Interfacing a TV's headphone's output to an amplifier's line-in inputs

Thread Starter

cmartinez

Joined Jan 17, 2007
7,083
So I bought an LG 50" LED TV, and I wanted to output its audio to a cheap-o brand amplifier to have quality sound.

  • The TV has two audio outputs, one is an optical output, and the other one is for headphones.
  • My cheap-o brand amplifier does not have a remote, but the TV does.
  • The TV's optical output volume cannot be controlled through the TV remote, but the headphone's output volume can.

So what I did is I connected the TV headphone's output directly into the amplifier's RCA line-in inputs... and it works... but with an ugly background hum ... it's a low level hum, but it's easily perceived, and I'd like to get rid of it.

Concerning sound systems, I ain't no over the top, demanding audiofool, but I don't like a hummy, dirty sound coming out of my amplifier either.

I'm guessing that a proper interface circuit for this sort of thing is extremely easy to do, but I have no experience in the matter. Perhaps there's a already a commercial product out there for this sort of thing, but I'm no aware of it.

Any suggestions?
 

#12

Joined Nov 30, 2010
18,210
Had this problem w/ my TV. Radio Shack sold a stereo audio isolator transformer. Fixed it right up.
Comes with RCA connectors.
 

crutschow

Joined Mar 14, 2008
25,419
It may be a ground loop and you could use audio transformers such as this to provide isolation of the two signals and break up the loop.
If the amp has a safety ground on the power plug, you could test to see if that would help by using a cheater plug to temporarily remove the ground.

Edit: #12 beat me by 10 minutes

Otherwise, do you have plenty of volume adjustment, i.e. for a good volume level, is the TV audio set to a relatively low level?
If so then you may be able to add an attenuator to the signal, which would lower the hum level. You just turn up the TV volume to make up the difference.
 

Thread Starter

cmartinez

Joined Jan 17, 2007
7,083
It may be a ground loop and you could use audio transformers such as this to provide isolation of the two signals and break up the loop.
If the amp has a safety ground on the power plug, you could test to see if that would help by using a cheater plug to temporarily remove the ground.

Otherwise, do you have plenty of volume adjustment, i.e. for a good volume level, is the TV audio set to a relatively low level?
If so then you may be able to add an attenuator to the signal, which would lower the hum level. You just turn up the TV volume to make up the difference.
Thanks, I was jut looking at audio transformers at Amazon, when I found this ... which only costs a hundred bucks ... :confused:
Your $10.00 dlls option sounds far more appealing :D
 

#12

Joined Nov 30, 2010
18,210
Another method: I put a 10 ohm, 10W resistor in series with each speaker and connected the speaker driver output to the isolator transformer.
Why? because all the external audio output jacks had no volume control. The speakers have volume control, so I hooked them up to the isolator. Besides, the speaker output has higher voltage and lower impedance than a headphone output.
 

Thread Starter

cmartinez

Joined Jan 17, 2007
7,083
It may be a ground loop and you could use audio transformers such as this to provide isolation of the two signals and break up the loop.
If the amp has a safety ground on the power plug, you could test to see if that would help by using a cheater plug to temporarily remove the ground.

Edit: #12 beat me by 10 minutes

Otherwise, do you have plenty of volume adjustment, i.e. for a good volume level, is the TV audio set to a relatively low level?
If so then you may be able to add an attenuator to the signal, which would lower the hum level. You just turn up the TV volume to make up the difference.
Question, would this also work?

EDIT: This other option looks promising too.
 

#12

Joined Nov 30, 2010
18,210
Two things I doubt. A car type noise filter might be about alternator whine. Not what you want in a home stereo.
2) Adding an attenuator then turning up the gain. Signal to noise ratio is not improved by this method.

No use having a 120 db SPL stereo system if you can't plug into it!:D
 

Thread Starter

cmartinez

Joined Jan 17, 2007
7,083
Two things I doubt. A car type noise filter might be about alternator whine. Not what you want in a home stereo.
2) Adding an attenuator then turning up the gain. Signal to noise ratio is not improved by this method.

No use having a 120 db SPL stereo system if you can't plug into it!:D
Hey, I remember now. As you've just said, those things are designed to filter noise caused by a car's electrical system, and have nothing to do with isolation transformers... thanks for the clarification.
 

Zofz

Joined Aug 29, 2016
2
If the TV is not grounded by mains socket, and the amp is, make the test and unplug the antenna cable.
If that was removing the hum, you can use HF ground loop remover - usually much cheaper device.
Alternatively, you can first route the antenna cable thru the amp case, where the opened cable shield contact the amp ground, and then to the TV.
 

#12

Joined Nov 30, 2010
18,210
I have no objection to looking for ground loops, but my TV was born defective. I actually phoned Sanyo and they told me the problem was in the design of the TV, so I had no choice but to use an isolation transformer. Try looking for ground loops, but don't get crazy if that isn't the cure.

The hum level increases if I increase the amplifier's volume, but not if I increase the TV's volume.
That seems to argue for the highest voltage you can get out of the TV because the signal to noise ratio is cleaner there.
I know I crossed swords with crutschow, but I can't imagine the scenario to which he is referring. The one where attenuation followed by more gain fixes this. With this new information about the worst hum being in the amplifier, that seems to nail down the right procedure.
 

Thread Starter

cmartinez

Joined Jan 17, 2007
7,083
With this new information about the worst hum being in the amplifier, that seems to nail down the right procedure.
Yup... also, the amplifier has no hum until I plug the TV to its inputs. I think the audio transformer's oughta fix the problem.
It's only a $10.00 dlls bet anyway.
 

GopherT

Joined Nov 23, 2012
8,012
The audio amplifiers (Even the headphones) output of most modern TVs are full-bridge Class D amplifiers so neither pin is referenced to ground. Test with a continuity meter - you will likely see no conductivity to ground on either pin when the TV is off (or very high impedance).

If it is a full bridge class D driver, you are taking your headphone output and grounding one pin to your old receiver ground and using the other as a signal. This will be a problem. Mostly from the short you are creating from the class D amplifier to ground on the old amplifier. I think that is creating a buzz. An isolation transformer is in order.
 

Thread Starter

cmartinez

Joined Jan 17, 2007
7,083
The audio amplifiers (Even the headphones) output of most modern TVs are full-bridge Class D amplifiers so neither pin is referenced to ground. Test with a continuity meter - you will likely see no conductivity to ground on either pin when the TV is off (or very high impedance).

If it is a full bridge class D driver, you are taking your headphone output and grounding one pin to your old receiver ground and using the other as a signal. This will be a problem. Mostly from the short you are creating from the class D amplifier to ground on the old amplifier. I think that is creating a buzz. An isolation transformer is in order.
Thanks, Gopher... you've just earned the "Best Explanation Yet" award :)

I'm thinking about buying either this, or this.

Question, the specs on the second option state that:
  • Female 3.5-mm input
  • Male 3.5-mm output
Is that important? I'm guessing that, since it's a 1:1 isolation transformer, either side could be used as an input or output, whatever is preferred. Am I right?
 

GopherT

Joined Nov 23, 2012
8,012
Thanks, Gopher... you've just earned the "Best Explanation Yet" award :)

I'm thinking about buying either this, or this.

Question, the specs on the second option state that:
  • Female 3.5-mm input
  • Male 3.5-mm output
Is that important? I'm guessing that, since it's a 1:1 isolation transformer, either side could be used as an input or output, whatever is preferred. Am I right?
I think it will work.
 

Thread Starter

cmartinez

Joined Jan 17, 2007
7,083
Right or wrong, you're going to have to change one connector...right?
Home stereos don't have male connectors on their inputs.
No they don't, but I already have a male 3.5mm stereo jack connector to two RCA male connectors adapter. That's how the TV's hooked to the amplifier.

41JGQR4TJML._SX355_.jpe
 

wayneh

Joined Sep 9, 2010
16,399
Just to expand your thinking - you may want to consider taking this as the time to go to a full home theater setup. I've got an 8 channel system and it's nothing short of awesome! A 6 channel system (or 5.1, as they designate it with the subwoofer being the 0.1) would take full advantage of anything you get out of the TV tuner. The extra 2 channels are relevant to Blu-ray content.

A modest A/V receiver that can handle the channels, signal decoding and such is not very expensive these days. Then it comes down to speakers. That's where the money goes, but in my case I had a number of speakers on hand. I got new ones for the fronts, where dialog clarity is important.
 
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