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Unable to eliminate ground loop interference (car audio)

Discussion in 'Audio' started by clipper, Mar 11, 2018.

  1. clipper

    clipper

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    Dec 19, 2017
    I have adapted an old radio / cassette to accept a line in (aux) from my mobile / cell phone. Thread here:

    https://www.electronicspoint.com/threads/adding-line-in-to-an7465s-multiplexor.287201/#post-1759797

    On connecting the phone to the stereo in the car and reproducing audio from it, there are two issues:

    1) When the phone is connected via the 3.5mm stereo jack, the phone audio plays over the top of the radio audio, i.e. I am hearing both audio feeds through the speakers simultaneously. This issue can be "easily" solved however by plugging the charger into the phone. It seems that for some reason which I do not understand, when the phone has a power source, the audio signal is strong enough to "switch" the audio feed selected by the audio processor and silence the radio.

    2) The problem with this is that upon connecting the phone charger I am getting a significant amount of engine speed related whine and crackle.

    I am not so bothered about the phone needing to be connected to the charger as I like to take the opportunity to charge the phone in the car, but I cannot live with the ground loop interference.

    I assumed that the noise was due to the fact that I had the phone charger plugged into the cigarette lighter, whereas the stereo is connected to a seperate dash loom, so I installed a second lighter socket which takes its 12V+ from the same switched 12v supply as the stereo and also grounded the new socket to the stereo earth (in the dash loom), but this has made very little improvement (some, but not enough).

    I then suspected a poor earth through the dash loom, so I ran a new wire from the common stereo / charger earth to the car body earth lug (nice and clear bare steel), but this made no difference.

    The audio ground of the aux cable is connected to the stereo ground, and I have considered disconncting this, leaving only the positive audio channel feeds connected, but on the test bench this gave a lower volume audio feed from the aux input so I am reluctant to do this, but would it potentially help?

    Short of installing a ground loop isolator (last resort), does anyone have any ideas of anything else to try?
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    It sounds like your new input doesn't disconnect the existing radio signal when you plug in your phone.

    I would revise your modification and ensure it is correct.

    Plugging in the cable at the radio end only should shut off the radio and make the output go quiet (aside from a little nose perhaps).
     
  3. Bluejets

    Bluejets

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    Oct 5, 2014
  4. clipper

    clipper

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    Dec 19, 2017
    Thanks, but maybe I wasn't clear in my original post, but the new input does disconnect the existing radio signal as long as the phone which is plugged in is also connected to a charger.

    If, for example, I connect my phone with the media player reproducing a song, and lower the volume of the phone media on the phone itself to zero, I do not hear any radio audio in the background.

    But I do hear a great deal of noise when the engine is running.
     
  5. clipper

    clipper

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    Dec 19, 2017
    Thanks but no thanks!

    FM transmitters are a poor solution due to several issues (e.g. low emission power available to consumers without a licence). Also as I live in a busy city, there are very few free frequencies, and these are poorly regulated. Any attempt to use an FM transmitter in a car would simply result in a poor quality audio and frequent interruptions from pirate / unauthorised radio stations or other overlapping broadcasts.

    I can "solve" my issue by throwing money at it in better ways (e.g. an in line noise filter) but this isn't really solving it, it's masking it.
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    You were perfectly clear. I read the post where you describe your modification. The fact that you hear signal from both sources when the phone is plugged in tells me you haven't wired it up correctly.

    When you plug the charger in, the radio signal is probably shorted to ground through the power supply of your phone (again, because your mood was not done correctly -- you probably have signal and ground swapped). The fact that you also couple in noise is not surprising.
     
  7. clipper

    clipper

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    Dec 19, 2017
    Thanks!

    So when we are talking about performing the modification "correctly", we need to take into account that I am assuming that the help received in my original post is in fact correct. I sadly do not have the knowledge to verify if what was suggested to me is the optimal solution or not.

    The only part of the modification that I did not follow as per the suggestion in the original thread is the use of capacitors at the connection point, I do not know what the function or purpose of such capacitors would be so I left them out thinking that I could add them in later if needed.

    What I can be sure of is that the L & R audio signals are connected as per the schematic provided. I did not receive any guidance on connecting the audio ground, I just assumed that it could be connected to any ground pin on the PCB, could this be my mistake?
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    24,861
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    Jan 21, 2010
    The information you asked for, and received in that previous thread seems fine.

    However, what you report as a feature in your last post is actually a bug. You say that when there is a signal from the phone it overrides the signal from the radio, but as you've found, this isn't true -- they're just mixed together.

    With RCA sockets there's not much you could do without adding additional switching. With a 3.5mm jack, you should find that there are contacts that are opened when the plug is inserted (not all sockets have these). The idea being that inverting the plug disconnects the original source. For stereo, there needs to be two of these contacts.

    What you need to do is to break the original signal path, and route it via these contacts in a manner such that the contacts on the 3.5mm plug, when inserted, connect to the correct place.

    If, after wiring this up correctly, connecting the charger causes something odd to happen, it may just be that signal ground is separate from power ground -- something that may actually be more likely that I originally assumed. This may prevent you from using the phone while on a charger when connected to your amplifier.

    I'm not in a position to assist you with the nitty-gritty of this work at the moment, but maybe @73's de Edd can assist?
     
  9. Bluejets

    Bluejets

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    Oct 5, 2014
    I would have thought a filter would be a perfectly acceptable solution to noise.
     
  10. clipper

    clipper

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    Dec 19, 2017
    Thanks again *steve*, I appreciate your help so far. I respond to your post in the hope that someone else can help!!

    I'm not to sure what you mean by the 3.5mm jack "having contacts that are opened when the plug is inserted", my jack is a very simple 3.5mm jack with no internal switching, soldered directly to the three wires of the audio cable which is connected to the PCB as follows:

    Signal channels pass through two mirco-switches which I use to isolate the aux cable feed when I want uninterupted radio playback without having to physically disconnect the 3.5mm jack from the phone.

    Audio ground is connected directly to a ground pin of the PCB (same ground as the power ground of the car stereo. At the moment I have disconnected the audio ground at the 3.5mm jack end to see if that helped, but it made no difference.
     
  11. clipper

    clipper

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    Dec 19, 2017
    Yes, I am beginning to think the same!!
     
  12. clipper

    clipper

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    Dec 19, 2017
    A quick update on this:

    After rewiring everything again from scratch to make sure I had done everything correctly, I was still faced with terrible interference.

    I tried with the steel wire shield of the aux cable connected as a ground and without it connected, but still, not much difference.

    So I bought a ground loop isolator, but sadly the way that the line in is connected to the stereo's circuit means that the line in only overrides the radio signal when the connection is direct. So when the filter is used, I lose the ground loop interference, but I have both the aux signal and the radio signal playing through the amp simultaneously.

    The positive side of this discovery is that there are now two solutions open to me:

    1) Eliminate the gound loop noise so that the line in can be used without a filter (this is proving to be very difficult) or
    2) Find a way to "force" the stereo to mute the radio signal when the line in is used.

    Can anyone help me with implementing a manual mute function to the radio signal?
     
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