A low hum from audio equipment is one of the most common complaints in home listening setups. It almost always comes from one of a small number of sources. This guide explains where audio hum originates and what can be done about it without requiring specialist equipment or mains wiring work.
What hum actually is
The hum you hear is typically at mains frequency — 50 Hz in most of the world, 60 Hz in North America — or a harmonic of it (100 Hz, 120 Hz). This is a sign that mains electricity is coupling into the audio signal somewhere in the chain. It is distinct from hiss, which is broadband noise, or interference from radio transmitters, which has a different character.
Ground loops
The most common cause of hum in a connected system is a ground loop. This occurs when two pieces of equipment are connected by an audio cable and also share a ground path through the mains earthing system, but at slightly different electrical potentials. A small current flows around this loop, and the audio cable picks it up as hum.
The standard fix is a ground loop isolator — a small passive transformer that breaks the direct electrical connection while passing audio. These are inexpensive and available for RCA and 3.5 mm connections. They work by coupling the audio signal magnetically rather than electrically, which breaks the loop without affecting the audio significantly at normal listening levels.
Do not defeat the mains earth connection on any equipment to solve a hum problem. This removes a safety feature and is not an appropriate fix.
Cable routing
Audio cables run parallel to mains cables, or looped over a mains transformer, can pick up interference inductively. The fix is physical: route audio cables away from mains cables and power supplies, and cross them at right angles rather than running them alongside. Keep cables as short as practical.
Unshielded or damaged cables
Unshielded audio cables act as antennas. Replacing a long, unshielded cable with a shorter, well-shielded one reduces interference pickup. Check cables for damage — a break in the shield, or a partially broken inner conductor, can introduce hum. Wiggling the cable while listening will reveal a damaged cable: if the hum changes, the cable is the cause.
Equipment with a shared power supply
Cheap switched-mode power supplies (wall warts) radiate more interference than linear supplies. If hum appeared after adding a new piece of equipment, try powering it from a different socket on a different circuit, or replacing its power supply with one of better quality. A mains filter or power conditioner can help in a noisy electrical environment.
When to call a qualified electrician
If none of the above resolves the problem, and hum is present even with all audio connections disconnected, the issue may be in the mains wiring — a faulty earth, a wiring fault, or equipment with an internal fault. These require a qualified electrician and in some cases a service technician for the equipment itself. Do not investigate or modify mains wiring yourself.
Frequently asked questions
I only have one piece of equipment connected and it hums. What is wrong?
With a single piece of equipment and no audio inputs connected, hum suggests either a fault in the equipment itself or interference from its own power supply coupling into the audio stage. A ground loop isolator will not help here because there is no loop. If the equipment is under warranty, return it. If it is a vintage piece, a service technician can assess whether its power supply capacitors have degraded — a common cause of hum in older equipment.
Does the number of things plugged into the system affect hum?
Yes. Each additional piece of mains-powered equipment in a connected system adds a potential ground loop. Hum that only appears when a specific device is connected usually means that device is creating a loop with the rest of the system. A ground loop isolator on the cable from that device is the standard fix.