audio - USB/Sound issues = ground loop?

  • leander

    I've got a few strange symptoms on a machine I built:

    • The machine won't boot (sometimes) with certain USB thumb drives plugged into USB ports; disconnecting the front panel USB/firewire/audio seems to have helped, but I'm not sure...
    • On-board audio has noticeable static/hum; it seems to only happen when ethernet is plugged in. I bought a Sound Blaster X-Fi XtremeGamer (wanted a nicer sound card anyway) and that seems to have resolved this issue, but...
    • ...the new sound card occasionally loses track of the speaker jack and gives me only SPDIF out (and occasionally an "internal aux" output shows up in the available outputs list). From what I understand, modern cards often automatically switch based on measured resistence

    I think I've got a ground loop, maybe at my cable modem.

    I found an article on solving ground loops, but I'd love to confirm that these symptoms are all related to that issue, and make sure I'm not dealing with multiple loops.

    I've got an ohmmeter around somewhere, but not a lot of experience using it.

    (I assume I should be stripping the machine down, disconnecting and unplugging it, and adding components one at a time to test...)

    Any general advice on hunting down the issue(s)? Anything I should be paying special attention to? If it is only the cable modem, what's the best way to solve it -- an isolator?

  • Answers
  • jerryjvl

    If you already suspect the modem, I'd suggest that rather than stripping the machine down and adding one component at a time, it may be easier to first remove the modem and see if that fixes it.

    No concrete suggestions on what to measure tho.

  • atroon

    I had a computer a couple years ago which wouldn't reboot if a flash drive (or anything USB with storage) was plugged in. That part turned out to be a BIOS setting, it was trying all 'removable drives' before the hard drive, and the hard drive turned out to be the boot device of last resort after a 10-minute time out (it was a freaky BIOS, let me tell you).

    The sound card issue could be grounding, but a simple check would be the volts (not ohms) between the bare metal of the case and a known ground, like an outlet ground pin if you know that's good or a water pipe. Then you could check voltage between your PC and the cable connector as well. Running your ethernet cable through a UPS 'filter' should match that ground to the PC's as well.

    The audio connection part: Are you running Vista/Windows 7? Windows post-Vista contains 'protection' routines that disable your analog audio outs when 'protected' (hi-def) content is played with a media player. Only digital (in this case your digital out and an internal Aux) outputs are supported so that you can't redirect audio data to a tape recorder (the 'analog hole'). More here.

    So, at least I've given you the possibility that all three of your issues are completely unrelated. I certainly can't prove that they aren't related, but these are just the first three things that popped into my head.

  • David Pearce

    Something (a head) once fell onto my computer and hit a USB drive plugged into the front. Now, whenever something is plugged into that port, the computer restarts. That USB port is broken beyond repaid

  • Related Question

    case - Front USB Port Grounding Issues
  • Matthew Ruston

    I think I have a static electricity problem.

    I have two computers in my house and a fiancé that wears fuzzy socks. Sometimes, when she plugs in USB devices into the front ports of her desktop computer (or my computer), the computer will instantly lock up. Often the LCD will go a solid green or purple color after this happens.

    She noticed this occurs most often after walking across the carpets, so I am confident that the front USB ports on both of our computers are not properly grounded. I checked USB port connections to the motherboard and the ground wire is mapped properly on all computers.

    Is there anything I can do to add extra/proper grounding to the front USB ports on a computer? I am afraid that she is going plug in her iPod one day and poof, her entire motherboard will be fried.

  • Related Answers
  • quack quixote

    The lockup is troubling. An answer to this related question indicated the poster fixed the issue by physically replacing the problematic front USB port. If the grounding is connected poorly on the USB port end, it won't matter how correct the motherboard header connection end is.

    Another thing to do is make absolutely sure that she's not inadvertently touching the metal of the USB plug when connecting. If she's gripping the plug by the rear, plastic-covered portion, any static she's generated shouldn't be conducted into the system.

    Finally, you could set up a grounding strip near your computer station, and both of you get into the habit of using it. This example shows the installation of a flooring strip, but the principle would be for just about any other solution. You could mount such a strip on the underside of the computer desk, on the desk leg, on a wall nearby, etc.