I have one tiny 40W amp powering the front and roll bar speakers which is under the drivers seat. I have a 400W amp powering the 10" sub in my trunk (the amp is under the rear seat). Both amps are grounded to a bolt under the center console, direct to the chassis. I don't recall what gauge wire I used, but I will check.

In my installation, I have a tiny cheap amp powering the front speakers, and larger expensive amps powering everything else. Although I've reduced my noise problems to my satisfaction (by reducing the amp gains), I'm pretty sure the existing noise problems are all due to ground loops between that cheap little amp and everything else. If I remove that amp from the mixture, all the noise goes away completely.

The next upgrade I make to my system will be to replace the front speakers and the front amp.

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Tony Fabris