Hardware & signal path
Ground loop
A ground loop is a hum caused by two pieces of connected equipment being earthed at different points, so current flows through the audio cable's shield. It sounds like a steady low buzz at mains frequency.
In practice
A steady hum that appears only when two devices are connected is a ground loop, not a faulty cable.
Find it by disconnecting rather than by replacing. Unplug audio connections one at a time until the hum stops; the last one you removed is in the loop. Then fix it properly: put everything on the same power circuit, use balanced connections where the equipment offers them, and attach the turntable ground wire to the mixer post — a turntable without its earth lead is the single most common cause in a booth. Never defeat a mains earth pin to solve it. That does stop the hum, and it removes the protection the earth is there for.
50 or 60 Hz
What you are hearing
One at a time
How to find it
Never lift mains earth
The one rule
Where these come from: Electrical convention and mains frequency, not catalog figures. For what we can measure, see the connection that resists it.
Why it matters
It is the most common noise problem in a booth and the one most often mistaken for a broken cable. The hum appears when two devices are connected, disappears when either is unplugged, and nothing about either device is faulty — the loop is the fault.
It matters more than it sounds because it is constant. A steady hum sits underneath everything, and a room will feel tired without anybody identifying why; on a recording of the set it is unmistakable and cannot be removed afterwards without taking the bass with it.
Frequently asked
How do you get rid of a ground loop hum?+
Put everything on one power circuit, use balanced connections where you can, and connect the turntable ground wire to the mixer. Never remove a mains earth pin.
Why does my turntable hum?+
Most often the ground wire is not attached to the mixer post. That thin separate lead is not optional — it is the return path the cartridge signal needs.
What frequency is mains hum?+
50 Hz in the UK and Europe, 60 Hz in North America, usually with harmonics above it. It sits right in the low end, which is why it is felt as much as heard.
Related terms
Balanced and unbalanced
A balanced connection carries the signal twice, once inverted, and cancels anything picked up along the way. An unbalanced one carries it a single time, so interference picked up by the cable stays in the signal.
RCA and XLR
RCA is the unbalanced two-channel connector on players, turntables and consumer gear. XLR is the balanced three-pin connector used for long runs and microphones, and it locks into place.
Line level
Line level is the standard signal strength equipment sends between devices — a player into a mixer, a mixer into an amplifier. It is far stronger than a microphone or turntable signal, which need preamplifying first.
Booth monitor
The booth monitor is the speaker pointed at the DJ, fed from its own output with its own level control. It is separate from the master, so turning it up changes what you hear and nothing the room hears.
Explore: Mixing guides · Sub bass · Booth monitor · All DJ terms