New amp, static noise subwoofeers?

Jonas84

CarAudio.com Newbie
Nov 21, 2022
6
2
So in my bmw there is a little oem amp in the trunk that drives the oem subwoofers under driver and passenger seat.

Decided to use a amp i know works lying around.

Spliced the input wires to rca and hooked all up and used a separate battery to test out. Played Great.

So i went on with installation.

Checked ohm to speakers 2.5 on both.
Checked ground and positive 0.2 and 0.1 loss in ohm.
0.2 loss Voltage to amp unloaded.

But as soon as I put on the ignition and factory unit starts upp there is a constant humming?

If i use external battery all is good.
If I use rca to amp from ex my laptop but cars battery all is good.

It just dont wanna play together... Why is it working with stock amp?

One thing i noticed i have no ohm reading on either rca i spliced in from head unit, neither positive or negativ.

On the widow i put ignition on and off.. You can clearly hear When i put it on, and how it picks upp all noice after ignition is off from all modules.
The sound before is probably a electric disturbance When filming with my phone, because a ear cant hear that.

 

Water Bear

Senior VIP Member
Oct 17, 2017
207
11
I recommend you check your grounds. If the head unit has a different ground than your amp that could cause issues. The stock amp might also have a different ground (chassis vs. engine vs. direct to battery) and those three may not be connected well from the factory.
 
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Jonas84

CarAudio.com Newbie
Nov 21, 2022
6
2
  • Thread Starter Thread Starter
  • #3
Well i tried straight to chassi,the negative under hood for jumpstart, aswell the one going to neg on car battery.

I even tried the ground from the oem wire to oem amp.

How is it possible the stock amp works with same input/cable.

And yes i buy it that headunit may use a another earth source.. But shouldnt it still in the end connect to chassi?

Never had so much problem just hooking up a amp.. Specially When i just took over the stock one with a new one.

Wanna understand how in the holy the stock amp can perform on same -+, rem, input, output and the new just backfires.

In the ens i accept if i need to run a new ground to chassi for the stock headunit.. But Wanna understand it all before Doing so... Its not the funniest one to remove.
 
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Jonas84

CarAudio.com Newbie
Nov 21, 2022
6
2
  • Thread Starter Thread Starter
  • #4
Just tried removing ground from the rca and suddenly no noise, exept if i raise the bas on amp "gainer?" a little pop when shut ting of system.

Would a ground loop noise isolator do the trick or do i need to take out head unit and make another ground?
 

Jonas84

CarAudio.com Newbie
Nov 21, 2022
6
2
  • Thread Starter Thread Starter
  • #7
I took the wires from the oem harness, so the oem amp is not plugged in.

So signal from headuint is spliced to rca.
 

Water Bear

Senior VIP Member
Oct 17, 2017
207
11
Just tried removing ground from the rca and suddenly no noise, exept if i raise the bas on amp "gainer?" a little pop when shut ting of system.

Would a ground loop noise isolator do the trick or do i need to take out head unit and make another ground?
Hmm. Backing up a step here, it sounds like the splice to RCAs may have some kind of issue. I've never done one like that - are you sure you did it right? I hate to say it but you might also want to check the amp itself at this point. If the inputs weren't wired right it's possible the amp was damaged (I hope not!). If you have a multimeter, check the output voltage coming from the factory wires and make sure it's within spec at all relevant volume levels for your amplifiers RCA inputs. Eg. If your head unit is outputting 8V or something and your amp is only rated to accept 4V, you may have damaged the amp.

Wanna understand how in the holy the stock amp can perform on same -+, rem, input, output and the new just backfires.

In the ens i accept if i need to run a new ground to chassi for the stock headunit.. But Wanna understand it all before Doing so... Its not the funniest one to remove.
Can you tell us a little more about how exactly it's wired? When you say "the same -+" are you saying that you are using the same power / ground wire from the factory harness for the new amp? Input is the RCA splice, output is the factory out, etc.
 
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Jonas84

CarAudio.com Newbie
Nov 21, 2022
6
2
  • Thread Starter Thread Starter
  • #9
Il sure, used ground to the round outer shield on rca, positive to the pointy part. Checked them multiple times.

Check the output? That would be the one to the speakers. That gives good signal.
As for maybe damaged the amp i dont think so since if i plug the signal to a external source and letting car power it.. Works great.

If I turn it around and let the car give signal and power from external source, aswell good.

Since i dont get a ohm reading from ground cables on input signal i believe as many said. They dont share the same ground.

Wiring from amp.

Grounded to chassi with 0.2ohm loss
Connected to battery with fuse, 0.1ohm loss.

Signal is spliced from the oem harness to rca. The rca is really short. Perhaps 5-6"

The oem output is extended about max 10" with the same size cable and wired up to amp.

Remote is taken from 12v outlet since it shuts down earlier then oem. I tried oem rem, and for testning now run it straight from battery.

I bought a ground loop isolator and will try it out later today.

Remember When cutting the negative on rca splice and connecting om rca it didnt have the interferance, connected the other rca that still had both positive and negative and straight away the subwoofer connected to that rca started humming.

Throwing in that i cant get a ohm reading from ground input signal anywhere in the trunk, battery or the ground you use for charging under hood would be a clear one that the oem head unit dont share the same ground.

Cant see any other problem.
 
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Water Bear

Senior VIP Member
Oct 17, 2017
207
11
Unfortunately we're out of my personal experience at this point. Hopefully someone with more experience can step in and help you out. Good luck!

Edit: Also check out this thread
 
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dragon.breath

Senior VIP Member
10+ year member
Nov 22, 2007
943
256
I would try the ground loop isolator. That sounds like the problem to me. Their is a difference in the grounds between the head unit and the amplifier.
 
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