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Laptop mains hum/noise when headphone output plugged into mixing desk

I have been using an AJP laptop to play music (out of the headphone jack) out of a small P.A but when I plug the power into the laptop a lot of noise comes through. This doesnt happen in all the buildings I have played in just certain ones? I need the power in because the battery life is very short. The noise is a crackling and humming with a repetitive heartbeat like pulse.

I think the problem is mains noise and the way the building is wired up but is there anyway of counteracting the hum/noise?

Thank you.
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CorrArts
Asked:
CorrArts
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1 Solution
 
rindiCommented:
Get a Powersupply brick that is capable of supplying more power.
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CorrArtsAuthor Commented:
Do you mean the power supply to the laptop? How would this help?
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rindiCommented:
Yes. A more powerfull powersupply which is also better quality will have less interference. A powersupply that is running at it's limits will also have more interference.
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CorrArtsAuthor Commented:
I see. but I dont get the noise everywhere I 've been just in certain buildings, are you sure a new power supply will solve it? also will a more powerfull supply not affect the laptop in some way?
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rindiCommented:
It can also be caused if the mains supply isn't very good. Again a better powersupply can filter that out. A more poersupply won't affect the laptop, as long as you don't go too high with the voltage. The voltage should be the same as what is needed for the laptop, the input voltage of the powersupply can be in the range of 100 to 240 Volts. If your laptops requires 4 Amps to function, make sure the powersupply can deliver well over that, like 5 or 6 amps or more. The laptop itself will only draw what is necessary for it.
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jhanceCommented:
I really don't think this is a power supply problem.  While that's possible, if it were the root of this issue you'd hear the same noises if you plugged in a set of headphones and listened.  Presumably you do NOT of you'd know what the problem is.

I think the problem is a line level or impedance MISMATCH between the headphone output and the input to your PA system.  

A headphone output is designed to drive a set of headphones and is a relatively high-level signal equipped to drive a load of about 1000 Ohms.

A PA system input is either a line level or microphone level and has a much higher input impedance.

So when you send your headphone output to the PA, the signal is:

a) Much too strong for the input.  So you have to turn it way down.  This reduces the signal you want and effectively accentuates the underlying noise.

b) Mismatched impedance which also has the effect of accentuating the noise.

What to do?

1) If your laptop has a LINE OUT, use it instead of the headphone out.  This will be a much better match for the input to the PA both in terms of level and impedance.

2) Get a direct box like http://www.whirlwindusa.com/dirbox.html.  These are impedance matching devices which also take the single-ended headphone signal and convert it into a balanced-line signal common with PA amplifiers.  They also have an attenuation circuit which would be useful for your situation.

3) You could also try an attenuation or "pad" box.  Or you could make one.
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HKComputerCommented:
I think the previous post may be correct, but I wouldn't rule out a "cluttered circuit."  You may need something that can filter noise on AC.  I'm not an electrical expert so I can't say a lot more than that.

I compare it to my experience with this:
My Citizens Band speaker in my vehicle outputs excessive noise when the Air Conditioner runs
My cordless telephones picks up noise when the microwave runs
My PA amplifier system also outputs more noise when its on a circuit with certain devices such as a microwave
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SoyYopCommented:
Like HKComputer, I think the problem is noise.

Had you ever tried to use a cell phone and wireless headset at a time? You got a lot of interference...

Noise on power line can be the cause, or electromagnetic noise from equipment, too. Do you have radio equipment nearby?

Maybe a small noise filter on the line solves the problem, or a different power source. But if the problem is related to "air" interference (like comm equipment), well, sorry :(


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fredshovelCommented:
Hi CorrArts,

Whilst years ago audio guys would have shuddered at the thought of using the headphone output of anything to supply audio to a PA it's fairly common practice today, even in professional audiovisual presentations -- mostly because that's the only output some laptops offer. Of course 'line out' or 'aux out' is the way to go if you've got it.

I can't see you driving the impedance down on the line level input of your PA as these type of scenarios have been well catered for over the years, with a lot of manufacturers offering 100K Ohms at the input. The mixer on your PA will almost certainly have two types of inputs. Balanced microhone inputs (three pin XLR) and unbalanced line level inputs (usually mono jack 6.5mm).

You may simply have a faulty lead or the wrong lead -- you've sure got a lot of noise happening.

Your connection lead should be a stereo mini-jack to mono 6.5mm jack to connect to the unbalanced line level jack input on your mixer (if this is your case). -- I hope you  didn't make your own lead out of unshielded speaker wire.

Your noise could certainly be an earth loop (60 cycle or 50 cycle hum [depending what country you're in]). You can break this by inserting a hum eliminator on the input to the PA.
http://www.ebtechaudio.com/he-2des.html

You can also, as jhance suggested, use a DI or direct injection box, which is usually mono so your lead has to be stereo minijack to mono jack 6.5mm. This will match the impedance to (and also pad the signal) to  balanced microphone level -- it is then plugged into the microphone input (three pin XLR) of your mixer.  DIs also have an earth lift -- which may be useful in your situation.


 








 
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sound2manCommented:
Also check to see if your laptop and mixer are plugged into the same circut. It can cause problems if you are plugged into another circut.  I also strongly agree with the suggestions to get a good DI box.  Another thing to try is a ground lift adapter.  These lift the ground pin from the electrical socket and can sometimes stop ground loops that cause hums.  THIS IS A TEST ONLY. DO NOT USE AS A PERMANANT FIX.  If this helps you problem, consider getting an ebteck hum filter as one of the other posters has already suggested.
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scrathcyboyCommented:
I beg to differ, the problem is the laptop itself, most likely.  Many sound cards on laptops are not designed for quality recording, listening, or playing.  We have NEVER been able to get a laptop sound card to record high quality stereo music free of background distortion.  It is a problem endemic to laptops, sorry.
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sound2manCommented:
If I understand rindi correctly, he is only having this problem when plugged in mains power - not when running off of battery.  If this is not the case, then my answer is moot and void.  If this is the case, I stand by it.
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sound2manCommented:
Sorry, I guess the OP was CorrArts, not rindi.
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CorrArtsAuthor Commented:
Hi Experts!

Sorry its been a while. I have done a couple more shows and tried some of your solutions. I tried using different power packs, a DI box and balanced cables but all did not work, although the DI box did stop the noise for short periods it still came back? Am I flogging a dead horse?
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rindiCommented:
I don't know. My colleague recently had a similar problem with an IBM notebook. He didn't use the original powersupply, but another one from an older laptop which was less powerfull, and with that powersupply the sound was disturbed. With the original Powersupply for that notebook which was more powerfull he didn't have that noise. It doesn't just need to be the power of the supply causing this, but also if the components used weren't good that could also cause more noise.
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fredshovelCommented:
It's clutching at straws time: The actual minijack on your laptop may be worn or even faulty -- this is quite common.
Sorry that all the solutions have you running out and buying stuff but another solution may be to buy a PCMCIA soundcard for your laptop.

Before you do this, if it's possible, you may want to do a double blinded test with another laptop when you get the noise -- because the culprit may be another piece of equipment (say a projector) putting out excessive RF (radio frequency).

"There will be an answer." (McCartney).  
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SoyYopCommented:
I had exactly that problem with a power conversion device (a cheap unit) I had to use because my laptop's plug did not fit on the wall (non-us). I connected my audio output to the datashow. The noise was brought into my laptop by the power line... I thought our data was broken :(

I got curious and Once I got a regular adaptor (just the adaptor, no transformer) it tested it again. The noise disappeared! I checked back plugin in the power conversion unit, so I'm sure was it.
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CorrArtsAuthor Commented:
Thank you for all your help and comments. I think I will look into the power supply situation some more!

CorrArts
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