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How do you do....it? Thread, Audio Feedback in Technical; Ok, another one for you. Apologies if im talking out of my bottom. It turns out, from what i can ...
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    Little-Miss's Avatar
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    Audio Feedback

    Ok, another one for you.

    Apologies if im talking out of my bottom.

    It turns out, from what i can gather, Toshiba laptops are pretty poorly earthed and unfortunately we have a lot of them in school.

    The side effect of this is when a member of staff sets their laptop up in the hall for assembly, plugs in the sound and then plugs in the power to charge it (cus they probably havent bothered before hand), you get a very loud annoying hum!

    Now someone has suggested tin foil which i guess makes sense as something needs sheilding....but what?

    Is it the audio cable the runs from the audio socket on the wall to the laptop or something nearer the amp!?

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    witch's Avatar
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    I have had this issue and my short fix was to make sure their laptop was fully charged (I have a couple of spare batteries) so they don't need the charger.
    Someone said if you plug the laptop into one of those big reel extensions, but not unwind it, that works too.
    This
    LAPTOP HUM ELIMINATOR - GROUND LOOP ISOLATOR - eBay (item 260483878860 end time Jan-22-11 01:23:52 PST)

    might work too

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    laserblazer (4th January 2011)

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    Has anyone tried the hum eliminator?

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    robk's Avatar
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    Oh yes, the hum eliminators work really well. I have found that sometime changing the power supply for a different unit helps, with cheap replacements being worse than the originals. (I am thinking HP laptops with this).

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    Little-Miss's Avatar
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    Oh....glad its not just me then!! lol

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    Little-Miss's Avatar
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    I've ordered a Hum eliminator. Worth a try for less then a fiver.

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    3s-gtech's Avatar
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    It's to do with the earth of the laptop power supply being at a different potential to the earth of the amp. When you connect the two earths via an audio lead, current flows between them, causing interference with the audio signals. Common across most laptops to be fair to Toshiba, though some make better efforts to shield it than others.

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    dhicks (4th January 2011), Little-Miss (4th January 2011)

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    robk's Avatar
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    You can also get "switching noise" from the power supply which sounds likes hiss and is digital noise off the PSU. hum eliminators are a really good start.

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    dhicks's Avatar
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    Quote Originally Posted by 3s-gtech View Post
    It's to do with the earth of the laptop power supply being at a different potential to the earth of the amp. When you connect the two earths via an audio lead, current flows between them, causing interference with the audio signals.
    Intriguing. So how does plugging in to a large coil of power cable help? I understand the coils start to induce currents in each other, and can heat up to (seemingly) dangerous levels, but does that also help balance out ground levels or something?

    --
    David Hicks

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    3s-gtech's Avatar
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    Some info on ground loops - sometimes this interference is not caused by ground loops though, as robk mentioned:

    Wiki link

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    dhicks (5th January 2011)

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    Andrew_C's Avatar
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    Firstly, it isn't "feedback", that is an entirely different problem that I can waffle on about if you'd like.

    This issue is a real pain all the way through the "Presentation Industry". Ground Loop Isolation is the way forward, some are better than others, but most (even the cheapest) will be fine for school use. Some laptops are fine, some aren't, even from the same manufacturer. The isolators will help a lot with the digital hash.

    However, a warning; NEVER disconnect the earth in ANY earthed appliance. This might clear the noise, but leaves you open to serious injury or even death in the event of a fault. It may seem blindingly obvious, but there are many dolts out there who regard this as "normal practice". It isn't.
    Last edited by Andrew_C; 4th January 2011 at 11:38 PM.

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    dhicks (5th January 2011), robk (5th January 2011)

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    john's Avatar
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    I always carry a few CPC Ground Loop Isolators in my stock pile and have a couple personally in the car to clear up these hums on events I'm at as nothing annoys me more than the humm of earth loop.

    I have someone I work with who's partner is a "music expert" as they have a band and teach at a Music College, and they has never had these problems ever in there career and its all my sound systems fault according to them, and I got a poor designed system from one of the areas well known reputable AV Firms.... Yet most people I talk to that use laptops via sound system, unless they have Ground Loop Idolaters hidden / integrated into the system somewhere in the bowls of it, have had it with Laptops, Toshiba and HP being the worst I have found. My Samsung Netbook isn't bad at all. I also found some Desktop PCs now are starting to do it for some reason, it seems to be ones with very cheap PSUs in them though (home brew not big name I will add) so this may become a bigger issue in future.

    As has been said, if you unplug the mains from your laptop it will go away quite nicely.

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    dhicks (5th January 2011)

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    3s-gtech's Avatar
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    Other thing I do is use a fairly powerful ultra-portable (Lenovo X100e) for running AV. Fully charged, it'll do a whole evening (2 1/2 hours) of Powerpoint on batteries. The Intel CULV ones are even better for this. Bit of a cheap getaround, but effective!

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    Marci's Avatar
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    I cheat and use a wireless audio transmitter (Sennheiser) from laptop to mixing desk. Resolves all such issues instantly.

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