Quick answer: An unbalanced audio cable carries the signal on one conductor and the return on the shield, so any noise voltage induced in that path is added to the signal without cancellation. It is fine for line-level runs under about five metres in a domestic setting. Hum usually comes from a ground loop between two earthed devices rather than from the cable, and no amount of copper fixes a loop.
Almost every consumer audio lead is unbalanced: RCA, 3.5 mm, TS instrument cable. They work because the signal is large compared with the noise picked up over a short distance. When the distance grows or the environment is electrically noisy, the same cable becomes the weak link.
How the circuit works
Signal and return
The centre conductor carries the audio, and the shield carries the return current back to the source. Noise induced in the loop appears in series with the signal because the receiving end measures the difference between the two, and the two are not treated symmetrically.
What balancing changes
A balanced cable sends the signal on two conductors with opposite polarity and a separate shield. The receiver subtracts them, so noise that appears equally on both cancels. That is why a microphone can run 30 m on XLR and an RCA lead cannot.
Practical limits
| Run | Environment | Outcome |
|---|
| 1 to 3 m | Domestic | Quiet |
| 3 to 5 m | Domestic, away from mains | Usually quiet |
| 5 to 10 m | Office or stage | Hum likely |
| Over 10 m | Any | Use balanced or digital |
The ground loop, explained without jargon
Two devices plugged into different sockets are connected by two paths: the audio cable and the mains earth. If those earths sit at slightly different potentials, current flows through the audio shield, and the receiving device hears the voltage drop along it as hum. The classic test is to unplug the charger from a laptop: if the hum stops, the loop involves the mains.
Solutions, in order of preference: put both devices on the same outlet strip, use a ground loop isolator with a transformer, or convert the link to balanced. Lifting the earth in a mains plug is not a solution and is dangerous.
Choosing and routing
Choose a cable with a braid or foil-and-braid shield, keep it away from mains leads and dimmers, and avoid running it parallel to power cable for several metres. Where a cable must cross a mains lead, cross at right angles rather than running alongside.
For instruments, a TS cable is unbalanced by design and the amp input is high impedance; that combination is fine at 5 m and noisy at 15 m. The balanced alternative uses three conductors, and the XLR connector types reference explains how the pins map. For line-level links between a computer and an amplifier, the RCA patch cables guide covers domestic options, and converting to TRS and XLR balanced wiring is the durable answer for long runs.
Practical scenarios
- Laptop into a hi-fi: 2 m RCA lead, both devices on one strip, laptop charger on the same strip to avoid a loop.
- Keyboard into an amplifier 6 m away: use a DI box or a balanced connection rather than a long instrument cable.
- Projector audio return: short unbalanced lead if the run is under 3 m, otherwise extract audio at the source.
- Stage floor run: balanced XLR to the desk, unbalanced only for the last metre to the instrument.
FAQ
Can I use an unbalanced cable for a long run?
It will pass signal, but hum and treble loss become likely beyond five to ten metres depending on the environment. A balanced connection or a digital link is the correct solution, not a thicker unbalanced cable.
Does a shielded cable prevent hum?
It prevents induced radio-frequency noise, but a ground loop current flows in the shield itself, so shielding does not help. Only breaking the loop or balancing the connection removes that hum.
Is a mono jack unbalanced?
Yes. A TS plug has tip and sleeve only, so the return is the shield. A TRS plug can be balanced mono or unbalanced stereo depending on the equipment, which is why the same connector appears in both roles.
