Accord loud "hum" using high level input

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BreadLord

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Hey guys, I got my girlfriend a subwoofer setup for chirstmas and I got it installed today but was met with issues. The amp is capable of taking high level inputs into the RCA and is also able to switch on when the radio is when configured this way so no need for a remote wire. Anyway I got everything in, RCAs tied to the rear speakers (In correct polarity), and everything played fine. Sounded good, but when I closed the trunk the sub immediately started playing a very loud tone. I checked both the ground to body at battery and ground to sub in trunk, both are bare metal. I have no idea what's causing it, granted I'm new. So recap, RCA is taking in high level input from rear speakers, and everything works fine so long as the trunk is open. Grounds are both bare metal.

I also put my own battery in the trunk, disconnected the amp from the car power and used the other battery for power, same thing. 

I also disconnected the RCAs and connected an RCA to 3.5mm jack to my phone and amp, sub played fine even when using the car power and trunk closed.

That tells me it's the high level input setup (I think) but I don't know why. I even took alligator clips onto each lead of the rear right speaker and put them onto a stripped RCA connector. Put this RCA into a splitter and then that into amp. I then used a jumper from 12v to remote to turn on the amp. Same thing happened but with trunk open and radio off. (In all instances engine was off).

The bolt being used in trunk is a factory grounding bolt for the taillights. I brought it to bare metal under the bolt. Same thing I did in my friends corrolla which worked fine. 

Probably (99.9%) my fault, but I'm having a hard time and I cant find help elsewhere. 

 
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when the trunk is closed, the supports can affect wiring, causing shorts or other issues. 

when you connect the RCA wires, they need to be insulated with electrical tape so each wire is protected, meaning they cannot touch positive and negative.  This can happen when wires are moved by the trunk supports if they aren't taped properly.   pics would help.

 
when the trunk is closed, the supports can affect wiring, causing shorts or other issues. 

when you connect the RCA wires, they need to be insulated with electrical tape so each wire is protected, meaning they cannot touch positive and negative.  This can happen when wires are moved by the trunk supports if they aren't taped properly.   pics would help.
I isolated both RCA wires as best I could, even used a multimeter to test for a short on them. 

I've ditched that system and got a more proper setup with a pac oem1 LOC. Works great, but there is still somewhat of an issue. The same loud tone will be produced If amp is turned on, connected to the LOC via RCA and the radio is off. I jumped the rem to 12v with the radio off and it immediately made the tone. However using the rem wire from the LOC works perfectly as it's only on when the radio is outputting signal. (Trunk can be closed fine now) 

So basically the noise only happens when the RCA (coming from the line out converter that's plugged into the stock harness) is plugged in and the amp is manually turned on. Doesnt happen when the remote wire from the LOC turns on the amp as it is then recieving input. Still troubling as it still has the potential to happen and I dont want there to be an accident due to a sudden deafening noise. Again, both grounds are bare metal. Is there anyway or device that can stop this? Thinking about a pac sni-1 noise isolator but unsure if the problem is actually a ground loop. Seems rca related so it might work.

Like to add that the trunk issue is gone. 

 
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It's not uncommon to have noise when the amp is on but the HU is off, but it shouldn't be a very loud noise.  Keep your gains on the amp as low as possible and get gain as early in the signal chain as is possible (i.e. use most of the head unit volume range and use gain at the LOC. 

Are you forcing 12V back into the LOC remote output when you manually turn the amp on or did you disconnect the LOC when you did the test?

 
It's not uncommon to have noise when the amp is on but the HU is off, but it shouldn't be a very loud noise.  Keep your gains on the amp as low as possible and get gain as early in the signal chain as is possible (i.e. use most of the head unit volume range and use gain at the LOC. 

Are you forcing 12V back into the LOC remote output when you manually turn the amp on or did you disconnect the LOC when you did the test?
I had the LOC gains up to about 65-75% (was a rough setup just to test for issue) and had the amp gain far down (below a "normal" marker on the dial) probably 20 to 30 percent, bass boost off. Sounded nice and clean. Powered off car and powered on amp, noise happened again and is still pretty dang loud but obviously not as loud with amp gains down. It may be some sort of weird ground loop I have no clue (not a lot of experience in this field). 

But yeah HU bass EQ 0, LOC a rough 75% and sounding clean, AMP gain way down for testing. Works perfect when radio on. 

If I'm not mistaken I didnt have the remote wire ran when I tested for the noise the first time. Pretty sure I had the RCA running outside the car into the front window and into the LOC. Car was off and I powered on the amp. Still issue

Edit: I have an oscilloscope (cheap one) if testing for specific wave forms would help at all. Ground loop noise isolator is in the mail

 
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Well just to follow up that the pac sni-1 noise isolator took care of the issue!. I installed it today and, with the car off, powered on the sub and no noise! Guess maybe it was a ground loop? It was quite loud and my grounds are bare metal...I don't honestly know a lot about ground loops though. Either way the isolator fixed the issue. 

Another quick question: the sni-1 has two brown wires that are only to be used if there is no output from the sub/amp. I'm not using them and I am getting output but a decent amount quiter. To compensate I have to turn my amp gains *almost* too high to be comfortable with. Would using these 2 ground wires bring my level back up where it used to be before it got to the amp? Thanks 

And thanks keep_hope_alive for taking time to respond. 

 
Well just to follow up that the pac sni-1 noise isolator took care of the issue!. I installed it today and, with the car off, powered on the sub and no noise! Guess maybe it was a ground loop? It was quite loud and my grounds are bare metal...I don't honestly know a lot about ground loops though. Either way the isolator fixed the issue. 

Another quick question: the sni-1 has two brown wires that are only to be used if there is no output from the sub/amp. I'm not using them and I am getting output but a decent amount quiter. To compensate I have to turn my amp gains *almost* too high to be comfortable with. Would using these 2 ground wires bring my level back up where it used to be before it got to the amp? Thanks 

And thanks keep_hope_alive for taking time to respond. 
The noise isolator is reported to attenuate the signal.  Glad it solved your issue, ground loops are difficult to resolve sometimes.   OK to increase output from the LOC to compensate.

 
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