At home, I have the following setup:


(The schema is in Norwegian, let me translate for you:
Forsterker = Amplifier
Vegguttak for antenne = Wall outlet for antenna
ikke jordet = not connected to earth
jordet = connected to earth)


Amplifier and VCR is not connected to earth, I guess that's usual? VCR is connected to amplifier by regular audio cables. Antenna (Cable-TV) is connceted to the VCR.
PC is connected to earth. PC is connected to amplifier with regular audio cables.

With this setup there is a humming-sound in the speakers. Louder the more I turn up the volume. If I disconnect the antenna from the VCR, the humming stops.
If I disconnect the PC, and reconnect the antenna, there's still no humming.

It seems like the combination of both PC and antenna connected makes the humming noise. (Of course, if I disconnect the VCR from the amplifier, the humming stops, because the antenna is no longer connected to the amplifier. But if I instead connect the antenna to the TV, which is connected to my DVD-player by SCART, which in turn is connected to the amplifier by audiocables, it hums again. So there's nothing wrong with the VCR.)

Amplifier and VCR is connected to the same power outlet. The computer is connected to a power outlet in the opposite corner of the room. Both are on the same circuit in the fuse-cabinet.

Has anyone got an idea what's the cause of this, and how I may be able to solve it?

Sounds very like an earth loop.

Standard fix is to connect a piece of wire from a screw on the back of the computer to a screw on the back of the amplifier. Wire stripped from an old piece of mains cable is suitable, your amplifier may alredy be fitted with an earthing terminal.

UJ

There's a ground loop loop in your system caused by different ground potentials of the antenna and the main electrical network.

A quick fix would be getting an antenna isolator. That device goes between the antenna and the VCR. It's basically a 1:1 transformer which cuts off the galvanic connection between the antenna and the VCR. There's a problem however: the isolator is extremely sensitive to external noise and significantly lowers the signal level thus causing noticable quality loss.

A more permanent and high quality fix would be complaining to your TV provider until they fix the potential difference issue. That'll probably never happen though.

That's good advice but it's the VCR that I would ground rather than the computer, although having the whole system at the same potential wouldn't hurt.

Thanks for the advices.

Instead of connecting the amplifier ground to the PC chassis, I connected it to the wall outlet (I made a grounding wire of a cable. I isolated the blue and brown wires of course, with some tape).

This solved the problem. I fail to see why...now I have three units that leads to ground, instead of two.

Can someone explain why grounding the amplifier solved the problem?

A quick search on Google for "ground loops" will offer many explanations.

To sum up, your various pieces of equipment were at different potentials (i.e. their references to ground varied). An eletrical signal travelled through your audio cables to make up for this difference in the form of a low voltage AC sigal (either 50 or 60 Hz). Although it's low voltage as far as power supplies are concerned, to an audio pre-amp it can exhibit itself as a noticeable hum. By grounding your other piece of equipment you've eliminated this potential difference.

You wouldn't experience this problem with just consumer audio equipment because none of them are grounded. Adding a PC is the first ground, and probably still OK. Adding a second ground like a TV antenna, which should be earth grounded, can create a ground loop. I experienced this same problem when connecting my PC and cable box to my amplifier.

Because it removed the ground current from where it was causing trouble.

The hum was caused by current flowing along the cable that connected the PC to the amplifier. The potential difference was caused by the antenna connection. What happened when you earthed the amplifier was that the current flowed straight to ground instead of through the PC on it's way to ground. Basically you shortened the path from:
Antenna->VCR->Amplifier->PC->Ground
to:
Antenna->VCR->Amplifier Casing->Ground
Which stopped the current having an effect on both PC and amplifier.

Thanks

By the way, after I grounded the amplifier, it now hums if I choose another channel than the channel that's connected to the PC.
Also, if I disconnect the PC, the other channels hum even more.
They don't hum at all if I remove the amplifier ground.

I have the same problem, for me it's the radio cable antenna.
So, assuming you don't want to record the TV's or VCR's audio directly to harddisc, you can either
-disconnect the TV antenna
-or disconnect all electrical connections between PC and Hifi/video equipment
-or disconnect computer and power supply system
For me it's mostly the second choice. Interesting: The electrical digital out of the audio CD player doesn't matter, I can leave it connected. It seems to have something like the transformer niktheblak mentioned.

Had hums in my competely non-grounded system until I connected a TV card, that now grounds pretty much everything through the coaxial (TV) cable. Are there any risks just having the TV cable ground as groundpoint?

can it hurt to gruond every piece of equipment ?

If all your components stand next to each other, then it can't hurt.
But if there are long distances (=long cables) you could get hum through
a 'ground loop'.
If you have long cables, they behave like an Antenna and Receive a 50Hz/60Hz-signal (depending on your country you're living) from all the powercords witch are around you (and also them in your walls and other rooms etc.)
The powercables around you behaves like a huge radio-sender with a frequency of 50/60Hz.

Professional audio equipment have symmetric inputs. That means, that the signal is not relativ to ground. There is a positiv signal and a negative signal input. If there comes a hum into the signals, the hum is the same on the negative and the positive signal. The input circuit of these components substracts the negative signal from the positiv signal and the result is twice the signal without hum:
(+signal + hum) - (-signal + hum) = 2*signal

There is another elegant (and high-end) way to connect your computer to your audio-equipment. If you have an amplifier with optical digital audio input or you
have a sepparated D/A-Converter connected to your audio-equipment you can connect your computer (with optical audio-output) with an optical S/P-DIF-cable
to your audio equipment. Beside this nice solution you get higher quality, cause
the analoge audio output from the computer is full of noise and other crap.
Also good sound cards deliver average sound quality on the analoge output.

Thanx for the reply

my Equipments i right next to each other. and im'using the sameground cable.