***Health & Safety Warning and Disclaimer***
Before you try this, I must issue a Health and Safety warning on the electrical front, and also a disclaimer that I can accept no responsibility whatsoever if you try this and it a) borks your equipment, b) kills you or anyone else.
PLEASE NOTE THAT ALL SOLUTIONS MENTIONED HERE ONLY APPLY TO LAPTOP PLUG-INS AND NOT TO DESKTOP COMPUTERS WHICH REQUIRE 3-PRONG PLUGS.
If you bypass the grounding system of a desktop computer, you will put human lives at risk of fatal electrocution.
That said, you should bear in mind that if you have a 3 pin power plug (UK) and you go abroad to the continent or the USA, you would use an adapter to remove the earth connection, and all your power supply and laptop are double insulated for your safety. many DJs have found the need to do something along the lines listed below to get rid of interference when using laptops
You have a laptop with a separate power supply, you live in the UK or a location that provides you with a three pin power plug for connecting to the mains. You want to output the sound from your laptop via the headphone socket to an amplified source.
If you try this you will most likely get a big pile of sound interference from the power supply and laptop. This is caused by earth loop from the power supply. If you unplug the power supply from the laptop you will find the noise goes away. But if you need to keep the power on what do you do?
1. Remove the earth pin from your power plug
2. Remove the earth pin/connector from the other end of your power cable
3. Paint or cover the earth pin with paint or nail varnish or tape
4. Find a two pin connector lead to go in the back of your power supply
5. Use the alternative power lead for continental use that may have been supplied with your laptop with an adapter
6. You can try fitting additional filters to the power supply lead
7. You can try putting your power supply in a metal box or cage that is earthed
8. If you can, turn the plug around by 180 degrees so you only plug in the two pins (not really recommended but it did work for some)
9. Try moving the power brick away from the laptop
8. And the preferred easy and cheap (@ £4) option, go and buy two adapters from your local electrical retailer. One for plugging the three pin power plug to a two pin for continental use, then you plug that into the second adapter for a continental appliance for use in the UK (or a shaving adapter) which has a three pin plug (and usually the third pin is covered in a plastic covering to prevent electrical contact) to plug into the mains.
For additional protection against electrical spikes, use a surge protection device between the power supply and the mains. Many extension four way type leads have this built in.
Now try outputting sound, and you will find the interference has gone.
Note: very often you will find that power outlets (especially on extensions or 4/6 ways) have a protector in place that is deactivated by the earth pin, so you have to have an earth pin to plug in.
Again, you try this at your own risk, and I cannot accept any responsibility for the outcomes.