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This is a question of curiosity. I have a '98 Acura Integra, a cheap car stereo with a 3.5mm aux in, a cheap inverter and a smart phone. If I have the smart phone audio hooked up to the car stereo and the inverter charging the phone I get a high pitched tone through the speakers at around 4k RPM that increases in pitch with the RPM of the engine.

I know the solution is to not use the inverter, there is no reason to be doing that, but I'm curious why the pitch of the noise fluctuates with the engine RPM. Shouldn't the output of that DC outlet be mostly consistent?

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3 Answers 3

up vote 5 down vote accepted

Most of the time a ground loop is the cause of this problem. This problem is common especially if your stereos speakers are amplified, and can be fixed my making sure that your stereo unit and/or amplifier have a good ground connection. This can be done by either finding a more direct ground connection to the cars frame, sanding the area around your ground connection to remove any rust or paint that could be adding grounding resistance, or even running a ground wire from your stereo or amp directly to your battery.

In fact, it is most likely that you are constantly experiencing the whining sound. With the engine idling if you put your ear up to the speaker you will probably hear the sound, it just doesn't become noticeable until higher revs.

Note: Making sure any wires you add (+ or -) are properly insulated always helps too!

See this link for a full description.


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Good point - I hadn't even thought of ground loops in this context. +1 –  Rory Alsop May 5 '12 at 12:03

When converting the 12V DC from your battery to whatever voltage the inverter puts out, there are conversion steps internally to AC and back through switching the voltage.

The switching circuit frequency is probably sensitive to the voltage, and as this varies (can be from 11 to even 15v in some cars) that could change the speed of the switching stage.

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I'd check to make sure no power wire is near the spark plug wires. That's caused similar sound for me in my '88 civic wagon with aftermarket stereo. RPM dependant, almost like a turbo whine hahah. 3yrs old but maybe this could help someone else?

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