Hello, I'm new here and I'm looking for some help diagnosing an issue I've had for a while now.
I have a MB Quart FX4.50 amp plugged into my Clarion NX501 head unit and I get a hissing sound on pretty much any gain level I set, the hiss is less noticeable with the gain all the way down, but even then it is still present.

Here are somethings I've tried already.

1. Grounding HU to firewall
2. Setting lower gains on the amp
3. Different set of RCA cables
4. 3 Different HUs
5. Noise Filter on the 12 amp Accessory

Also, the hiss goes away when I use another audio source instead of the head unit, I tried connecting a regular dvd player and played a cd and there was no hiss present. This was done using the same RCA cable that runs to the head unit.

Is there anything else I can try to fix this? Any help will be greatly appreciated.


UPDATE:

I started messing with the 12 Amp InLine Noise Suppressor that I have and noticed that I forgot to ground it, so I did and the hiss became even louder than before. I would guess a good 3-4 times louder. This is when the engine is on, if I only turn on the accessories, then the hiss almost disappears.

UPDATE2:

I tried connecting the head unit's 12V directly to the battery and the noise almost disappears.

UPDATE3:

I ran a power wire from my power distribution block directly to the head unit and the hiss goes away but then I get some alternator whine. I'll just leave everything the way it was for now since I rather have a hiss than a whine on my speakers. I ordered a ground loop isolator and will give that a try tomorrow and hopefully that will fix my problem.

UPDATE4:

I gave up and bought a PAC SNI1 Noise Isolator and after installation all the noise went away along with a little bit of sound quality but that is something that I'm willing to give up in exchange of not having that annoying hiss.

 

What type of RCAs? Are they grounded at both ends? If so remove the ground at one end. You may have a ground loop.

 

I had a cheap one from ebay but I replaced it almost right away with a Krystal Kable (Knukonceptz) RCA cable.

How can I remove the ground from a RCA cable?

 

I solved a similar problem by buying a 2 gauge cable with lugs on each end and connecting one side to the negative battery terminal and the other to a sanded bare point down on the frame in the engine bay. I also took off the thick aircraft cable body ties under the cab and sanded the points where they attach for a very secure body to chassis ground. I used some sealant to cover the connections and protect from oxidation. I believe there is a stock ground cable that runs from the frame to the engine block...sand the contact points and once again seal them with something tough, I use automotive GOOP glue.

 

Based on your recommendation I have cleaned the terminals and contact points, and while I was at it I upgraded the "big 3" with 0 gauge wire, but I still get the same hiss noise as before. I'm kinda convinced that the noise comes from the head unit in some way but I haven't found the exact cause of it.

 

You can buy them that way and they will usually have an arrow on them telling you which way to use them or you can cut one end off and replace the connector so that only the center pin is connected.

 

Thanks. I removed the ground from one of my old RCAs and the noise remained the same, I could even say that it was just a little bit louder than before.

Also, I tried connecting the head unit's 12V directly to the battery and the noise can be barely heard. I would like to connect both the power and ground wires directly to the battery but I don't have enough wire to do it.

Does anyone think it would be a good idea to get power and ground directly from the battery?

 

Thanks to everyone for your advice in solving this issue, but I finally gave up. I went and bought a PAC SNI1 Noise Isolator and after installation all the noise went away along with a little bit of sound quality but that is something that I'm willing to give up in exchange of not having that annoying hiss.

 

If you now set up your gain correctly you may get some of the sound quality back.