Filter Line Level Input

I have two amplifiers connected to the same line level input. One of the amps is creating noise on the line. The amp creating the noise is going to a sub woofer and it is filtered out via the low-pass filter so you do not hear the noise in the sub. However, the noise leaks back down the line level wire to the amp which then eventually makes its way over to the car door speakers and sounds like a high pitched whine that goes up and down as I accelerate the car engine. There are probably other ways to fix this problem, but what I am wanting to do, is just filter the signal from the sub amp from back-feeding to the door speaker amp. My first thought was to put a diode on the line level going to the sub amp so any noise cannot travel back down the line, but I read some where that this would mess up the sound of the sub. Does anyone know how I can filter out the noise so it doesn't travel back down the line?

Im think you have a ground loop between the two amplifiers either because their grounds or their +12V is fed from two physical locations. If both amplifiers have their power wires at the same locations then there will be no ground loop.

I have one ground wire that goes into a splitter connector then out to the amps.

jocanon said:

I have one ground wire that goes into a splitter connector then out to the amps.

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... And presumably a ground in the audio cable linking things.
That's all you need for a ground loop, more than one possible electrical ground path between two devices.

You can get an isolation transformer to go in line with the audio connection, which breaks the loop and will hopefully cure the problem, if that is the cause.

There are many types and they are not that expensive:
eg. https://www.amazon.com/Isolator-BOSS-Audio-B25N-Systems/dp/B000LP4RMG

And a few more:

jocanon said:

sounds like a high pitched whine that goes up and down as I accelerate the car engine.

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Sounds like alternator or ignition noise, much more common a few decades ago. As above, an iffy ground connection could be the culprit. Or, you might need an L-C filter on the power input to the amp.

ak