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Mains noise

admin_exported

New member
Aug 10, 2019
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A lot of my music library is on Mp3 files. The quality from my old desktop computer was fine, but having transferred to laptop, I am beset by mains harmonics from the rubbish PSU (running on battery is OK, for a very limited time!). What's the best option that doesn't cost too much ?
 
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Anonymous

Guest
Your laptop PSU is undoubtedly a switch mode PSU and the noises you are experiencing are most probably not coming from the mains but rather from the PSU itself. A switch mode PSU is very good at generating noise and has extensive filters on the incoming mains and outgoing DC power to bring the levels of noise down to levels that conform to the EU CE directives.

I suspect you have a faulty PSU, see if you can get in changed.

Mr K.
 

Tony_R

Well-known member
Oct 20, 2008
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18,520
Is the laptop connected to any other mains powered device? For instance do you have it connected to an amplifier?

I've had instances of laptop PSU's having a slight amount of leakage, due to a capacitor being connected between the raw mains and the DC ground of the laptop PSU (seems to be a bit of a design flaw...!). This results in a tiny amount of (AC mains) current flow between the DC (negative) output terminal of the laptop PSU and ground. If your laptop is connected to an amplifier this could possibly be causing some hum due to some part of the amplifier / hifi system being earthed. (you state in your post "harmonics" - I presume you're referring to mains hum?).

The above explanation can also explain why you can sometimes feel a very, very slight tingle if you touch the PSU DC plug and an earthed device at the same time.

I've had the above happen with quite a few portable switched mode PSU's for monitors / laptops etc.

What brand is your laptop?

Tony.
 
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Anonymous

Guest
Hi Tony,

Thanks for your reply. The laptop is an Acer Extensa 5620, and it is linked to the hi fi system via it's headphone jack/screened cables/aux amp input. The harmonics are not a mains hum as such, rather a mixture of white noise, harmonic ringing and staccato morse code ! And totally intrusive. So the PSU (presumably switched mode) is almost certainly the culprit, as it is the same if I plug it into the mains on an extension lead from next door !

Do they make suitable low noise PSUs ? It's output is 19v, 3.42A

Regards
 
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Anonymous

Guest
Hi Mr K,

Thanks for your reply. I forgot to mention that I am connecting into the hi-fi system via the headphone socket/screened cables/aux amp input. The noise is a mixture of white noise, harmonic ringing and staccato morse code (!) and is just the same if I plug it into the mains on an extension lead from next door, so I am sure you are right. I will consult the manufacturer if I can, it's an Acer Extensa 5620 and the PSU is 19V @ 3.42A with international approvals including CE

Do you know if anyone makes suitable low noise alternatives ?

Regards
 

John Duncan

Well-known member
Jan 8, 2008
2,027
13
19,695
steve marnier:
Hi Tony,

Thanks for your reply. The laptop is an Acer Extensa 5620, and it is linked to the hi fi system via it's headphone jack/screened cables/aux amp input. The harmonics are not a mains hum as such, rather a mixture of white noise, harmonic ringing and staccato morse code ! And totally intrusive. So the PSU (presumably switched mode) is almost certainly the culprit, as it is the same if I plug it into the mains on an extension lead from next door !

Do they make suitable low noise PSUs ? It's output is 19v, 3.42A

Regards

My laptop makes an unholy racket when plugged in. Get an airport express and get the audio output out of the laptop altogether. Course, that does limit you to iTunes unless you add Airfoil for $25...
 

Tony_R

Well-known member
Oct 20, 2008
17
0
18,520
steve marnier:

Hi Tony,

Thanks for your reply. The laptop is an Acer Extensa 5620, and it is linked to the hi fi system via it's headphone jack/screened cables/aux amp input. The harmonics are not a mains hum as such, rather a mixture of white noise, harmonic ringing and staccato morse code ! And totally intrusive. So the PSU (presumably switched mode) is almost certainly the culprit, as it is the same if I plug it into the mains on an extension lead from next door !

Do they make suitable low noise PSUs ? It's output is 19v, 3.42A

Regards

This does sound like a faulty PSU - however, I would have thought that if it's generating that much noise, you would be getting some kind of interference elsewhere.

If you plug headphones (only - leaving your amp disconnected) directly into the laptop, does the noise persist?

Also, does the laptop have a low power mode, as used when watching DVD's etc.? You could try switching it to that mode and see if the noise reduces.

The noise you describe doesn't actually sound PSU related ( I do realise you stated it's fine on battery though! ).

What I suspect is happening, is that in battery only mode, the laptop is "stepping down a gear" ( i.e. the clock / bus speeds are all slower ) and hence no interference - but when you run off mains, it switches to full power ( CPU speed increases / bus speed increases etc. ) resulting in the noise you describe.

Can you try the above experiment with the headphones and give some feedback on the result?

Tony.

P.S. I'm not aware of any special low noise PSU's but I'm sure they must be available.
 
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Anonymous

Guest
Tony,

Very curious, it's fine on headphones, which suggests it's being picked by the subsequent transmission line. Yet the same jack/cables/aux amp input was fine when using the old desktop PC. The laptop power save options vary the noise tones slightly but don't reduce the overall level
 

Tony_R

Well-known member
Oct 20, 2008
17
0
18,520
steve marnier:
Tony,

Very curious, it's fine on headphones, which suggests it's being picked by the subsequent transmission line.

Steve,

That really does sound like some kind of a loop to me. (a mains / earth loop).

Does your amp have an optical input (well more to the point: does your laptop have an optical output)?

I don't think you're going to cure this easily, without some kind of (galvanic) isolation between the laptop and the amp.

You can get audio isolation transformers - but that's a lot of fuss to go to really and you need really good ones to maintain sound quality...

It would be easier to optically couple it to your amp, or use a seperate DAC.

Or.. is your amp (or some other device connected to it) earthed to the mains? Although I don't advocate removing earths - but that may well cure the problem. (but introduce others - such as the risk of electrical shock).

Does your home have RCB's or fuses? If you don't have RCB's, it's highly likely you already have some kind of electrical fault, which is being exacerbated by the connection of the laptop (an earth leakage RCB usually trips for minor leakage faults).

EDIT: Incidentally, your old PC would have been earthed firmly to mains earth via the mains plug. This may well explain why you had an inteference free experience.

Tony.
 
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Anonymous

Guest
Thanks John, in the light of my other replies, that alternative might be worth considering - what's an airport express, and will it connect to my external hard drive ? And I don't run iTunes so what's Airfoil ??

Regards
 

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