Many DJs use laptops and hard-drives as part of their set but when using these it may cause their sound system to hum. Even controlled power packs of lighting and video projection can also cause a hum if you tend to hook up these systems to your main sound system.

In computer and audio equipment, tiny currents and voltages can cause noise in the circuits and hamper operation. If you have a buzzing or humming problem on your speaker outputs, it's caused by what is known as a ground loop. This is where two pieces of equipment are operating; usually using different outlets or they have no true earth. The hum is caused by the mains power trying to balance using the only path it has, the earth line. As it cannot equalise it creates a loop, hence "ground loop interference".

There is many suggested ways of solving this problem, but the most simple and safest solution for a DJ is to use what are known as ground loop isolators. These devices look like a little plastic case and have inputs and outputs that are designed to reduce electrical noise, and prevent unwanted hum. Simply plug them in between equipment outputs and mixer inputs. You will need to buy one of these for each stereo channel with a hum, and going into the mixer.

A low quality power supply unit can lead to ground loop noise, especially on laptops. To solve this issue the user will have to disconnect the laptop from the power supply so that it runs on DC power, or use a ground loop isolator. 

If you are going to use a replacement power supply unit, make sure that its specifications meet the ones required by your computer in order to avoid permanent damage.

Always connect your devices to the same power outlet. Even if the power supply of your laptop computer is not connected, your setup may still be vulnerable to ground noise from an external device with its own power supply. All devices that are connected to each other should share the same power outlet.

To see where the ground loop surfaces, disconnect all your devices from the computer and mixer, but not the amplifier or speakers. This also applies to surrounding devices you may have connected to your computer such as external data storage. To check the amplifier or active monitors, create the ground loop, monitor your audio signal from the headphones output of your audio device .If you find the device that introduces the ground loop then an isolator may solve this issue. Never lift the ground of the power connection, maintaining a good safety ground is always essential to prevent electrical shock.

If possible, always use balanced audio cables to connect your audio device to your mixer or speakers. If your mixer provides both unbalanced and balanced inputs, always connect your balanced sources to the respective inputs on the mixer.