So we've recently started watching missed TV programs via the internet. From my laptop, I'm running an audio cable from the headphone jack to my pre-pro and an S-video cable to a video input on the TV. I'm getting a fairly loud hum in the HT system that comes and goes when I plug in or unplug the video cable. With the video cable plugged in and the pre-pro on, plugging in or unplugging the audio cable has no effect on the hum.
Any recommendations?

Hi a401,

Just to verify the ground loop, try unplugging the AC/DC charging adapter from your laptop and run the laptop on the internal battery. I suspect the hum will go away.

The ground loop is likely a voltage differential between your incoming cable-TV box (do you have one?) or satellite receiver.

Try connecting the video cable from the laptop to one of the video inputs on the pre-pro. If the hum goes away, great. If not, unplug the incoming cable to your cable TV box. If the hum goes away, then you may have to install a ground isolator on the incoming cable to the cable-TV box such as this:

http://axiomaudio.com/groundisolator.html

Note: this can't be used on a satellite receiver, only regular cable-TV.

What is the AC/DC adapter for your laptop plugged into? Try connecting it to the same power strip or circuit that supplies the TV power.

Some combination of these should work.

Regards,
Alan

Hi Alan,

I'll have to try unplugging the AC adapter, which is plugged into a different wall outlet than the rest of my AV stuff. Or plug it into same one... I should be able to get 45 minutes out of my battery.

I have that ground-loop isolator on my TV cable already - no box - and it works great.

Thanks for the update.

Since you have the ground-loop isolator on your incoming TV cable, then I'm sure the loop is happening through the AC/DC adaptor to your laptop and the rest of your gear. Running the laptop on the battery with the adaptor disconnected will confirm that if the hum disappears.

Then you can experiment with different AC plugs and your equipment.

Good luck!

Finally had a chance to play a bit... Unplugging the power adapter stops the hum, but there's no where I can plug it in without humming again. Oh well, the battery should last about an hour...

Scott, presumably using a "cheater" plug for the power adapter would stop the hum. If I followed your connections, everything would still be grounded through other connections, although the apparent ground loop coming in through the power adapter would be cut off.

That did it! Thanks.

You're welcome, Scott.

Edit- Wait.......what?