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?