Hum

In audio systems, hum usually originates from power line interference. Most electrical systems operate at 50 Hz (in Europe and Australia) or 60 Hz (in North America), and when stray electromagnetic fields from mains power couple into audio circuits, they induce a tone at that same frequency, often accompanied by harmonics at 100 or 120 Hz.

The most common cause of hum is a ground loop, which occurs when two or more pieces of interconnected equipment share multiple paths to electrical ground. Small differences in voltage between these ground points create circulating currents that introduce hum into the audio path. Improper shielding, unbalanced cables, or proximity to power transformers and fluorescent lighting can also contribute.

In analogue systems, hum can enter through mains power supplies, cable shielding, or inductive coupling between nearby cables. In digital systems, it may appear if analogue connections are poorly isolated or if power supplies share noisy grounds.

To eliminate hum, engineers typically use balanced connections (such as XLR or TRS cables) to cancel common-mode interference, ensure proper grounding practices, and separate audio cables from power lines. Ground lift switches, isolation transformers, and DI boxes can also break unwanted ground paths.

Persistent hum can indicate deeper issues such as faulty power supply filtering, deteriorated capacitors, or incorrect system grounding. Addressing hum not only removes the audible noise but also ensures system safety and optimal audio fidelity.