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Balanced Audio Line, a balanced audio line has 2 hot wires, a pair of wires both carrying an audio signal. Neither wire is
grounded or has a path to ground. A shielded balanced cable will have 3 wires, two audio and 1 shield wire. The signal on the pair of wires is such that the signal is identical but out of phase with each other. On a balanced audio line, it is usually possible to get audio from either wire to ground although it would then be unbalanced audio. Balanced audio lines must connect to balanced outputs and balanced inputs in order to remain balanced. This can be done by using transformers or active circuits such as integrated circuits. |
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Higher quality microphones have a balanced output and must connect to a balanced input to take advantage
of using a balanced line. The main advantage of using balanced audio lines is the ability to run very long cables that are less prone to pick up noise. This is because any noise picked up will be induced onto both signal wires identical and because the noise signal is identical on both wires the noise will cancel at the balanced input. Balanced audio outputs tend to be low impedance. A balanced input may be low or high impedance. For extremely long cable runs low impedance is best to avoid high frequency loss, 600, 300 or 150 ohm impedance. Balanced audio cables are usually shielded but it is possible to use balanced audio cables without shielding. The telephone company ran high quality audio over miles of unshielded cables for years providing audio service for radio and TV stations. Balanced audio is usually found on professional equipment. Balanced audio level will normally have a reference level of about +4db, consumer home equipment is usually referenced to -10db. |
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This story gives an example of how well a balanced line and balanced amplifier input works. I was called to a church to
fix a problem in their PA system. Seems that ever since the church was built a year earlier, the PA system would pick up
our Fm station. What prompted them to finally call was one Sunday morning when the radio station played "Stairway
To Heaven". I got to the church and looked all around the front and asked where the amplifier was. I was informed the amplifier was installed at the back of the church so it could be kept locked up. All the microphone cables ran from the front of the church overhead over the ceiling to the amplifier in back. This was a big church and those cables had to be at least each 300 feet long. I tried and I tried and nothing would stop the RF from getting into the microphone inputs. I was puzzled because it was a good Altec amplifier and all the microphone inputs were transformer balanced in. Then I got to thinking about the cable shields. I disconnected all the shield wires from the connectors at the amplifier input and the RF was gone. All those shield wires were picking up RF and making the amplifier chassis hot with RF. And all the microphones worked perfect, no noise or hum. |
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