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After some tests I'm pretty sure that this subtle hiss that I mentioned before is coming from some OEM module of the car when it is on, as the other PDX amp (4.150) that I have is also showing some hiss after re-testing.
It seems to me now that when I tested the 4.150 that module was off (the car was off), so it made me think -incorrectly now- that I have a bad 4.100. But additional tests with different scenarios showed that regardless of the amp there is a very subtle hiss and even some times a faint whine that goes on and off at different times, like there is something else turning on and off even when the car is not running. None of these noises vary with engine speed or they increase with volume, so alternator or fuel pump interferrence is out of the picture. All this testing is done without any inputs to any of the amps...
There are still a couple of things that I can do to try to further minimize the hiss to acceptable levels, with better cables, shorter grounds and speaker wires, and better layout of all the components. I will not be able to completely eliminate the hiss as I'm using the Cleansweep remote volume at 75%, so I can use the OEM volume controls but that's acceptable for me.
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