Sena SM10: Noise in audio system


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dtrider
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Sena SM10: Noise in audio system

Postby dtrider » Sun Mar 20, 2016 6:18 pm



My wife and I recently got new helmets and we decided to go back to using wireless bluetooth headsets for them instead of moving the J&M wired headsets over from our old helmets. We went with the Sena 20s headsets (lets us use the intercom and listen to music at the same time) paired with a Sena SM10 dual bluetooth transmitter so that we can listen to the same music source. I have the SM10 mounted on the left handlebar and use an adapter to connect it to the bike's audio system through the driver's headset connector.

Everything was working fine when I first set it up, but today we started to pick up some noise from the bike. With the engine running there is a whine that changes frequency with the engine speed, and with the engine off but the ignition on or in the ACC position I can hear a steady clicking. If I switch back to the wired headsets there is no noise.

The noise isn't horribly loud and when on the road with the music playing it's barely noticeable, but it is there. I would like to find a way to eliminate it if possible. Any thoughts? Would moving the SM10 to a different spot help? Do I need to find a better way to tap into the bike's sound system?

Thanks
/dwight


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Re: Sena SM10: Noise in audio system

Postby dtrider » Sun Mar 20, 2016 7:16 pm

One update - I just found that if I disconnect the power from the SM10 so that it is running on it's internal battery, the noise goes away. So apparently I'm picking up a signal through the power system. Any ideas on how I can isolate it? Currently I have it connected to a USB power outlet that is tapped into the bike's power system under the left-side glove box.
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Re: Sena SM10: Noise in audio system

Postby WingAdmin » Mon Mar 21, 2016 7:05 am

dtrider wrote:One update - I just found that if I disconnect the power from the SM10 so that it is running on it's internal battery, the noise goes away. So apparently I'm picking up a signal through the power system. Any ideas on how I can isolate it? Currently I have it connected to a USB power outlet that is tapped into the bike's power system under the left-side glove box.


That was going to be my first suggestion - disconnect the power.

What this indicates is that you have a ground loop.

In a perfect world, everything that is ground would be exactly the same. So the ground wire that is part of the power supply for the SM10 would be exactly the same as the ground wire that is on the headset connector, as part of the audio signal.

In vehicles, this is never the case. Because of EMI, the resistance of wire, and other reasons, there will always be a difference in potential between two "ground" wires on the bike. This difference can be very tiny - in the range of millivolts (1/1000 volt). However, audio signals themselves can be measured in millivolts - so if you have a difference between the grounds (say, injected from the alternator), this will be picked up in the audio signal and amplified. This is called a ground loop.

The solution is to use a ground loop interrupter. This is essentially a transformer, that allows the audio signal to be transferred across, but without actually connecting the ground wire - thereby breaking the ground loop.

This simple and cheap device should fix your problem:

http://www.amazon.com/Mpow-Ground-Isola ... ingdocs-20

Plug this device into the audio input on the SM10, and plug the wire coming from the headset plug into this device. Your noise should go away.

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dtrider
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Re: Sena SM10: Noise in audio system

Postby dtrider » Mon Mar 21, 2016 10:07 am

Thanks. I was kind of thinking that it would turn out to be a ground loop, but since I've never run into one before I wasn't sure. What really threw me was that the buzz only showed up yesterday even though I installed everything a couple weeks ago. I was thinking that something had come loose in my power connection and I was going to have to redo that to add isolation there.

The device you posted the link for looks like a simple enough fix. But before I go and order one though, is there a harness that already incorporates one? As I mentioned, I currently have the audio input to the SM10 connected to the driver's headset connector with a 5-pin to 3.5mm adapter. It works, but looks pretty cheesy. Adding this isolation will add yet another module that I'll need to secure somewhere.

What I would really like to have is a harness that would let me tap directly into the bike's sound system without going through the headset connector. I saw that Sierra Electronics has something close: http://www.sierra-mc.com/proddetail.asp ... A-GW-SMH10. If I understand it correctly it looks like the main idea of this harness is to use an SMH10 headset as a dongle that is hardwired to the bikes audio system and then paired by bluetooth to the headset on your helmet. It also has a cable to connect to an SM10 for stereo music streaming. The entire harness is overkill for me since I don't really need the SMH10 side of it, but a harness for just the SM10 audio connection would be nice in order to keep the wiring clean. Maybe I need to call Sierra to see if they have any suggestions. They may even already have something that would include the ground loop isolation.
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Re: Sena SM10: Noise in audio system

Postby dtrider » Mon Mar 28, 2016 11:56 am

Just wanted to follow up on this. I got the ground loop interrupter that WA suggested, and it seems to have completely eliminated the problem. We took a short ride on Saturday and my wife didn't notice any engine whine at all in her headset (her hearing is much better than mine). So she is happy again and as we all know, a happy passenger helps to make a happy driver.

Thanks.
/dwight


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