Audio Hum with 12" PBook

Discussion in 'Macintosh Computers' started by Glossybear, Nov 14, 2003.

  1. macrumors member

    My Rev-A 12 inch powerbook has a weird "bug" that I can't figure out, and I'm hoping SOMEONE will have a suggestion.

    I often use my powerbook to listen to my iTunes Liberary though my stereo system (Phillips 5.1 tuner.) I have an RCA cable running out from the "CDR/Tape" input that terminates in an RCA to 1/8th-inch converter.

    When I plug this into the headphones jack on my powerbook I get a HIDEOUS audio hum. It is like fingernails on the chalk-board of my soul and quickly induces a blinding headache. (I must be extra sensitive to hum because it doesn't bother my wife.)

    This in and of itself isn't too weird lots of things could be causing the hum. But the weird part is if I plug my iPod or a diskman into the same cable there is no hum. And if I plug headphones or computer speakers into the powerbook there is no hum. Only when the powerbook is plugged into the stereo.

    Any thoughts? It is driving me slowly insane.
  2. macrumors 68000


    Sounds like some kind of ground loop problem. You can buy adaptors at radio shack that should solve it, if in fact that is what's going on. Hopefully someone with better experience can give you a more definitive answer.
  3. macrumors 6502

    Are the computer speakers powered? Is your PB plugged in to AC when you have it running into the stereo? If it is a relatively low frequency hum then MoparShaha is probably right, might be a ground loop issue.
  4. macrumors member

    Not quite sure I get it...

    Is the ground problem with the Powerbook or my stereo?

    It hum happens if the 'book is plugged in and charging or running off the battery.
  5. macrumors 68000


    Your PowerBook is not sending a properly grounded signal to your stereo. One of those things I was talking about provides a proper grounding signal. I ran into a similar problem with my home stereo. Most home stereo's have a grounding jack on the back, where you can connect a wire and attach it to a grounding point in your house (something metal). Obviously, you can't do that with the PB, but one of those ground loop devices will artificially provide ground.
  6. macrumors 68010


    Your powerbook is looking for ground via your stereo. Your stereo is probably grounded and therefore the minus of the audiocable connectig your powerbook to your stereo is grounding your powerbook on the minus of the stereoinput. That's the hum you are hearing.

    Simply put your stereo powercord in another ac-socket, which isn't grounded. Or ground your powerbook. That would do the trick! Ofcourse you can also buy a adopter whcih was mentioned by Mopershasha, but this doesn't always solve it (sometimes you need even two! one at the beginning (powerbook headphones out) of the cable and one at the end stereoinput).

    I had the same thing with my G3 (which was grounded) and my stereo (wasn't grounded) (this is probably the reverse situation). Anyhow just by putting the G3's powercord in a non grounded socket, the hum was history.
  7. macrumors regular

    Re: Audio Hum with 12" PBook

    I've had this problem too. The guys have suggested the grounding which will definitely eliminate some of those things - but what I also found out is that a lesser quality RCA cable will cause a hum. I had some piece of crap cable I dug up from the garage which hummed with my iBook but not with my discman or anything else I plugged into it. I changed the cable for a better shielded cable and the hum went away. A friend of mine also seems to think that lesser-quality RCA cables are prone to accepting interference from outside objects (airport, cordless phones, microwaves, etc). I guess when their core technology was created we didn't have any of these devices.

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