Audio signal grounding in audio equipments
Grounding in consumer audio is somewhat fragile because there is no overall plan. Each piece of equipment takes a slightly different approach. As long as there are only a few units involved, there are few if any problems.
Once the system grows beyond a certain point the lack of a grounding plan catches up with the consumer and there is hum or worse. The exact point where trouble begins depends on the specific units involved and the local power and cable TV system wiring. Ideally, you want the audio system grounded at a single point.
The grounds of the input and output connectors in typical audio equipment are typically wired to common ground in the equipment. That common ground is typically connected to equipment case.
If the equipment has a grounded power connector, then the case ground is connected to mains grounding connector. If the equipment does not have the gounded power connector, that ground is not connected to mains ground (case is floating).
That is tyhe typical case in home audio equipments and also in many professional/semiprofessional equipments. Many commercial amps uses balanced inputs for hum reduction, but that is not available in home audio equipments.
With home equipments the grounding situation gets worse for audio, when equipments with grounded plugs are added to the system - as the 'ground' potential can vary through various circuits in your home. This differing 'ground' level will be impressed on the low-level signals as common-mode hum.
Usually the the real trouble maker in your system is the connection to the cable TV system. The antenna connector shield is grounded typically in the equipment to same point as all the audio inputs (equipment case), but the other end of antenna cable can be grounded to the common ground with the mains ground somewhere quite far away. When there are multiple ground paths they are virtually never at the same potential and therefore some current will flow through your equipment grounds often causing the hum you are experiencing.
Tomi Engdahl <Tomi.Engdahl@iki.fi>