How to kill ignition noise

Thread Starter

mikejp56

Joined Jun 14, 2015
69
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
 

MikeML

Joined Oct 2, 2009
5,444
"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.
 

Thread Starter

mikejp56

Joined Jun 14, 2015
69
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
 

MikeML

Joined Oct 2, 2009
5,444
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?
 

MikeML

Joined Oct 2, 2009
5,444
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?
 

Thread Starter

mikejp56

Joined Jun 14, 2015
69
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
 

CGingras

Joined Dec 29, 2015
2
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.
 

MikeML

Joined Oct 2, 2009
5,444
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.....
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...
 
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