Mains interference

When I plug my lapop into my hifi its fine, except when its on charge and then I get alot of interference. I was wondering if this gives an indication of the amount of noise coming through my mains system?

 

just to add....the noise comes through regardless of the volume knob. This is out of the headphone jack into the back of the amp by the way.

 

Its a ground loop problem. Cut the ground wire at one end of the cable, or put in a 2.7 ohm resistor (or near value).

 

Oh ok. I'm no electrician but I guess it makes sense that there's a loop formed when I plug it into charge which causes the problem. I don't think I'll start snipping wires as its not really a problem but good to know whats caused it. Cheers.

 

As this happens when on charge, it could well be 'noise' from an SMPS charger.

 

Quite likely but it shouldn't get through unless there is a ground loop issue.

Don't you have an expendable cable you can snip the earth on and try it out?

 

Well, its not really a problem as I rarely plug it into my hifi anyway. I only really mentioned it because a friend had told me that he had bought himself a new multisocket thing with mains screening on it and it had really improved the sound of his system. I wondered if it indicated that there was alot of noise coming from my mains supply as maybe the laptop didn't filter it like an amp might. As you can tell...I don't know the technical side very well!

...plus I'm not sure how to go about cutting the ground wire at one end of the cable Its a headphone jack into red/white plugs at the hifi end. Which wire would I cut?

 

If you can open the phono plugs then you need to cut the wire going to the outer part of the plug so only the center pin is connected.

The reason you get the noise is because the mess coming out the laptop power supply doesn't know whether to go to ground via the connection to the amp or directly through the mains cable.