Whenever I plug the power cord and the speaker cable in my laptop there's a hum that can be heard in the subwoofer. However, when I take out the power cable, it goes away. It also goes away whenever I plug in a VGA cord into the laptop (with the power cable plugged in as well.)

Why does this happen?

 

Whenever I plug the power cord and the speaker cable in my laptop there's a hum that can be heard in the subwoofer. However, when I take out the power cable, it goes away. It also goes away whenever I plug in a VGA cord into the laptop (with the power cable plugged in as well.)

Why does this happen?

Ok, this is merely a 'shot in the dark'... It seems as the hum is introduced by the PS (60Hz crosstalk 'through' the SMPS...) --- Perhaps capacitance in the VGA cable (or VGA display if connected) filters off or cancels said noise...

Responses to the following would be helpful:

1) Is the hum present when the the power cable is attached to the laptop but disconnected from the mains?
2) Is the VGA cable 'open'? -OR- Is 'the other end' connected to a VGA device?

Best regards
HP