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How eliminate this earth loop?

Discussion in 'Electronic Equipment' started by Jon D, Apr 24, 2006.

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  1. Jon D

    Jon D Guest

    I am in the UK.

    I have a hi-fi cassette player and I feed the outputs to the line-in
    on the motherboard of my PC.

    I get a horrible hum on the recording I make on the PC. The
    recordings are of phone conversations so the overall quality of what
    I am recording is not great but the hum is still very distracting.

    (1) I am told that an audio isolating transformer should work.
    However I have learnt that basic transformers with only a small metal
    core are prone to picking up hum. So I figure any such isolation
    will remove noises I don't have but will be liable to getting the hum
    I want to get rid of!

    (2) I believe another way of reducing hum is to take off the earth
    from either the source or destination equipment. However the
    cassette player does not have an earth. Would it help to take the
    earth off the PC?

    (3) Or should I actually add an earth to the cassette recoder? -->
    Maybe take the mains earth from the mains socket and connect it to
    the outer of the signal lead? Or to the brushed aluminium case of
    the cassette recorder?

    I get this hum if I take the signal from the line-out or the earphone
    sockets of the tape player. Changing the input socket on the PC to
    the mic socket does not help. The lead is not damaged because I can
    use that lead to replay from a battery-powered dictation machine into
    the PC without any hum.
     
  2. Ethan Winer

    Ethan Winer Guest

    Jon,
    liable to getting the hum I want to get rid of! <

    Unlikely. If you're on a budget Ebtech makes a line of "hum eliminator"
    transformers. I wouldn't call these units "audibly transparent," but they're
    okay for what they cost. There are a few other companies that make similar
    low-cost devices. If you want better quality and are willing to pay for it,
    look into Jensen Transformers:

    http://www.jensen-transformers.com/

    --Ethan
     
  3. GregS

    GregS Guest

    You don't want a core to introduce diferential hum signal, but its unlikely to introduce
    common mode hum, which is what an isolator is trying to acomplish.
    Reversing the plug on the player might work or help.
    You could try running a wire from the computer case to the player case.
     
  4. Arny Krueger

    Arny Krueger Guest

    I just did some bench tests of Radio Shack's "Ground Isolator" 270-054 using
    test signals that maxed out around 2.5 v RMS. ZSource = 150 ohms, ZLoad =
    10K ohms.

    The measured performance was truely amazing for a pair of transformers case
    and cables selling for only $16.65.

    All IM, THD, and noise artifacts were at least 80 dB down with most in
    the -100 dB range or better. Frequency response showed a 2 dB peak at 20 Hz
    and then 10 dB down at 10 Hz. There was a 3 dB peak at about 51 KHz falling
    to about 10 dB down around 100 KHz. +0.5 dB at 20 KHz.

    No problems with hum pickup were noted.
     
  5. GregS

    GregS Guest

    Amazing since its dual channel.

    I never measured the 600 ohm transformer, but some day I will.

    greg
     
  6. Arny Krueger

    Arny Krueger Guest

    Since this part may vary, I should point out that the UUT differed at least
    cosmetically from the picture at:

    http://www.radioshack.com/sm-see-all-needs-and-wants--pi-2062214.html

    The cylindrical case on the sample tested is more truely cylindrical, with
    sharper corners far less nomeclature. The markings are on a black sticker
    and lack the pictorals of signal connections and text related to them. A
    RCA (female) to 3.5 mm TRS adaptor was provided.
     
  7. GregS

    GregS Guest


    The caveat here is the driving requirment. A typical CD player or tape player would
    have several hundred or more ohms output, and this unit would not
    work. Just how well would it work??

    greg
     
  8. Arny Krueger

    Arny Krueger Guest

    I don't think that is a given.
    Presuming that mutual inductance is high across the audio band, the load on
    the transformer is equal to the sum of the source and load impedances. IOW
    in the case of my test, the total load was 10,150 ohms, which is the
    stronger effect. How that is broken up between the input and output would be
    the weaker effect.

    I did a test where I added an additional 10K in parallel with the output of
    the transformer. This reduced the bass peak to about 2 dB, and reduced the
    treble peak to 0.6 dB. Both referenced the midband response which was flat
    within 0.1 dB from 50 Hz to 10 Khz.
     
  9. Terry

    Terry Guest

    The cassette player has an earth unless it is battery powered. Are you
    saying it is battery powered, or are you saying it has just a 2-prong
    plug, not a 3-prong plug. A 2-prong plug has a connection to earth on
    one of its two wires.
    1) If you are running the cassette player from AC, try reversing the
    AC plug.

    2) Try plugging the cassette player into the same power strip the
    computer is plugged into.

    3) Ccan you run the cassette player from batteries? If so, that should
    eliminate the problem.

    4) Have the cassette player actually play the recording (over it's
    speaker), and plug a microphone into the computer to re-record it.
    Since it is just telepone conversations, the loss of qualtiy by
    re-recording is not really a problem.

    5) Isolation transformers should work for this problem, but I would
    try the above first.

    Terry
     
  10. Terry spake thus:
    Yikes! No, it should not. That would be a recipe for disaster, man. No,
    2-prong devices have a chassis ground that is in no way connected to
    either of the incoming line leads.

    Even with polarized plugs (which can sometimes be inserted backwards),
    this would be asking for trouble.


    --
    Pierre, mon ami. Jetez encore un Scientologiste
    dans le baquet d'acide.

    - from a posting in alt.religion.scientology titled
    "France recommends dissolving Scientologists"
     
  11. kony

    kony Guest


    Agreed, it's most likely a two prong plug then a step-down
    transformer isolating it.
     
  12. Terry

    Terry Guest

    Well, maybe we're just used to different language on different sides
    of the pond. But "earth" does not mean "chasis ground" to me.
     
  13. Jon D

    Jon D Guest

    My cassette player/recorder (made by Hitachi for the UK market) does not
    have an earth.

    The mains lead to it has only two wires: live and neutral.
     
  14. Franc Zabkar

    Franc Zabkar Guest

    You may be thinking of the "mains earthed neutral" (MEN) system. In
    such cases, even when there is no earth pin, the appliance can still
    be affected by earth loops via capacitive coupling.

    See http://www.jaycar.com.au/images_uploaded/humloop.pdf

    - Franc Zabkar
     
  15. Perhaps even more significantly, the lead probably ends in a
    non-polarised plug. The unit must be fitted with a double-insulated
    internal power unit. There is no electrical connection from the
    mains-side of the unit to the low voltage side.
     
  16. sQuick

    sQuick Guest

    Read above! (as lots haven't)
     
  17. Jon Elson

    Jon Elson Guest

    Are you sure the hum is created when the tape is played to the computer?
    Have you listened to the tape directly from the player (into stereo, into
    headphones, etc.)?
    That may help. Worth a try, for sure!

    Jon
     
  18. Don Kelly

    Don Kelly Guest

    ----------------------------
     
  19. Tex

    Tex Guest

    I've seen that a couple of times on my system if the input jack is not
    fully and/or correctly plugged in. What I get is pure 60 Hz hum. I
    find, when it occurs, that I have moved something and partially unseated
    one of the jacks. You might want to check to make sure this is not
    happening and/or your cords/jacks match and aren't giving you a phase to
    ground connection of some kind.
     
  20. Tex

    Tex Guest

    Re: my post immediately above this -- I guess I should not have used the
    term "phase to ground" as that sounds like I'm implying 120 VAC across
    the connection. What I really meant was some kind of current flow from
    power to ground and/or an inductance that causes/allows the ac sine wave
    pattern to be present at the audio input.
     
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