You connect speakers, interface and computer, and a low, continuous hum comes out of the speakers even with everything stopped. The volume rises with the gain, and sometimes it changes when you touch the laptop or plug in the charger. It is one of the most common problems in home studios, and it almost always has a specific cause: a ground loop.
Recognizing electrical hum
In Europe the mains supply runs at 50 Hz. Electrical hum therefore appears as a low tone at 50 Hz, often accompanied by its harmonics at 100, 150, 200 Hz and beyond. When the harmonics are strong, the sound becomes buzzier, like a "bzzz".
Other electrical disturbances have different characteristics:
- Clean, low hum: typical of a ground loop or a long unbalanced cable.
- Hum with high-pitched whines that change when the computer is working: interference from the power supply or graphics card.
- Rhythmic ticking: often interference from a mobile phone near cables or microphones.
What a ground loop is
Audio devices share a common reference: ground. When two devices are connected to each other with an audio cable and, at the same time, each is connected to the mains with its own earth conductor, a closed path forms: a loop.
The ground at different points of the system is never at exactly the same potential. The small difference makes a current flow in the audio cable's shield, and that current, at the mains frequency, adds to the signal. The result is hum.
The typical home case is this: a computer plugged into the wall, an interface powered over USB from the computer, and active speakers plugged into another outlet, perhaps through a different power strip.
Isolating the cause step by step
A systematic approach avoids random attempts:
- Turn all gains down and disconnect the interface inputs. If the hum remains on the speakers, the problem lies in the connection between interface and speakers.
- Unplug the laptop charger and run the computer on battery. If the hum disappears, the loop runs through the computer's power supply.
- Plug all devices into the same power strip, which in turn goes into a single outlet. This alone often greatly reduces potential differences.
- Disconnect one device at a time (external monitor, network cable, other computers) to find out which one closes the loop.
- Check the cables: a long unbalanced cable between interface and speakers is a frequent suspect.
The proper solutions
Balanced cables
A balanced connection uses two conductors carrying the signal in opposite polarity, plus a shield. At the receiving end, the input subtracts the two conductors: the signal adds up, while interference picked up equally by both cancels out. If your interface and speakers have balanced inputs and outputs, use XLR or TRS jack cables instead of TS jacks or RCA cables.
One outlet for everything
Grouping the audio system on a single good-quality outlet or power strip reduces the differences between grounds.
Ground isolators
There are transformer-based audio isolators to insert on the connection between interface and speakers: they break the direct electrical connection while letting the signal through. For interference arriving over USB there are also USB isolators, useful when the problem originates in the computer.
What never to do
Never remove or break a device's earth connection, for example with adapters that bypass the earth pin. The hum disappears, but you eliminate a fundamental safety protection against electric shock. Proper solutions act on the audio signal path, not on the system's earth.
What if the hum is already recorded?
If the hum has made it into the recording, a de-hum plug-in can remove the 50 Hz fundamental and its harmonics with very narrow filters. It works well when the hum is stable; the voice loses little, because the filters act on extremely thin bands. It is still a correction: the priority is to remove the cause before the next session.
In my work, when hum shows up only with the laptop charging, the first test is always the simplest: record on battery. If the hum disappears, the diagnosis takes a minute.
In short
- Electrical hum in Europe is at 50 Hz, often with multiple harmonics.
- A ground loop forms when two interconnected devices sit at different ground potentials.
- Work by elimination: battery, a single power strip, one device at a time.
- Balanced cables and transformer isolators are the proper solutions.
- Never break the earth connection of your equipment.
