Ground loops are usually perceived as a hum or buzz in your audio signal. They generally emerge when an electrical system consisting of two or more electrical devices is connected to the electrical ground in multiple ways. If the ground of the different power sources does not have the same reference level, an equalizing current will run through the audio connection and intersperse to the audio signal.
This can also be caused by devices that introduce an electric potential to the ground connection, including peripheral devices connected to your computer. On Laptops low quality power supplies often lead to ground loop noise.
With this in mind, the following points should help to eliminate the ground loop:
Shared Power Outlet
Always connect your devices to the same power outlet (e.g. with a multi-outlet power strip). Note that 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 physically connected to each other should share the same power outlet. You may also have to separate polarized plugs (two-prong) from grounded plugs (three prong), by connecting them to separate circuits.
Disconnecting your devices
In order to find out where the ground loop arises, disconnect all devices from your computer and/or mixer, but not the amplifier and/or speakers. This also applies to other peripheral devices you may have connected to your computer (external data storage, CD burners). To check if the amplifier or active monitors create the ground loop, monitor your audio signal from the headphones output of your audio device (while the amplifier/active monitors are disconnected). If you find the device that introduces the ground loop, a ground lift of the audio connection may solve the issue. Never lift the ground of the power connection!
Note: Contact the manufacturer of your device before lifting the ground to find out if ground lifting is supported on your device. Some Devices offer a switch for this, with others a modified audio cable or a DI-Box (see below) needs to be used.
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.
If you cannot fix the ground loop with the tips above, you can try to put DI boxes in between the connections of your audio devices, your mixer and/or your active loudspeakers. This will break the ground of the audio connection. With a DI box you can use balanced audio cables for unbalanced audio sources. Additionally, most of these Di boxes feature a Ground Lift switch.