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2009 Subaru Outback 2.5i PZEV Special Edition
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Discussion Starter · #1 ·
Hey all audio nerds!
My Scosche BT-FM transmitter unit emits a ringing noise/high hum that seems to be related to the RPM of the engine, the higher my RPM the higher the pitch of the hum and vice versa - lower RPM, lower hum. This doesn't happen if I plug it into the AUX port of the car, only if it is FM transmitting mode. Does this mean that there is a larger problem in the electronic system of the car (grounding?)
TIA
Bobby G
 

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2015 Outback Limited 2.5 & 2019 Outback Touring 2.5
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122 Posts
Is it possible to try a different frequency? If not, usually you could try ferrite clamps, capacitors, or various noise filters.

Noise Filters
 

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2009 Subaru Outback 2.5i PZEV Special Edition
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Discussion Starter · #3 ·
Is it possible to try a different frequency? If not, usually you could try ferrite clamps, capacitors, or various noise filters.

Noise Filters
I didn't even know 12v noise filters existed, Thank you!
 

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2012 OB , 2017 Impreza
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3,672 Posts
If the noise follows the RPM of the engine, it is likely noise from the ALTERNATOR. (ignition-noise is more like a "static" sound)

Back in my younger days, when I was into car-stereos, I would have to deal with alternator-whine. An electronic "filter" between the +12v feed and chassis (as close to the stereo as possible) would dissipate the noise.

Often, a properly-valued capacitor between +12 -and- ground would be sufficient. Sometimes, I added a choke (tuned to the frequency of the alternator-whine) in series with the +12 feed.

If you are not into electronic circuit design, you can simply purchase a premade "filter" to install on the power-feed of the radio. The main specification to be concerned with is the current-rating for the "filter" is higher than the radio can draw.

Use your favorite web-search tool to look for "12v noise suppressor" or "12v noise filter"
 

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2010 2.5 CVT Limited
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1,924 Posts
If you are not into electronic circuit design, you can simply purchase a premade "filter" to install on the power-feed of the radio.
Wouldn't it be better to put the filter on the power feed of the BT-FM adapter? Nip it at the source.
 

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2009 Subaru Outback 2.5i PZEV Special Edition
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21 Posts
Discussion Starter · #7 ·
If the noise follows the RPM of the engine, it is likely noise from the ALTERNATOR. (ignition-noise is more like a "static" sound)

Back in my younger days, when I was into car-stereos, I would have to deal with alternator-whine. An electronic "filter" between the +12v feed and chassis (as close to the stereo as possible) would dissipate the noise.

Often, a properly-valued capacitor between +12 -and- ground would be sufficient. Sometimes, I added a choke (tuned to the frequency of the alternator-whine) in series with the +12 feed.

If you are not into electronic circuit design, you can simply purchase a premade "filter" to install on the power-feed of the radio. The main specification to be concerned with is the current-rating for the "filter" is higher than the radio can draw.

Use your favorite web-search tool to look for "12v noise suppressor" or "12v noise filter"
I thought that the alternator might be the cause of the problem. I will look into adding one in once the weather is warmer lol
 
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