[jonywalker]

I posted this problem before, and its been getting more and more annoying day after day since I replaced the factory amp with alpine PDX-4.100. I tried turn the input sensitivity of PDX to minimum, and conntect the ground of headunit and amp together, but it doesn't seem to work. This HISS didn't exist before at least not as strong as it is now. However the level of this noise is constant in relation to the headunit volume, therefore the lower the volume I turn the more demonstrative it is. However, if I turn the volume of headunit to zero there is no noise output from amp anymore. But as soon as I turn the volume larger than zero the noice appears and keeps constant.
1. If the noise came from amp why there is no noise if the headunit doesn't output?
2. If it were from headunit why it was not present with the factory amp?

The alpine PDX has a S/N only 76 dB, maybe this is the reason. Did anyone who also has PDX amp experience with the same problem?
I could not imagine that the alpine has so bad signal to noise rate, it is a downgrading!

Attached is my installation diagram: