Audio hum

[Danj]

I've had my TV's audio output connected to my computer's line input for a while now, so that I can listen to the TV and hear any notification sounds from my computer at the same time. Recently, however, the aforementioned output has gained a nasty humming noise on it. My dad thinks it's some sort of earthing issue, so I tried several things:

<ul type="square"> Plugging headphones directly into TV - no hum, but then I can't hear the computer (obviously)
Rearranging power connections so that the TV draws its power from the UPS, which the computer is also plugged into and therefore should have a common earth - still hums
Swapping over the computer and monitor power cable - still hums
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This basically seems to leave the computer's PSU, which did sort-of fail recently - the fan stopped going round, and it was overheating, but after replacing the fan it "appears" to be working OK.

My question is, is it likely that my dad is correct and it is an earthing issue? If so, is there any sort of workaround? We can't afford to replace my computer's PSU right now, but wouldn't it be possible to build some sort of device to go between the TV's output and the computer's line input in order to eliminate the hum, as a temporary solution (my dad has a bunch of electrical components in his loft workshop, so this is a possibility)?

If it's not an earthing issue, what is it likely to be?

Also, if you're wondering why I don't just use a TV card, it's basically so that a) no extra CPU load is placed on my computer and b) no monitor space is taken up, I have precious little as it is. As well as that, TV cards typically cannot accept RGB input such as that on the SCART cable of my PS2.

[Sensuifu]

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Danj said:

If it's not an earthing issue, what is it likely to be?


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Is the PSU fan noisy? It could be something from the PSU, or the fan generating ambient noise. Some sleeve-bearing fans get noisy when they're dirty, off-balance, etc. Can happen to ball-bearing too, just make sure the fan is clean and running smoothly.

[Danj]

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Sensuifu said:
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Danj said:

If it's not an earthing issue, what is it likely to be?


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Is the PSU fan noisy? It could be something from the PSU, or the fan generating ambient noise. Some sleeve-bearing fans get noisy when they're dirty, off-balance, etc. Can happen to ball-bearing too, just make sure the fan is clean and running smoothly.

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It's not ambient noise. The noise only appears when I connect the cable between the TV and the computer line in. Without that cable connected it's fine, there's no noise at all. Also, note that we did replace the fan after the old fan failed, and at the moment the fan seems to be running fine.

[Sensuifu]

Have you tried other cables? Test the cable using the mic-in, make sure you lower the volume control before you do though. If you still hear something it might be a ground loop. You might have to ground either the TV or computer to something else other than the power outlets.

[JackProton]

Are you using the TV's headphone output or the line level RCA audio outputs on the back of the TV to connect to the PC's line input?

Try disconnecting other cables connected to the TV one at a time -- cable TV coax, VCR, DVD, PS2, etc.

[quenelf]

Try unplugging the cable from the TV, but leave it plugged into the line-in. Do you still get a hum?

Also try plugging it into something else other than the TV (anything else will do - headphone socket of a walkman or game boy, for instance), and play some audio. Do you get a hum then?

If the answer to both is 'no' then the cable is OK and the PC is ok, from line-in to PSU to whatever. (And you know the TV is ok because you tried that separately already.) In that case it must indeed be some kind of grounding issue, I'm not familiar with how to solve these.

--quen

[TAS]

It sounds like a ground loop. When a TV is part of the setup 9 times out of 10 it's the antenna/cable hookup. Try unplugging the cable TV/Antenna lead and see if the hum is gone. If it is you can buy/make an isolation transformer. You might also want to check the grounding block, if it's outside the ground wire may have rotted.

[Danj]

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JackProton said:
Are you using the TV's headphone output or the line level RCA audio outputs on the back of the TV to connect to the PC's line input?

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Headphone output. This TV is a 10-year old 14" PAL Sony TV, it doesn't have RCA outputs.

[ QUOTE ]
quen said:
Try unplugging the cable from the TV, but leave it plugged into the line-in. Do you still get a hum?

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Nope.

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Also try plugging it into something else other than the TV (anything else will do - headphone socket of a walkman or game boy, for instance), and play some audio. Do you get a hum then?

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Tried plugging it in to my PSP, works fine.

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TAS said:
you can buy/make an isolation transformer.

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What sort of parts are needed to make one of these?

[TAS]

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Danj said:

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TAS said:
you can buy/make an isolation transformer.

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What sort of parts are needed to make one of these?

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A 1:1 audio transformer.
http://www.epanorama.net/circuits/audio_isolator.html

A prebuilt cheapo unit from Radio Shack 270-0054 $17 (listed for automotive use, but should work fine for your computer too)

More expnsive higher quality unit $59
http://www.jkaudio.com/pureformer.htm

[JackProton]

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Danj said:
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JackProton said:
Are you using the TV's headphone output or the line level RCA audio outputs on the back of the TV to connect to the PC's line input?

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Headphone output. This TV is a 10-year old 14" PAL Sony TV, it doesn't have RCA outputs.


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Ah. You may get better results using an attenuating dubbing cord since there's an impedance mismatch and the signal can overload your audiocard's input.

[quenelf]

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JackProton said:
Ah. You may get better results using an attenuating dubbing cord since there's an impedance mismatch and the signal can overload your audiocard's input.

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It won't, though /images/graemlins/happy.gif Well, I guess it depends on volume settings, but in practice, in my experience, connecting headphones to line in always works, from whatever device's headphones socket. I know it kind of shouldn't, but it always does /images/graemlins/happy.gif

(Anyhow, an overloaded input would probably be distortion, not a hum.)

--quen

[JackProton]

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quen said:
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JackProton said:
Ah. You may get better results using an attenuating dubbing cord since there's an impedance mismatch and the signal can overload your audiocard's input.

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It won't, though /images/graemlins/happy.gif Well, I guess it depends on volume settings, but in practice, in my experience, connecting headphones to line in always works, from whatever device's headphones socket. I know it kind of shouldn't, but it always does /images/graemlins/happy.gif

(Anyhow, an overloaded input would probably be distortion, not a hum.)

--quen

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Its true that it will work in practice without using an attenuating cable but you should, at least in theory, get less distortion with it. Plus, some of the older TVs I've had actually had had a constant low-level background hum on the headphone output just because the audio section was't designed all that well. The hum didn't change with the volume level and so using an attenuating cable allowed me to cut down the hum level relative to the audio.

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TAS said:
A prebuilt cheapo unit from Radio Shack 270-0054 $17 (listed for automotive use, but should work fine for your computer too)


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I have one of these (a bit older version) installed between my SB card line out and my denon 3803 input. Before installing, it would hum badly. After installing, sounds like another component piece hooked into the system.