Noise only when Engine is running (Buzzing sound - increase with gain boost)

NorthStyle

A few things:

1) If the buzzing/noise (interference) was there before, adding another amp just amplified it (no pun intended).

2) You can't ground the audio outputs, each speaker has a +/- that needs to be connected to complete the series. You removed it, didn't complete the series, and thus lost sound (not sure how that would work if you grounded both the input on the speaker side and the output on the headunit/amp side, but never been tempted to find out). That being said: I'd replace them with good quality RCAs from the headunit to the amps.

3) Check your grounds and their distance. I've always been taught that grounds should be as short as possible. Where are your amps located? If they're somewhere in the trunk, and you're running your ground to a seat, that's a pretty long distance. Also, I'd ground each amp to a different point.

4) If you're going to run your amps off of the same cable, use a power distribution block to separate them. For example: I only have a single amp, but my compressor and crossover require power as well; so I have a 0 gauge (fused, forgot the size) from the battery to the distro block, then a 4 gauge (fused) to the amp; and two 8/10 gauge cables (each, fused) to the compressor and the crossover. Each has a different ground. The only thing that is shared is the remote/signal wire from the headunit to the amp which then feeds the crossover.

My advice would be to trace the initial source of interference. I'm sure BigAudioFanat or RonRizz will chime in later as both of them are leagues ahead of me knowledge-wise.

SouthArk370Z

They both have a lot of experience with modern systems and have been very helpful to others. Although the basics haven't changed over the years, some of the details have (we never had to worry about such things as LOCs back in the Good Old Days).

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bangity

I placed the armp (it's quite small) right under the passenger seat. So the ground cable is about 1ft in length. I used a 4 gauge cable for power and ground even though 8 gauge is adequate for the amount of RMS (100watt RMS constant, 300 RMS max). The sub itself has only 100 watt RMS. So the 4 gauge should be adequate for a total of 200watt RMS max.

I have never used the RCA inputs. I will try that today as I have 2 pairs of RCAs.

I am also considering ground loop. There is a ground bolt on the chassis under the kick plate of the passenger side. Maybe move the ground point to here? This however will lengthen the ground cable to about 3-4ft.

I have no idea where the source of interference is. When I remove the high level input from the HU to the amp, there is no sound and thus no buzzing. Therefore, it's probably somewhere from the HU to the high input of the amp?

What's the different between Low Level (RCA) vs High Level (audio +/- cable)?

I have turned the gain all the way down but it's still there. As is now, I can't use any gain at all.

Thank you for your time.

Edit: Just ordered 3 RCA to 3RCA. The Sub(Coax) will be with a Y-splitter to the Sub as it accepts 2 channels. Also ordered a ground noise isolator just in case for the RCA.