A constant hum can be heard that cannot be influenced by the volume control. It remains audible even with the volume control turned all the way down.
On a television set or monitor, ground loops can cause wave-shaped picture interference.
Cause: An audio/video system must be regarded as a closed system that may be grounded at only a single point.
A ground connection is established, for example, via the protective earth contact of a grounded mains plug. If an antenna connection is now made, e.g. via a cable TV outlet, the system is already grounded at two points. Between these — often widely separated — ground points, a voltage difference can develop, which is then heard as hum. Even when no antenna is connected, hum loops can occur if some of the devices are connected to one electrical circuit but the rest of the devices to another.
Remedy:
A hum loop can only be eliminated by interrupting one of the two ground connections or by providing a common ground.
The simplest way is to interrupt the ground connection in the antenna cable. For this purpose there are so-called sheath current filters or ground isolators, which let the antenna signal pass unhindered but effectively interrupt the ground connection. Of course, a sheath current filter must be used for each antenna line (radio and TV). By simply turning them, the devices fit universally for radio and TV.
If the cause of the hum loop lies in the devices being connected to different electrical circuits, the problem can only be solved by connecting the devices to one circuit.
If this is impossible for aesthetic reasons, e.g. when operating high-quality active speakers, your electrician can easily provide a remedy by rewiring the corresponding outlets.
Will a sheath current filter help? A simple test:
Please disconnect all antenna connections from the devices. If the hum disappears, you can easily solve the problem with sheath current filters.
If the hum remains, you must proceed systematically.
Remove all connections from the amplifier (or receiver) except for the loudspeakers. Now no hum should be audible. Then reconnect all cables one by one and check for hum (please turn the amplifier down or switch it off while connecting!)
