Ground loop hum: how to find it and fix it safely

How a ground loop forms

The earth pins in a house are not all at the same voltage. Current in a cable's live and neutral wires induces a small voltage in the earth wire running beside them, and research on building wiring shows this happens even with dedicated "isolated" ground circuits, so two sockets in the same room can sit at slightly different voltages.

Plug a source and an amplifier into two such sockets and join them with an unbalanced interconnect. The cable's shield now links the two earth points, and together with the mains wiring it closes a loop. That small voltage difference drives current around the loop, and the current develops a voltage along the shield's resistance. In an unbalanced connection the shield is also the signal's return path, so the input has no way to tell that voltage from the music.

The loop also has an area, and a mains-frequency magnetic field passing through it induces current directly. That is why routing matters: a cable draped over a power supply or coiled beside a transformer can hum where the same cable run elsewhere is silent.

What hum sounds like, and what it tells you

A steady low tone at the mains frequency, 50 Hz in most of the world and 60 Hz in North America, points at a voltage difference between earths or at magnetic pickup.

A harder buzz, rich in harmonics above that tone, usually means the current in the loop isn't a clean sine wave, which happens when power supplies draw current in brief pulses or leak switching noise to ground. Chargers, computers and televisions on the same circuit are the usual suspects.

Where the hum comes from is the next clue. A hum that appears only when one input is selected has localized itself to that source and its cable. A hum that stays with every interconnect unplugged is not a loop. And a hum you hear from the amplifier's own case, with the speakers quiet, is mechanical: the audiophile tweaks roundup review says that kind of transformer hum is often caused by tiny amounts of DC on the mains, and found the iFi DC Blocker+ made it far quieter.

Finding the loop, step by step

Work through the chain in order, changing one thing at a time. The first change that silences the hum shows you where it lives.

1. Unplug the interconnect at the amplifier's input and leave everything else connected. If the hum stops, the loop runs through that cable and the source behind it. If it carries on, the amplifier, its supply or its speakers are the place to look.
2. Disconnect anything that brings in an outside earth: a cable or satellite TV feed, an aerial, a shielded network cable. A coaxial TV feed in particular can sit at a different potential from your mains earth.
3. Plug every audio device into one socket or one power strip. This isn't about filtering; it gives every device the same earth reference, and on its own it cures many cases.
4. Run the source on battery if you can, a laptop unplugged from its charger or a portable player. If the hum goes, the mains path is involved.
5. Move signal cables away from power cables and transformers, cross them at right angles where they must meet, and keep the loop between two devices' cables small.

Fixes that break the loop safely

A loop needs a conductor in the signal path, so the reliable fixes take that conductor away or make the input ignore it. A transformer-coupled isolator on an analog interconnect passes the music and blocks the ground connection. A USB isolator does the same between a computer and a DAC, a common version of the problem on a desk. An optical link carries no conductor at all, so replacing a coaxial digital run with TOSLINK can't form a loop, and the same holds for network gear: the tweaks roundup review describes the iFi LAN iPurifier Pro turning the network signal into light and back to cross a physical gap.

Balanced connections, where both ends offer them, read the difference between two signal conductors and ignore what they share, so loop current in the shield stays out of the signal. That depends on the equipment terminating the shield at its chassis. When a maker connects it to the circuit's ground inside the box instead, shield current flows through the circuitry and brings the hum back, a design fault known as the pin 1 problem that the AES48 practice exists to prevent.

Some equipment builds in a switch for this. The Monosaudio SP-USB filter has one that connects or disconnects its ground from the chassis, and the Monosaudio AS-EU08 + SP-USB review advises usually leaving it connected but trying both positions, since the better one varies from system to system. Switches like this are designed to separate a signal ground from the chassis inside a device, which is a different thing from removing the mains safety earth.

Never lift the safety earth

Don't remove, tape over or bypass the earth pin on a mains plug, and don't use an adaptor that does it for you. The earth wire is what gives fault current a low-resistance path back to the fuse box or breaker panel if a live wire ever touches a chassis, so the fuse or breaker trips. Lift it and the first fault can leave the metalwork you touch live. It can also send the fault current through the interconnects to the next device, which can overheat a cable enough to start a fire.

Lifting the earth often does stop the hum, which is why the advice keeps circulating. Breaking the loop in the signal path stops it just as well and removes none of the protection.

When the hum isn't a ground loop

If the noise is unchanged with every interconnect unplugged and only the speakers attached, look at the amplifier itself. A failing smoothing capacitor lets through ripple at twice the mains frequency, and in a valve amplifier an aging tube can hum on its own.

If it rises and falls as you move a cable or turn a component, it is usually magnetic pickup, and distance and orientation cure it. If it is a fizz or a whine that changes when a phone or a charger comes near, it is radio-frequency interference reaching an analog stage, and shielding or a different route will do more than any earthing change. Noise that arrives through the mains itself is the subject of the power conditioning page, and the best hi-fi accessories guide ranks the filters and conditioners we've tested.