laptop to P.A. Amp PULSES

[sam duncan]

WHEN I CONNECT MY LAPTOP TO A P.A. SYSTEM TOMPLAY MUSIC WHEN EACH TRACK ENDS I GET A PULSING SOUND COMING THROUGH THE P.A. HOW CAN I STOP THIS. I HAVE TRIED TWO LAPTOPS AND TWO P.A. SYATEMS BUT THE SAME PROBLEM EXISTS

[Tomi Engdahl]

What kind of laptops have you tried (brand, model)?
What kind of PA system did you use?
How have you wired your laptop to the PA system?
Do you have any wireless devices nearby (cellular phone, internal wireless data card, WLAN in use etc..) that could cause interference?
Is there some network connection (Ethernet cable) from laptop being used?
What accessories do you have connected to that laptop?

Here are some tests which could help to resolve the problem if you can provide answers to them:

Does the pulsating noise happen when the laptop is powered with the mains adapter?
Does the noise happen when the laptop is powered from the internal batteries (mains adapter disconnected from laptop and mains power)?
Is that noise still there when the power from laptop is off (mains adapter still connected?