock. He is a freakin genius. Here is his seminar handbook on balanced and unbalanced connections. This is where it all started making sense to me.