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.
Enjoy!
