earth loop noise?

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This topic contains 5 replies, has 4 voices, and was last updated by Profile photo of kentlowt kentlowt 10 years, 6 months ago.

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  • #22520
    Profile photo of Knoxford
    Knoxford
    Participant

    I have my laptop setup to record our church service. Nothing fancy just the line in mic on the laptop to record L and R channels to Audacity. Very low tech.

    A couple of months ago I setup a Stereo Aux 1 (STAX1) to use as an output to an iDec so I could control the amplitude of the output signal. The other benefit is I can have a whole different mix for the iDec (iPod).

    Anyway, I am sending that same STAX1 Left and STAX1 Right to two of the 1/4 inch outputs on the back of the surface (T112) that were not in use.

    I have a cable that is 1/8 stereo to two RCA Male connectors. I then plug on two RCA female to 1/4 inch tip sleave male plugs.

    When I plug in the two 1/4 inch plugs and then plug in the 1/8 stereo plug to the mic jack in the laptop I get a generated noise that sounds like a 50 or 60 hz earth loop problem.

    This noise is not present in the iDec (iPod) recordings. That unit is built into our portable surface rack and I only get to see the equipment on Sunday morning for 2 hours because we are portable church. Anyway, I don’t have time on Sunday morning to pull of the covers and check out how the iDec is wired.

    My Laptop Power Supply was plugged into the surface external outlet on the right side of the surface box. Dell XPS M1530.

    I have a sample file I recorded into Audacity but I don’t see how to upload that to the forum.

    Does anyone have any ideas how to stop the noise?

    Thanks,
    Knoxford

    #26690
    Profile photo of martijn
    martijn
    Participant

    try to loose the earth on the laptop
    Otherwise use a proper transformer Box (DI) to solve your problem!

    #26693
    Profile photo of kentlowt
    kentlowt
    Participant

    A better way to check this is to run the laptop on battery power. It is never a good idea to remove an earth ground. If the hum goes away on battery then you will need isolation via a DI. If it doesn’t go away you have a bad cable or audio curcuit in your computer.

    112T/IDR48/IDR16

    #26694
    Profile photo of sraymond
    sraymond
    Participant

    I’d agree with Kent. You may need some transformer isolation. I’ve seen comments elsewhere about Dell having a poor grounding design, at least on older laptops. I haven’t seen anything about whether they’ve gotten better on newer models. I have an older Inspiron 8200 that was bad this way. Even a high quality USB Pre interface had hum when recording into Audacity. Running on battery or (cough) lifting the ground on the Dell got rid of it. A stereo DI also fixed it. I also can’t recommend lifting the ground as it’s there for a reason. Mine uses a lump so its low voltage at the laptop as most are but the ground may offer static discharge protection for the laptop. Can’t say for sure though as I have a Thinkpad with only a two prong edison. Running on battery when recording would be the obvious cheap fix if the battery is good and has sufficient capacity for the needed time. You possibly could buy a high capacity battery for less than the DI or isolation transformers.

    #26698
    Profile photo of Knoxford
    Knoxford
    Participant

    Thanks for your responses.
    I was thinking I will go with something like this

    Radial JPC™ Stereo PC-AV Direct Box

    But it appears that all these DI like these are more designed sending audio from devices like iPods, audio out on a pc etc. TO the console.

    I want to go the other direction from the console to the PC. I do have an email into the Radial Tech asking about this.

    Thanks again,

    Knoxford

    #26699
    Profile photo of kentlowt
    kentlowt
    Participant

    The device you want is a line level shifter with isolation or just a plain isolation transformer.

    112T/IDR48/IDR16

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