Powering on my LCD causes my amp to hum through the speakers

[TylerDurden]

I have managed to narrow down the cause of the humming through my speakers to my TV. When it is switched off there is no hum, but when I turn it on there is a loud background hum.

This is a problem because the TV is actually the one and only source in this situation.

Can anyone help me get rid of it?

[Bone]

Youve probably got an earth loop. Disconnect the earth (common) on one end of the connecting signal cables.

[TylerDurden]

Originally Posted by Bone

Youve probably got an earth loop. Disconnect the earth (common) on one end of the connecting signal cables.

Thanks for your input, but I don't really follow what you mean. What should I disconnect and where?

[Mooly]

Just start again... is the TV connected as a signal source to the amp ? Not sure if you meant that or source of the hum

If the first, how is the TV connected. Is it with phono leads ?
An earth loop is where there is more than one ground return usually via mains leads etc.

Is it a hum ? which is a deep and very pure tone, or is it a buzz with harsher rasping sound ?

[TylerDurden]

Originally Posted by Mooly

Just start again... is the TV connected as a signal source to the amp ? Not sure if you meant that or source of the hum

If the first, how is the TV connected. Is it with phono leads ?
An earth loop is where there is more than one ground return usually via mains leads etc.

Is it a hum ? which is a deep and very pure tone, or is it a buzz with harsher rasping sound ?

I have an HTPC and a Freeview PVR attached to my TV via HDMI. The TV then connects to the amp from the TV-Out jack to Phono leads.

If I was to choose I would probably call it a buzz, it isn't a deep tone, but a mid to high harsher sound.

Thanks for your input.

[Mooly]

This probably isn't going to be easy to pin down.
Is this buzz affected by the setting of the volume control or is it present even with the volume on minimum ?

Very difficult without seeing the set up.
I would initially have the amp and TV connected but NOTHING else. That means no aerials... no PC, no nothing , just the audio lead between the two and see if the buzz is still present when the TV is on. Then try connecting items in the chain one at a time.

[TylerDurden]

The buzz is not there when volume is on minimum.

It is there with nothing but the audio leads between the TV and Amp are connected.

[Mooly]

OK. The next thing, and this might sound strange, is too just connect the outer (which is the ground) of the phono leads to the amp but not the signal (which is the middle pin). That's easiest done by unplgging L and R and then just hold the outer part of one or the other so that it touches the metal body of the phoo socket on the amp.

If you try that start with the volume on minimum and see if the hum still there as you turn the volume up.

If not then things get more complicated... it could either be entering as a "valid" signal meaning that the audio from the TV has this buzz on it for some reason... either a fault or "just the way it is".

Or it could be something more complex. It's a process of elimination.

What you could also try... safe to do but be aware of excess noise level, is to disconnect both L and R phono leads and connect a pair of headphones (just hold the plug across the phone plugs to connect across any of the 3 connections on a h/phone socket) and that will give audio in one or both earphones depending how you connect but it should let you see if the buzz is there. TBH that might be the quickest way to see if the audio is "clean" from the TV.

[Mooly]

That should read h/phone plug above

Just confirm what you said before.
"The buzz is not there when volume is on minimum". Is that with or without leads connected.

[benb]

Is the TV connected to a cable TV connection? If so, disconnect that and see if the hum stops. There's a simple RF isolation transformer you can add between the TV and the cable. I understand the cable TV connection is the cause of hum in many or most cases involving audio and TV's.