Loud radio signal interference when using 3.5mm jacks on iMac

Discussion in 'iMac' started by Verdanice, Feb 19, 2013.

  1. Verdanice macrumors regular

    Aug 13, 2007
    So in this office, there are a bunch of iMacs which all suffer the same problem. When using the 3.5mm audio jack with any headphones, there's some pretty loud radio interference - that is, there's a foreign radio channel that is very clearly audible and it's loud enough to make listening to music unpleasant.

    This doesn't happen when listening through a laptop or phone. It's only things plugged into the wall directly anywhere in the office, including a receiver. This actually happens at another office a few blocks away too, but the signal is much quieter.

    I've been led to believe this is a problem that a ground loop isolator could solve, but I'm not 100% sure how it would help. Would a device like this plugged in between the iMac and my headphones likely solve the problem?
  2. chuckiehina macrumors member

    Jul 29, 2007
    That device would help significantly with noise like pop, cracks and static but if you are really picking up "foreign radio channels" then I doubt that type of filter would help.
  3. Verdanice thread starter macrumors regular

    Aug 13, 2007
    Interesting - any concept of what can be done, or if it is in fact a ground loop related issue?
  4. seanm9 macrumors regular

    Dec 29, 2007
    Cape Cod, MA
    Not being an electrical engineer... I don't know why you are getting this only when plugged in AC in the office... But knowing what I know about RFI... It sounds like a shielding issue... I have seen all sorts of electronics suffer from direct pickup of a Local radio station... I have seen TV sets that when powered off and unplugged from the wall outlet, still playing the audio of a local radio station... My friend had a nice pair of floor speakers that picked up the same station while the stereo was off... And my favorite is I have a friend who worked for a tower company... They are able to bring a speaker up on some of the towers and by aiming the magnet of the speaker at the transmitter listen to that radio... It sounds like trying to add some shielding (tin foil) around the headphone jack and wire might be a good way to trouble shoot this... Could be the metal of the desk top is acting as an antenna while the laptops are plastic... What happens when you plug the laptop into the AC?
  5. bobcan macrumors 6502a


    Jan 8, 2007
    Sunny but Cold.. Canada
    As I work in the Pro-Audio industry, I am quite familiar with the term and causes of 'Ground Loops' (and what this piece can resolve) so I will just address that first.. Then I will try to toss out a couple of things to check if you are able to..

    A Ground Loop is essentially created when 2 or more electrically powered devices are connected that enable more than one path to an A.C. Power Ground source ~ ~ ie. If you had an issue with your iMac and a set of Powered Speakers, as opposed to headphones, then that device in the Link above, which likely has an isolation transformer inside it, might 'disconnect' the actual devices physically by passing the Audio Signal through the Transformer Coils.. Technically, 'the loop is broken' by having the Passive Coil remove the linking of the devices, the Audio passes through by proximity of the Coils to the Iron Core..

    Now your RF (Radio Frequency) Interference issue could be almost related in another way.. Bad Cables (especially if the are very long) and maybe even a Radio Station or Transmitter Device nearby could easily be the cause of you and others hearing this unwanted music.. As 'seanm9' noted above it may well be a common piece such as a Metal Desk, or even Metal Studs in the Wall, that are all combining to 'create an antenna' that is unfortunately sympathetic to the Frequency of some Radio being transmitted somewhere nearby!!

    All electronic equipment sold these days has a disclaimer that says what THEY put out and can be affected by as far as electronic interference (usually meaning they can be used near each other without creating issues) but most do not state how well they SHIELD from it though.. Chances are very good that if there are several computers affected similarly that it is a RF Field nearby, and that you will not easily fix the problem, but you never know.. You might try listening on the headphones while rotating the computer on the desk (if it is a true stray RF source I would expect that orientation of the device may affect the sound level, unless a REALLY strong signal)

    The World is currently awash in RF creating devices, which is why Portable Pro Audio Systems (such as you would see at any large concert) are always susceptible to so many 'buzz and noise issues' when they are Set Up and Torn Down in different venues days after day.. and why Audio Techs spend literally HOURS tracking down such issues these days, given the massive amount of Wireless Microphones, Instruments, and In Ear Monitor systems that are used in most concerts!!

    Good luck, I would first suggest trying to see what is a common component among ALL computers that have this issue, and if there are ANY that Do Not have the issue, what is different about them (same or different headphones, position in the room, etc) ~ If you were near to me, there are tools we have that will show RF Signals and would help to track it by it's Frequency ** If you listen and could learn the stations 'call number' or name you might find out more about The Source.. as stated earlier, you just might have a Transmitter/Repeater near you (if you are in a tall building downtown in a large city, there might be one on your roof) and maybe you all are just unlucky enough to be bombarded with RF that will NEVER go away!! :cool:

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