Balanced and unbalanced connections

Cables pick up interference from mains wiring, radio transmitters and lighting dimmers. Balanced connections are designed to reject that interference, which is why professional equipment uses them.

Unbalanced

An unbalanced connection uses two conductors: the signal and a ground (the shield). Interference induced into the cable adds to the signal and cannot be told apart from it. For short, low-noise runs, this is acceptable. It is typical for consumer equipment and instruments, with RCA or tip-sleeve (TS) jacks.

Balanced

A balanced connection has two signal conductors and a shield. The signal is carried as two versions with opposite polarity: one conductor is "hot" (+), the other "cold" (−). A differential input subtracts the two.

• The wanted signal is the difference between the conductors, and the subtraction doubles it.
• Interference reaches both conductors equally (common mode) because they run side by side in a twisted pair. The subtraction cancels it.

The ability of an input to cancel common-mode signals is its common-mode rejection ratio (CMRR), typically 60 dB or more for good inputs. Balance does not need the two conductors to carry exactly opposite signals; it needs the impedance to ground to be the same on both conductors, so that interference couples equally.

Connectors and wiring

The assignment of pin 2 to hot is standardised (IEC 60268-12, AES14). Older equipment may be wired the opposite way, which inverts polarity when mixed, see Polarity is not phase. A TRS plug can carry either a balanced mono signal or an unbalanced stereo signal, and mixing the two causes problems.

Cable length

An unbalanced line from a high-impedance source should stay short, a few metres, because of capacitance and noise. Balanced lines from low-impedance sources can run for hundreds of metres, see Impedances in audio.

Phantom power

Condenser microphones are powered through the same two signal conductors: 48 V DC is applied through two 6.8 kΩ resistors to pin 2 and pin 3 relative to pin 1. The same voltage on both conductors is common mode, so a balanced input ignores it. The microphone takes up to about 10 mA. Dynamic microphones are not affected if the cable is correctly balanced, but unbalanced or faulty cables can be.

Transformers and active circuits

Inputs and outputs can be balanced with transformers, which also give galvanic isolation, or with electronics. A transformer-balanced connection breaks ground loops, which cause hum when devices are grounded at several points.

Practical advice

1. Use balanced connections wherever you can.
2. Keep signal cables away from power cables, or cross them at a right angle.
3. If you hear hum, check the grounds before touching the shield; do not remove the safety earth.
4. Use a DI box to make an unbalanced source balanced.

Draft: written by an AI assistant, awaiting technical review.