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Active speakers with built in radio pick up local radio when AUX cable is plugged in

  1. Sep 21, 2012 #1
    Hey there,
    I have an iHome system and when I plug an AUX cable into the line out/in port(or for that matter just plug it in on one end and complete the circuit with my body) it sometimes picks up a local indian music station. While I don't mind the music, I am confused as to why it happens and my rudimentary knowledge of physics isn't enough to explain it. When I touch the little port that connects the iPod to the speaker system, it significantly lowers the volume of the radio. Is this because my body is acting as a resistor? When I connect a piece of tin foil to the same iPod port and touch the tin foil it has the same effect. However, when I plug just one end of the Aux cable into the iHome and touch the other end I get a very strong radio signal.
    Sorry if this question is too basic to answer, I just didn't know where else to get a knowledgeable response :)
    Kiril
     
  2. jcsd
  3. Sep 21, 2012 #2
    Re: Active speakers with built in radio pick up local radio when AUX cable is plugged

    The engineers who designed tube stereos 70 years ago prevented this by inserting a large resistor in series with the input stage. This created a lowpass filter when combined with the natural input capacitance of the first tube's grid.

    Now, products are engineered by selecting a chip that integrates as many functions as possible and then using the manufacturer's reference design directly. There may be little or no actual electrical engineering going on in the manufacture of your speaker. Or perhaps to lower the cost they hired an inexpensive inexperienced engineer who didn't know that the problem existed much less how to avoid it.

    Welcome to 2012.
     
  4. Sep 21, 2012 #3
    Re: Active speakers with built in radio pick up local radio when AUX cable is plugged

    Is there any pragmatic way for me to fix that problem? When I hold onto the port for the ipod it doesn't create the interference. Is there a way to jerry rig a resistor that i can attach to the ihome to use when I want to use an AUX cable?
     
  5. Sep 21, 2012 #4

    jim hardy

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    Re: Active speakers with built in radio pick up local radio when AUX cable is plugged

    i have no idea what is an ihome and heard once of ipods from grandkids.

    But it's apparent your port is detecting the radio station's signal. Let me guess- it's an AM station not far away?

    Your body is acting as a capacitor which shorts outthe radio signal when you touch the port
    and when you touch end of wire acts like more antenna

    see is a 100 picofarad (100pf) capacitor, probably 79 cents at Radio shack, connected across the "port" gets rid of the interfering radio station..

    perhaps somebody who's more familiar with modern stuff can help with mechanical connection details.
     
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