I installed a new HU and AMP in my car this weekend (MINI Cooper S). I now have the dreaded alternator whine through the speakers. So I need some ideas for things to try. Here are the particulars and what I've tried so far.

The battery in this car is in the rear, the amp is also in the rear.

I've run the preamp cables from HU on the opposite side of power (I ran a power cable from the battery to the HU).

The whine rises with the revs, starts around 3000 and gets louder.

If i disconnect the preamp cables to the amp, i still get the whine in the speakers.
Ditto if i connect an external source to the amp.

I get the same whine if i use the front premp cables or the rear preamp cables.

I've tried a few different ground points for the amp, the best one I found was the hatch release which is bolted directly to the body. HU is grounded to the body (screw behind HU).


This is the best clue, the loudness of the whine changes dramatically if I move the amp from the drivers side to the passenger side. It also changes if i rotate the amp 90 degrees. My guess here is there is a power source in the back of the car that is throwing off interference to the amp or preamp cables. I just don't know how to solve that!

any ideas?

 

If you disconnected the preamp cables and still have the whine, it's obviously not the cables. Turning the amp 90 degrees and changing the whine makes me think the amp is either not shielded very well, or is not grounded very well.

 

well since the battery is in the rear of the car, I've also tried grounding to the ground point for the battery. Which I would think is the best ground in the car? But maybe I'm off about that?

 

take some pics of your install and post, we can see from there, your grounding and wire routing is the issue

 

try grounding the amp directly to the - terminal on the battery. If the whine subsides...then it is your ground.

 

You could also have an resistance difference in your HU an amp ground points. If you connect an external source to the amp to test, it likely has no ground to the car at all, which could cause the same problem. Is the amp mounted directly to metal? I've seen this introduce noise into systems before because the entire case of the amp is grounded, effectively creating another ground point.

 

Sounds like you've spent some time and have done some logical testing already. Good work.

[QUOTE]Originally posted by SC_Highlander
[B]You could also have an resistance difference in your HU an amp ground points.

 

an update for you armchair car stereo detectives out there

i changed around the grounds for the amp (finally settled on using the same ground as the battery) and changed the head unit ground as well. when i did this i was able to remove the whine with the preamp cables disconnected but still had it with the preamp cables plugged up. i still was only picking it up through the front tweeters, so i tried running the front speakers off the rear preamp channels and still whine. (as a personal note, yes i did test this before, but there was obviously errors in my previous testing, since i should've nailed down the ground point for the amp first then look at the cables/head)

i took a second look at where the preamp cables were running, and at the point where they enter the rear quarter panel, they were meeting up with a power harness for the rear lights as well as a power run for the amplified fm antenna. so i ran the preamp cables under the rear seat instead of through the quarter panel trim. and success!

this has removed nearly all of my whine (99%) i think there's a super high frequency one still there, but it's hard to hear over the car so it might not even be there.

i would say modifry was kinda close (his comment about RF made me think about that boosted fm box). but moving the amp within the boot still makes a difference in whine coming in, it will come back if i slide the amp to the other side. i guess mounting the amp on top of the battery as well as the huge wire that runs from the battery to the front of the car has limits!

i'm going to leave everything alone for a little while to make sure the problem has gone away, but thanks for all the ideas!

 

I was having a similar problem with my S, and yes modifry is correct on all of the suggestions he gave. After doing all that also it minimized it 99.8%, so as a last resort I purchased some RCA Noise filters, this did the trick for me. That completely got rid of the noise. The problem with the S as I was explained is that there are a number of electrical components all over the car, so there is plenty of interference points. This noise filters go in the output of the radio and then goes to the input of the amp.

Hope that helps

 

The problem with these filters is they either lower the gain of the entire band (not likely), or worse, notch out and lower the gain on a PORTION of the band. Uncontrollable noise-shaping, not good.