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eliminate ground loop hum?

Discussion in 'Pro Audio Equipment' started by mugtastic, Feb 10, 2007.

  1. mugtastic

    mugtastic Active Member

    can someone explain how i get rid of the ground loop hum created when i plug my preamp into my interface. i assume this is what i'm experiencing. the preamp is powered by a 2 prong wall wort. the interface is usb bus powered.

    i've read that i have to lift the groung from one end of the chain and not the other but i don't know how to do that.

    the laptop its going to is plugged into a different outlet - but the hum only occurs when the preamp plugs into the interface (input and headphone gain not affecting the hum at all).

    thanks for any help.
     
  2. RemyRAD

    RemyRAD Well-Known Member

    mugtastic, I've had the same problem with my Hewlett-Packard laptop and my USB audio interface. There was no problem provided the laptop was running on its own battery. When I plugged its power supply in, which had a grounded 3 pin Edison plug, I got ground loop hum. No lifting of audio grounds seemed to help?

    Part of the problem with the laptops are the switching power supplies. You will have to lift the laptops power supply ground with a 3 into 2 AC adapter. Unfortunately, lifting the audio grounds with this scenario, just doesn't seem to work.

    You ALSO SHOULD NOT PLUG THINGS INTO DIFFERENT ELECTRICAL OUTLETS if you can possibly avoid it. These devices are low current and so, if you are not using high-powered amplifiers that are loading down the outlets, you would be better off trying to group the power for these devices.

    Beware, however. You could have an electrical wiring problem? Wiring mistakes are made, that's a fact Jack. So it is best to take an AC volt meter and 2 extension cords of equal length. Then connect the leads to the ground pins of both outlets. You should only see a small voltage? If there is a problem, be careful, you could see 117 volts between the two grounds! A small voltage won't kill you but will cause hum, when pieces of equipment are connected between those outlets, grounded to those outlets and grounded together via the wiring.

    BANG!
    Ms. Remy Ann David
     
  3. mugtastic

    mugtastic Active Member

    ha! i get home from the weekend away and switch everything around, try different outlets - nothing changes. then i unplug the magnetic power cord and the hum is gone. then i check the e-mail and its remy with the exact answer. why am i not suprised.

    so i'll go the 3 to 2 adapter route - but what are the dangers involved. i'm worried about the macbook first and myself second. unless i get a mildly ouchy shock at which point i'll cry and spit at the stupid computer.
     
  4. RemyRAD

    RemyRAD Well-Known Member

    mugtastic, I don't think you have much to worry about? If you are uncertain, you can always use an AC volt Meter. Just connect the test leads to the grounds of the different pieces of equipment and make sure that you do not see that are greater than just a few volts. If you happen to connect to grounds and find 117 volts. Very carefully remove the test leaves and switch off the equipment. You have a miss wire in your electrical system if that happens.

    I think one of the reasons why we have problems like this is because the outboard switching computer power supply do not any AC line transformer, which the couples the power supply wiring from the actual electrical connection. Whereas the switching power supply is a more active device that has no electrical isolation from a transformer. They don't use transformers. Because transformers provided inductive coupling, circuits are actually connected together that have no physical connection to one another. The signal is merely induced into the other windings, like magic. So I am a big supporter and advocate for transformers since they do things that no active electronic components can adequately mimic. So, give me the microphone transformers, give me the output transformers, give me the big heavy linear power supply transformers.

    Fwew.........now I can unwind
    Ms. Remy Ann David

    And thank you mugtastic ! Glad it helped!
     
  5. MrEase

    MrEase Active Member

    Remy, while I appreciate your humour and what you are saying is true, that line (mains) switching supplies do not have an AC line transformer as such, they do still actually have a transformer for isolation. It's just that these are run at much higher frequency than the 50/60 Hz mains frequency and so can be MUCH smaller for the same power transfer.

    So in fact the following statements are not true at all - they do use transformers.

    In essence WRT grounding, the scheme is much the same as "conventional" transformers with the ground being connected to (normally but not exclusively) the negative DC output on single output supplies. In essence, with regard to a true "ground loop" the behaviour of each will therefore be similar.
     
  6. RemyRAD

    RemyRAD Well-Known Member

    MrEase, while you are quite correct that switching power supplies do in fact have an internal transformer, it is not the first stage input transformer. And so, it does not provide isolation. It is used within the circuit in a different manner. I can't go into specifics since I don't design power supplies and particularly, don't like to even attempt to work on a switching power supply, so I don't. That's one of those disposable items you just replace when it fails. I'll fix a linear power supply but not those Hocus Pocus power supplies. I'd rather fight then switch!

    Switched
    Ms. Remy Ann David
     
  7. MrEase

    MrEase Active Member

    Remy, no they are not the first thing in line and a "switcher" has several hundred volts of non isolated DC in its primary circuit (even more so with my local 230V AC line voltage). So no, they are not "fun" to work on!

    However you are still incorrect in stating that they do not provide isolation.

    There are very strict regulations that (very sensibly) insist on this and this includes clear insulation performance on the isolating transformer and spacing requirements between isolated and non isolated parts of the circuit. I COULD go into more specifics as I DO, occasionally, have to design these things!

    As I said before, WRT ground loops the two types will be essentially identical.
     
  8. mugtastic

    mugtastic Active Member

    ok so one problem leads to another. now when the laptop power is disconnected im getting horrible buzz from my p-bass. plug in the power - buzz goes back to normal but the low freq hum returns. im assuming this is because there is no 3 prong plug anywhere in the setup. how can i fix this?

    if i go with a better quality firewire interface will this all be good? thanks.
     
  9. Mises

    Mises Guest

    Just as an aside, since we're talking about transformers.... a company called Tripp Lite, manufactures some very reasonably priced line isolating transformers which are pretty popular in faclities which require decoupling from the mains.

    http://

    Just glancing superficially at their specifications, I didnt see much of any difference (other than the amount of watts they are designed to handle) between their so-called medical ("hospital") grade equipment and their general purpose line isolation equipment... so unless theirs a real difference the standard grade would probably function good enough.


    Another model that seems interesting is the Furman balanced line transformer isolation unit:

    http://

    Looking at the price though ($1700 US Dollars), it looks like an overpriced ripp offand more exaggerate dhype. The Tripp-Late probably accomplished more or less the same thing at 1/4'th the price.
     
  10. MrEase

    MrEase Active Member

    I don't think a different firewire interface will really help as it appears your problem is with grounding. With nothing grounded it seems that your p-bass is able to pick up just about anything, hence the buzz. Does your pre-amp have a ground terminal on it anywhere? If so rig it with a ground, if not see if you can ground the case.

    Your PC supply is normally grounded and I would not recommend lifting that even though the supply is isolated. If you can ground the pre-amp somehow I would first try that with the PC grounded as well. If the hum is cured, then great, but if it gets worse you could try lifting the ground on the PC.

    To see a real engineers analysis of grounding, try the following link:

    http://www.dself.dsl.pipex.com/ampins/groundloops/grndloop.htm

    It gives a "real" world insight to how grounding really causes hum loops.

    Edit: Also I'm sure that you are aware that the pickups on your p-bass will always tend to pick up mains hum. When in a studio environment it pays to move around or turn 90 degrees to find a minimum for the hum pick up.
     
  11. mugtastic

    mugtastic Active Member

    thanks. what do i attach to what to ground the pre? i've temporarily solved the problem by putting an inferior pre in the chain with a 3 prong plug. that returns the buzz to normal levels and with the comp on battery power all is quiet and good, but its crazy to have that pre in there just to ground things.
     
  12. MrEase

    MrEase Active Member

    I thought I'd said this! If the pre has a ground terminal, use that OR connect the pre case (a metal part) to, well, ground....

    If you are still finding this confusing I recommend the link I've already given. It ain't rocket science! :<)
     
  13. mugtastic

    mugtastic Active Member

    i appreciate the time and expertise and i've got a better understanding of ground loops in the past few days and i read the page in the link - but dude - no one writes anywhere "go to store x and buy 3 feet of wire y with an end like a turntable ground wire. unscrew a chassis screw on the pre and stick the u end under it. tighten. then stick the other end in ground. the only location of which in an upstairs room in a house is the ground hole in an electrical socket. or run a wire to the pipes under the nearest sink. or whatever.

    until i get that info, "how to ground wall wart powered audio equipment" i can't really go about doing it.

    and the 3 to 2 adapter for the computer didn't make any difference.
     
  14. AudioGaff

    AudioGaff Well-Known Member

    You want straight forward and simple? Assuming that you do not have a cable or building wiring problem, the simple way to resolve ground loop type problems is to go out and buy a balanced power unit like those made by Monster, Furman or Equi-Tech. There are also other advantages to using balanced power other ground problems.
     
  15. MrEase

    MrEase Active Member

    Unfortunately I am not familiar with the wiring in your house, nor do I know exactly what gear you are using. Consequently I cannot tell you exactly what piece of wire to put where. Of course if I came round I would be able to see all this and hopefully sort out your problem. This is simply not possible so I am trying to do the next best thing. That is to try and give you the resources to find out HOW to go about it. You have read the link that I gave. If you understand from this what is going on then equally it should help you to sort out the problem. Also you will understand better and if you ever move then you would be able to sort out any new problems arising.

    Please remember that it is impossible for me to be totally specific in these circumstances.

    As long as you don't "ground" anything to the live terminal you should be able to experiment. If you can't sort this out with the help given, you may have to consider paying someone to come round and do it for you.
     

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