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Thread: Laptop Power Supply Noise through USB (ARRRRGGHHH!!)

  1. #1
    ido1957's Avatar
    ido1957 is offline 8K Gold Member
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    Laptop Power Supply Noise through USB (ARRRRGGHHH!!)

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    This is a companion thread for my computer frustrations...

    My mixdown path is ADAT->Analog Mixer->USB Converter->Laptop USB port.

    My problem is there is a high pitched noise coming out of my monitors when the power supply is used on my laptop. When I disconnect the power supply and run my laptop on battery power it is perfectly quiet. Everything is plugged into the same plug/circuit in the wall.

    This limits my time to a few hours which isn't always enough when doing mixdowns. Then I have to stop and recharge the battery for an hour before continuing.

    Help Me Someone!

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    cpl_crud is offline oh for an hour of sleep..
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    try puttingan isolating transformer or DI box in your analouge signal chain (ie, between your soundcard and your monitors).

    Laptop PSUs are a real pain like that. Because of the switching nature of their design (and the fact that they weren't designed with audio in mind).
    I run into this problem all the time. I'm not entirley sure, however I think it's a ground loop which is being switched by the PSU (Ie speaker earth->montior->laptop->psu->laptop earth), hence the difference from a "standard" ground loop
    www.shagtech.com
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    I spend my days getting paid to push around air.
    That fact is both amazing and depressing.

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    ido1957's Avatar
    ido1957 is offline 8K Gold Member
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    Isolating Transformer - found one on eBay, it says it needs to be installed by a professional electrician...anything simpler for us newbs?

    How would a DI Box work? Any examples of a DI Box and example set up for my situation?

    Open to any other suggestions also......

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    cawhite12 is offline Dedicated Member
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    I had the same problem with using a firewire pcmcia card and the firepod. What I did was get a 3-prong to 2-prong adapter to put on the laptop plug then plug it in to the wall. This will lift the ground out of the line completely and should fix your problem. Worked for me anyways.

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    ido1957's Avatar
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    That sounds like a pretty simple solution, now all I have to do is find one of those adapters and try it. I think I remember hearing they were discontinued because of safety reasons but I'll check around....

    We used to use them all the time in the little community halls that had old fashioned two prong wiring. SNAP! got a shock all the time when you touched the mic with your lips while you played guitar!

    Edit:
    Yep - thought so - from the Air King Site....
    WARNING: USE OF A THREE-PRONG TO TWO-PRONG ADAPTER IS NOT RECOMMENDED. IMPROPER CONNECTION MAY CREATE THE RISK OF ELECTROCUTION. USE OF SUCH ADAPTERS ARE NOT PERMITTED IN CANADA.
    Last edited by ido1957; 03-07-2006 at 19:52. Reason: Add to my frustration

  6. #6
    dgatwood's Avatar
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    Quote Originally Posted by ido1957
    That sounds like a pretty simple solution, now all I have to do is find one of those adapters and try it. I think I remember hearing they were discontinued because of safety reasons but I'll check around....
    They're a terrible idea. Most computers were NOT designed to operate without a ground, and will produce substantially elevated EMI/RFI if you do. The few laptops that were designed for such operation come with a two prong plug on their power adapter from the factory (and maybe not even then...). For that matter, in general, lifting the mains ground from any piece of equipment is not a safe thing to do. Those third prongs aren't just there for looks....

    First, you need to understand why you're getting noise. The noise caused by a ground loop (which what you describe probably is not, BTW) is caused by the grounds not being at the same voltage. This can be caused by a leaky power supply circuit, a poorly grounded device, etc., and is really common on devices with two prong adapters.

    When you float a ground like that, you have a 50/50 chance of actually increasing the noise. If you float the ground of the device whose ground wasn't sufficient to begin with, every bit of electrical energy that would normally have passed harmlessly to the mains ground (as a result of power supply leakage, etc.) is now being shunted through your audio cable's ground in addition to all the energy that was there before. Since this is a relatively high resistance path running parallel to your signal wires, you're basically dumping additional noise into your signal....

    The only way this won't happen is if you get lucky and lift the ground of the device that the current is flowing towards, in which case you're really just floating an electrically hot ground, and thus potentially creating a shock hazard if you touch any "grounded" metal part of that device while touching something that really is grounded. Oh, and you've also turned the case of the machine into a giant antenna....

    The right place to lift the ground is in your AUDIO cables, where the amount of current in the line is mostly harmless, and where you really don't want any current (apart from any noise induced in the cable from your environment) to be flowing. By doing this, you are preventing current from running between one device's ground plane and the other device's ground plane. In theory, the best place to cut the shield is probably in the middle of the cable, for minimum average resistance to ground, but nobody actually does that. They lift one end. Easy enough to do.

    That having been said, what you are seeing is probably a very different issue. A high pitch whine in a laptop power supply is almost certainly caused by a noisy battery charge circuit. That shouldn't be helped at all by floating the mains ground, but I suppose anything is possible....

    The correct solution for this problem is to physically isolate the power (and possibly ground) between the USB port of your laptop and your audio interface. The easiest way to do this, if it works, is to simply purchase a powered USB hub. In all likelihood, that should provide the isolation you need, assuming the power rail is at fault (which it probably is).

    If you find that the noise still continues, then the problem is likely the computer's ground not being sufficiently well grounded. The easiest solution for that is to force the computer's ground to earth ground by connecting a heavy gauge wire from a grounded part of your audio interface (silver screw, ground shield of an unused input or output, etc.) to a building ground (the ground pin on a wall plug). 10AWG should solve the problem.

    Forcing the ground plane of the offending device to earth ground with a suitable heavy gauge wire will take care of most ground loop issues, as well....
    Quote Originally Posted by Obi-Wan
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    cpl_crud is offline oh for an hour of sleep..
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    As someone who's been electrocuted, and had to pry people off trusses that have become live, I'll have to say this:

    DO NOT USE 3-2 PIN ADAPTORS ON YOUR POWER LINES!

    It's really, really dumb. Go the DI/Iso route.

    That isolating transformer isn't what you're lookign for.
    I was thinking somethign like
    http://www.jacksmusicfactory.com/def..._Rapco_ISOBLOX

    Or use any DI with a ground lift.

    What you're trying to do here is berak the circiut described above.

    AND DON'T DE-EARTH YOUR EQUIPMENT.

    Really, really dumb. Would you stick a screw driver in a power point? Then don't de-earth equipment.
    Plus, it can also harm your gear.
    www.shagtech.com
    bad name- average site...

    I spend my days getting paid to push around air.
    That fact is both amazing and depressing.

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    zazz's Avatar
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    ok ...its a bad idea but i also have the same problem....so disconecting the earth means the sound from my speakers has no more buzz which works because i tried it and thats what happens. With this in mind if i buy a usb hub between my usb audio interface and my laptop then this should have the same effect as removing the earth from the laptop power supply??

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    tallmanr is offline Newbie
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    I had the same problem and used one of these:

    http://www.radioshack.com/sm-see-all...i-2062214.html

    Fixed the problem.

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    I think your laptop has no connection to the same ground as the rest of your equipment and so "the Rest" pick up screen modulation amongst other things (high freq squeal sounding modulation). Some of the others have suggested filters/ transformers/ etc. but one (dgatwood) mentioned this problem. Connecting laptop ground to system ground should eliminate this problem and several others like DC offset voltage playing havoc with levels (and noise) without buying more stuff. Finding a good laptop ground is tough though, usually no ground point so next best would be connecting to "DC-" input , or PC Board ground (internal), or on a drive frame screw or such.

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