How to get rid of Interference on Car USB Adapter
October 23, 2010 11:54 PM   Subscribe

How can I get rid of the audio interference caused when I use a 12v charger with an mp3 player in my car?

I've got a late 80's full size ford truck that has a 3.5mm aux-in that I plug my mp3 player into. I have a 12v cigarette lighter to USB adapter that I use to charge the device.

I saw this thread, but it's doesn't answer all my questions.

When the device is charging, there is a high pitched hum that tracks with the engine rpms. So, it seems like some interference from the alternator. When the engine is off it charges fine and there is no interference.

So, my question is, which of the following would be the best option.

Are these Pyramid-NSXX items the right thing for the job?

This one in particular because of the high amperage.

Where would I install this? Can it go under the hood or do I need to get up in the dashboard?

This thing below seems to get rid of the noise after the mp3 player. Seems like a silly way to go about it. Do these things actually work compared to something that actually conditions the 12v power?

Then there is this one that uses RCA cables, but it has those two additional brown wires. What are those for?

Also, can anybody explain or give me a link to what is going on here? I'd like to know what, specifically , is causing the noise? What currents are actually going through the wire? Is it 12v DC plus some other AC current on top of it? Is that even possible? Also, what kind of components are in the devices linked above? Capacitors?

Finally, as a bonus question, does anybody know where one can find PDF versions of those Chilton repair manuals for ford trucks? I've searched high and low to no avail.

Thanks in advance!
posted by Popcorn to Technology (10 answers total)
look up "ground loops" - this is probably what's going on. it's small AC currents which happen to be in the audible range of human hearing, caused by the alternator as you have surmised.

here's a good thread.
posted by joeblough at 12:38 AM on October 24, 2010

Are you sure that the problem is the power from the cigarette lighter? Do you hear the interference when the car is on, and your mp3 player is playing but not charging? I'd imagine that an improperly shielded input line is more likely to cause audible interference than a wonky power supply. (Especially since most (all?) mp3 players should keep the audio data safe all the way to the jack, and that power fluctuations would just change the rate that the battery is recharged.)

Then there is this one that uses RCA cables, but it has those two additional brown wires. What are those for?

Those are so you can hook the filter directly into your car's speaker wiring (or your audio deck's output -- I can't really tell which way the device is supposed to be oriented).
posted by clorox at 12:40 AM on October 24, 2010

. . . power fluctuations would just change the rate that the battery is recharged.

Scratch that. After working it through a bit more, I think it might depend on how your mp3 player is designed (IANAEE).

All mp3 players have to process the digital info into a waveform, which is then sent to the speakers. To maximize battery life, this is probably done at very low power, and the signal is then amplified. In normal use (not plugged into anything), the amp draws on the battery, which provides a relatively clean power source. But your mp3 player may be designed to power the amp from USB when it's plugged in, instead of discharging and recharging the battery constantly.

Either way, check if you still hear the interference when you play an mp3 with the car on and the device not charging. If it's still there, then the output line is the problem; if it isn't, then the power supply is the problem.
posted by clorox at 1:04 AM on October 24, 2010

Alternators and ignition systems put out RF interference. It's probably that. Check and clean the grounds in the engine compartment.
posted by gjc at 6:04 AM on October 24, 2010

These problems can be hard to fix.

It's either conducted or radiated interference. From what you describe, it seems conducted. The fact that it changes with RPM suggests noise from the alternator or the rectifiers that turn its output into DC.

You may have to make some compromises to fix it. I would experiment with other chargers. The USB charger should not be a high current device. MP3 players usually have modest current requirements, so don't get anything huge. Designs vary, and if you can find one that has the parts you need in it, it's easier than explaining how to add them to your circuit to eliminate the noise.

Cars are not held to the same standards as other 'unintentional radiators' in FCC rules. It's a noisy environment.

Large chokes, capacitors, etc. are one way to reduce the noise, but it's vehicle specific and without looking at the source of the interference, I would definitely hit Office Depot or Best Buy, purchase 5 different units and choose the best. Return the rest. On to the next problem.

Good luck.
posted by FauxScot at 7:39 AM on October 24, 2010

I tried to fix this in my early days of building a CarPC about 10 years ago. The RadioShack Ground Loop Isolator helped, but did not fix the problem entirely. I eventually stuck with that, as the CarPC met an untimely stretch of "Wow, not really that useful to have a PC in my car."
posted by shinynewnick at 7:52 AM on October 24, 2010

A ground loop isolator inserted into the audio path between the MP3 player and the aux input took care of this in my car.
posted by Nothlit at 12:06 PM on October 24, 2010

More capacitors (separated by resistors) to make the 5v output far more buffered from the noisy input should do it (and would be cheap). It involves soldering (and acquiring the appropriate components), which from your focus on buying a product to do the job, seems like it's not a solution you're interested in, but it's a good solution.
posted by -harlequin- at 8:25 PM on October 24, 2010

Clorox, I'm sure it is the power from the the car that is causing the problem, when the engine is off the sound goes away and the device still charges. Also, the sound goes away if I unplug the usb cable when the engine is running.

I guess I'll give one of those RCA ground loop isolators a try first since they are so cheap. The fact that Nothlit says this completely solved the problem for him sounds encouraging. I'll give that a shot before trying anything that requires me to splice wires.

I'll report back on how it goes.

Harlequin, what do you think the current actually is at the cigarette lighter in the present state? Is it a rapidly oscillating wave that averages to 12v/R = I? What kind of amplitude and frequency are we talking about here? I'm assuming the amplitude is reliability small and the frequency is some multiple of the alternator RPM?

Thanks all!
posted by Popcorn at 9:47 PM on October 24, 2010

Also GJC, can you explain how that works with cleaning the grounds in the engine compartment? Are you saying that if this was a new version of the same car with a freshly wired electrical system that this wouldn't be a problem? What could be going on at the grounding points that would cause this? Would this be caused if the ground was totally disconnected? Or, is it that corrosion would increase the resistance to the current trying to get to ground? How does increased resistance allow an AC current to sneak onto the 12vdc?

I've heard people suggest that in the past and I've always been curious about the theory behind it.
posted by Popcorn at 9:53 PM on October 24, 2010

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