Many laptops are pretty well known for having terrible audio interfaces. There are usually noise and grounding issues.
A recent post Yet Another Dell Laptop Audio Grounding Problem blog posting documents a well known and horrendous audio problem found on many Dell laptops. I have faced similar noise problems also on other laptops as well.
Starting from Dell laptops. I did some analysis on one DELL INSPIRON laptop that used 3 pin power connector on the laptop. First I measured the resistance between laptop metal parts and mains lead ground and found out that the ground is pretty well connected to laptop case. The strange thing happened when I had the laptop running: I measured a around 200 mV DC voltage between mains ground and computer case! And that voltage difference can cause up to several hundred milliamperes of pretty noisy current through the audio cables that are connected to grounded audio equipment! For more details read my Dell laptop ground loop problems posting.
I think that that this kind of grounded low voltage side is pretty common on laptop power supplied.
Here is the way the laptop power supply grounding should have been built to avoid the problem:
In this configuration there is a separate wires on the cable to carry mains ground reference to laptop and to carry the DC power – side. This avoids the voltage losses on the DC- side to show on the ground potential. In ideal configuration there should be separate pins on the power supply connector for carrying DC- and ground, those then are connected together inside laptop in suitable location.
If the laptop power supply would have been built in this way, the noise performance should be pretty much comparable to a traditional grounded PC.
The reason why the laptop power supplies is most probably that fact that using my better grounding practice would make the power cable from PSU to laptop somewhat thicker and more complicated (expensive). There are also issued that more expensive power connector might be needed (that extra pin). There could also be some electrical safety and EMC issues that needs to be taken into account in this apprioach compared to the original grounded output configuration.