Bad hum when playing computer audio files thru receiver

[DavidIS]

I am trying to play audio files on laptop computer in my den to a receiver in living room, which is powering in-wall speakers used for home theater. I have in wall wiring by standard audio cable with RCA connectors on both ends - the run is about 40 ft total from den to living room. When I connect short 3 ft cable from laptop to the audio cable in den wall, everything works fine. Problem is I want to use laptop on desk in den that is about 20 ft. on the other side of the room from where the audio cable goes into the wall. When I extend the audio cable connection using a 25 ft audio cable from the wall to the desk where the laptop is located, I get a persistent hum and other noise through the living room speakers whenever the computer (and receiver) is on. When I play the audio files, the sound is muffled. Since all works fine when I use a short audio connection from laptop to wall connection, I presume the extra length is the problem. Any suggestions on what I can do? (Too much of a hassle to move laptop to other side of room and connect to audio cable each time). Thanx for any help.

[Whir]

You could try shielded cables.

40ft is a bit of a run as it is for non twisted pair signal cable. I'd never try to run analog audio over that kind of distance. Adding another 25' of unshielded cable definitely isn't going to do it any good.

[MD1032]

Sounds like a grounding problem. Make sure the connection to both the receiver and laptop from that cable is clean (alcohol the contacts with q-tips). Sounds like the cable is bad or isn't sheilded like the above poster mentioned.

[MaxVal]

You might try running a three pin power extension cord from the power strip of the home theater into the room where you want the laptop, and powering the laptop from it.
If the hum is reduced substantially, the trouble is caused by a "ground loop".
These occur when equipment is located in seperate areas powered from different circuits. Many times there are differences in ground potential (that is if you could measure the voltage between the ground pins of the two circuits there would be a measurable voltage difference between them.)
This difference in voltage appears across the shield wire (ground) running (in this case) between the laptop and the amp. The difference in ground voltage becomes part of the signal running from the laptop to the amp. Small voltage, small hum, large voltage...

If a ground loop is confirmed, you might try an adaptor to lift the ground from the laptop's power brick.
MAX

[RobPC]

Search on Amazon for ground loop isolator - you will find devices there you can insert in your signal path (via phono plugs) to remove the ground hum - price is about 5 pounds uk.

[ShyguyXPC]

though helpful... look at the date of this thread