How to kill ignition noise

Discussion in 'Automotive Electronics' started by mikejp56, Aug 7, 2015.

  1. mikejp56

    Thread Starter Member

    Jun 14, 2015
    49
    1
    Hi All,
    I have a simple problem. I am using an FM modulator to listen to music through my car stereo because my cassette player has bought the farm. I connected it to it's 12 volt adapter, and the ignition noise is awful. I rigged up a 3 volt regulator with 10uF and .1uF caps in parallel on both the input and output. I connected it to my car power with a USB cable. The noise was better, but still noticeable. Any ideas on how to kill the rest of the noise?
    Regards,
    mikejp56
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    "Ignition noise" is radio-frequency noise radiated from car's ignition wires/plugs to radio's antenna. Sounds like popping corn, except more regular rythm. Speeds up with increasing engine rpm.

    "Alternator Whine" is a more musical howl that increases pitch with increasing engine rpm. I think you are likely hearing whine.

    We need to determine which it is before recommending a solution.
     
  3. mikejp56

    Thread Starter Member

    Jun 14, 2015
    49
    1
    Hi MikeML,
    Actually it is neither. It is a constant noise that doesn't change whether I use the ACC position of the key, or if the car is started and idling, or revving. It is just there constantly. But if I use the modulator with its 2 AAA batteries the noise is almost gone. I guess the modulator is just a POS?
    Regards,
    mikejp56
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    Then it is not ignition noise or otherwise related to the car itself. Likely, the regulator you built is oscillating, possibly at an audible frequency. Did you use all of the bypass capacitors shown on the regulator's data sheet?
     
  5. mikejp56

    Thread Starter Member

    Jun 14, 2015
    49
    1
    I did. And the USB cable that I used has the inline toroids on both ends.
     
  6. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    Is the regulator switch-mode, or a linear like a LM317?
     
  7. mikejp56

    Thread Starter Member

    Jun 14, 2015
    49
    1
  8. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    You implied that the modulator makes some noise, even when powered with batteries?
    Does it do this if you take it in the house and listen on an indoor FM receiver?
     
  9. mikejp56

    Thread Starter Member

    Jun 14, 2015
    49
    1
    I do not know. I will try it when I get home from work later and post the answer.
    Regards,
    mikejp56
     
  10. ScottWang

    Moderator

    Aug 23, 2012
    4,854
    767
    Wow, Two Mikes conversations ... :)
    Talking about the noise, but I can't hear any sounds.
     
  11. mikejp56

    Thread Starter Member

    Jun 14, 2015
    49
    1
    Hi MikeML,
    I neatly ran the wires and tucked the unit under the dash. It made an improvement, so I am just going to leave it the way it is. It is tolerable.
    Thanks for your responses!
    Regards,
    mikejp56
     
  12. ISB123

    Well-Known Member

    May 21, 2014
    1,239
    527
    Try wrapping it in Alu. foil.
     
  13. TheButtonThief

    Active Member

    Feb 26, 2011
    219
    38
    Then it won't work at all =/

    ...unless you just mean the wires
     
  14. CGingras

    New Member

    Dec 29, 2015
    2
    1
    My experience with a cell phone with the headphone output connected to the car stereo "AUX" input playing very well, no noise at all. However, an annoying noise appear when I also plug the USB charger going from the cigarette lighter thru a small switching power supply then to the micro-USB connector on the phone.

    The noise is not related to the spark plug of running engine or any other device. It is actually a "ground loop". The headphone ground and USB ground are connected together inside the phone. This ground loop doesn't need more than a few miili-volt to be heard on the sensitive AUX input.

    The ground loop noise reveals a possible danger to damage the cell phone in the case where the cigarette lighter would be powering many devices, such as a power inverter. In case of bad contact to the ground side (cigarette lighters connectora are very prone to bad contact), the cell phone could end-up delivering many amp starting from the headphone connector, going thru the phone PCB, then along the micro-USB cable, finally going thru the power inverter, for that example. Cell phones are built to resist incredible abuses, so they may survive such brutal shock with some luck.
    Good luck all.
     
  15. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    A two-winding audio transformer (with galvanic isolation from primary to secondary) inserted in the audio wiring between the cell phone out and aux in will cure it. You buy one of these; they work like a champ to cure this problem...
     
Loading...