FM stereo interfence, Sensibulb LEDs and 12V Jacks - SailNet Community
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post #1 of 11 Old 03-13-2012 Thread Starter
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FM stereo interfence, Sensibulb LEDs and 12V Jacks

Hi folks,
I'm trying to troubleshoot some interference I get on the FM stereo, and see what I can do to fix it. First, some specs:

- Stereo, lighting, and 12V outlet/jacks are each on a separate breaker
- All 3 that are problem-causing are within a couple feet of each other

Second, the problem:

- When I plug a 12V phone USB charging adapter into 12V jack, then plug in my phone, I get interference in the FM as soon as the phone starts charging. This does not happen in my car. The charger does not cause interference unless the phone is plugged into it.

- I have four dome lights with Sensibulbs. The two that are in the forward cabin, far away from the stereo cause no interference. The two closer to the stereo do cause interference. It's very mild with one on, but much more noticeable with two on.

I bought Sensibulbs because of their reported low RF noise. I also have a MarineBeam LED light also very close to the stereo that seems to cause no noise on the stereo.

Soooo....my question is, are these two problems related? Is there something I should be doing to minimize the interference on the stereo side? Or do I just need different LED bulbs and a different 12V phone charger? Any way to troubleshoot this?

Thanks,
J

1984 Sabre 34 Mk I
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post #2 of 11 Old 03-13-2012
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Re: FM stereo interfence, Sensibulb LEDs and 12V Jacks

Your radio sounds sensitive to noise. On which lines, I don't know, but maybe the power supply wires - GND and 12V.

Does it have a shielded antenna cable or just a wire stuffed into the antenna socket (as mine does)? If the latter, switch to the former.

You could try putting a ferrite on the power supply wires, as close as possible to the radio.

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Re: FM stereo interfence, Sensibulb LEDs and 12V Jacks

The antenna cable looks like any standard antenna cable I've seen in a car or on a boat. That should be shielded, yes?

There are no ferrite cores installed on any of the stereo, if I recall correctly.

Certainly worth a try.

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Re: FM stereo interfence, Sensibulb LEDs and 12V Jacks

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Originally Posted by josrulz View Post
The antenna cable looks like any standard antenna cable I've seen in a car or on a boat. That should be shielded, yes?

There are no ferrite cores installed on any of the stereo, if I recall correctly.

Certainly worth a try.
Yes it should be shielded. Mine just has a thin piece of ordinary wire for an antenna.

So what you do is take both the live and ground wire, pass them through the hole in the middle of the ferrite, then wrap them several times around the ferrite.

Like the top picture here but more wraps : Filtering Out Interference Signals with Cable Ferrites

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Re: FM stereo interfence, Sensibulb LEDs and 12V Jacks

Thanks Mark. I'll give it a shot.

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Re: FM stereo interfence, Sensibulb LEDs and 12V Jacks

I have a similar situation with the 12V power supply for an XM MyFi satellite radio, which contains a very (electrically) noisy switching power supply. The situation is even worse if I use the hardware audio connection to Line-In on the stereo, despite several attempts at filtering. In other words, you may need more than a few loops through a ferrite.

Once upon a time car stereo shops had L/C (inductor/capacitor) filter assemblies to suppress alternator whine and such; you might have some luck with something like that if the ferrites aren't enough. Some info (and a caution on filters) can be found at http://www.carstereo.com/help/Articles.cfm?id=71

Apparently my high-frequency hearing is still pretty good, because several guests 10 years younger than I don't notice the whine, which my wife and I find intolerable. For now the workaround is to use the MyFi's internal FM-modulator to feed the radio tuner, but that's a compromise in quality and necessitates scanning and picking a new frequency as we move from place to place.

Next step will be more aggressive filtering, more direct power grounds (currently radio and accessories take their negative from the small busbars on the back of the breaker panels nearest their breaker). I think a heavy-duty common negative rail before the panels' pigtails would avoid some common-rail noise from running each "ground" all the way up to the breaker and back. I may try a pair of small 600-ohm audio transformers on the Line-In to determine if the noise is indeed coming in on the LineIn shields, vs power supply ground.

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Last edited by redline; 03-14-2012 at 03:56 PM. Reason: add filter link
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Re: FM stereo interfence, Sensibulb LEDs and 12V Jacks

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Phone chargers are some LED control circuitry are digital circuits these days, and they can make noise. If the noise if RFI, radio frequency interference that is broadcast through the air, that's hard to stop. If it is EMI, electromagnetic interference that is being carried out on the power lines, it can be easily filtered out.

Simplest way to tell which it is, is to bring another battery powered FM radio into the boat. Since that is not connected to the same power leads, it will have no problem if the interference is EMI. In which case you put noise filters on the radio's power branch, and on the ones to the charger and cabin lights as well. Those filters can be simple "noise filters" that you pick up at an auto stereo shop, or sometimes just 10mF capacitors will do the job well enough. (25c or so each?)

If the separate radio stills gets interference, it is RFI and you have to stop that at the source. A better charger and different lights might be called for, or moving the FM radios antenna further away from those noise sources.
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Re: FM stereo interfence, Sensibulb LEDs and 12V Jacks

Thanks everyone.

Hellosailor, that sounds like a great place to start. I have a battery-powered radio I can bring out easily to see if I still get the interference.

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Re: FM stereo interfence, Sensibulb LEDs and 12V Jacks

Hellosailor, for the purposes of this test, could the "other" radio be AC powered, from a power source not connected to the boat? Or is there some reason it would need to be battery powered?

Thanks!
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Re: FM stereo interfence, Sensibulb LEDs and 12V Jacks

AC with an extension cord, totally isolated from the boat's electrics, ought to work. AC can bring in other noise sources, battery radios used to be easy to find. And easier to move around, up close to other devices to see which is the culprit.
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