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Old 11-08-2019, 01:34 PM   #1
mega_stihl
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[Update] Alternator whine --> It's starting to piss me off now

So ladies & gentlemen. Here I am again, with the same problem. As some might know, I complained a lot about alternator hum. The situation is as followed: I wanted better audio but wanted to keep my OEM buisness radio. I did not have the HK option, so it was just OEM radio + thrash speakers.
So I installed an aftermarket (sony) amp and retrofitted the HK speakers. The sound is infinite times better than before. I recently had the amp disconnected and connected the speakers straight into the radio like before, and the sound was OEM-**** compared to when the amp is connected.

Now, what really ****ing bothers me is the alternator hum once the amp is turned on. I tried another amp --> same story. The wiring goes from the headunit to the left below my steering column, under the left door to the trunk where it's connected to the high/low converter, to the amp, where the speaker output wires go to the front again, to the speakers. The ground is taken in the left side of the trunk, where the OEM HK is supposed to take ground.

When I first started I wired from the right side, next to the big power/battery cable, and people said that this was the reason I had humming, so I rewired it to the left side of the car, but the problem was not solved.

I have tried different ground options, but this makes no difference at all. My high/low converters are not the problem, as when I eliminate them and connect it directly to the HLC of my amp, it sounds even 10x worse.

So I am starting to think it will be like this forever, but then I thought of something. What if this is because the outside of the amp makes contact with the grounding somehow (metal mounting frame of amp, ...), so I unmounted the amp to make sure it wasn't touching any grounding of the car --> same result. But what I noticed is that when I measure electrical resistance between the ground and the outside of the amp, I measure 60 ohm.

This was only there when the ground of the amp was connected to the ground of the car. So between the ground connector of the amp and the actual outside case of the amp, I measured 60 ohms (with everything turned off, and the 12V + disconnected). So there is some slight ground connection to my amp itself. Is this something that could cause this problem, or is this perfectly normal? I'm asking, since I have no clue what's inside an amp.

Also: when I connect the + and ground of the amp to a spare battery outside the car it's still the same. So this might also answer my previous question, as there can be no alternator hum coming from the ground as the ground (and +) comes from a spare battery.

Any advice will be appreciated, as this is really getting to me.

Kind regards

Last edited by mega_stihl; 11-13-2019 at 09:53 AM.
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Old 11-08-2019, 02:18 PM   #2
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Have you checked the engine ground strap?
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Old 11-08-2019, 02:29 PM   #3
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Only thing I can think of is to try changing your cable to shielded ones, if you haven't already used them. Problem with cables in audio circuits especially the longer ones is they will act like an antenna picking up stray signals that can introduce all kinds of noise on a system. You could try isolating the amp from the car body and running a ground directly to the negative battery post.
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Old 11-08-2019, 02:31 PM   #4
mega_stihl
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Quote:
Originally Posted by Silverdogz View Post
Have you checked the engine ground strap?
I have not. What is there to check exactly?

Quote:
Originally Posted by Newbimer View Post
Only thing I can think of is to try changing your cable to shielded ones, if you haven't already used them. Problem with cables in audio circuits especially the longer ones is they will act like an antenna picking up stray signals that can introduce all kinds of noise on a system. You could try isolating the amp from the car body and running a ground directly to the negative battery post.
I have not tried shielded audio cables yet. Well, afaik mine aren't shielded. I might give this a shot.

Grounding directly to the battery also made no difference.
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Old 11-08-2019, 02:47 PM   #5
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The engine ground strap should be near the right side motor mount,
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Old 11-08-2019, 03:27 PM   #6
mega_stihl
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The engine ground strap should be near the right side motor mount,
What I mean is: what specifically should I be looking for? Bad connection or something? Because if that is the case, shouldn't I have a lot more issues than simply alternator humming? Like several sensors that would give me the check engine light?
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Old 11-08-2019, 03:47 PM   #7
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Have you checked the engine ground strap?
If the starter has no problem of pulling over 100A during cranking then no reasons to suspect a loose engine ground strap.
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Old 11-08-2019, 03:53 PM   #8
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You didn't state but I assume the noise disappeared with engine not running. Yes?

Amp chassis with 60 ohms to its ground wire is not a surprise because some sensitive equipments like audi amp do not have chassis tied to power supply ground.

You had tested with an isolated external battery supplied the amp and still with same noise; this proved the noise is not on the amp power supply wires, but noise coupled to the amp audio input wires. What kind of audio input wires or cable you have? Best to use 2 twisted wire pair for each audio input channel to cancel out the common mode noise (assuming from the alternator).
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Old 11-08-2019, 03:54 PM   #9
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You didn't state but I assume the noise disappeared with engine not running. Yes?

Amp chassis with 60 ohms to its ground wire is not a surprise because some sensitive equipments like audi amp do not have chassis tied to power supply ground.

You had tested with an isolated external battery supplied the amp and still with same noise; this proved the noise is not on the amp power supply wires, but noise coupled to the amp audio input wires. What kind of audio input wires or cable you have? Best to use 2 twisted wire pair for each audio input channel to cancel out the common mode noise (assuming from the alternator).
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Old 11-08-2019, 03:56 PM   #10
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Originally Posted by Sapote View Post
You didn't state but I assume the noise disappeared with engine not running. Yes?

Amp chassis with 60 ohms to its ground wire is not a surprise because some sensitive equipments like audi amp do not have chassis tied to power supply ground.

You had tested with an isolated external battery supplied the amp and still with same noise; this proved the noise is not on the amp power supply wires, but noise coupled to the amp audio input wires. What kind of audio input wires or cable you have? Best to use 2 twisted wire pair for each audio input channel to cancel out the common mode noise (assuming from the alternator).
With the engine off, the sound is gone. It also changes when you rev the engine. I appreciate the input. I do use regular audio cables. Not twisted or shielded.

What is the point of a "twisted" cable? As in: how does this cancel out the noise? Also: is a shielded/twisted cable for the input enough, or should I do input & output?
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Old 11-08-2019, 06:18 PM   #11
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Audio channel inputs use 2 wire for each channel. Noise from any sources (lightning, alternator, phone) nearby can couple onto the channel 2 wires (any wire in the car and not just those audio input wires). With twisted wire pair the same noise signal on both wires would cancel out due to the wires intimately twisted into one.

But this is assuming the input wires -- from the head unit at console to the amp in the trunk -- is long enough to pick up strong noise signal from alternator. To prove this you might want to test with the Amp connect to the head unit with a few inches long channel wires. If the noise picked up by the head unit then the said twisted wire pair doesn't help much, and the noise would rear its head even with few inches long from head to Amp.
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Old 11-08-2019, 06:26 PM   #12
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Originally Posted by mega_stihl View Post

What is the point of a "twisted" cable? As in: how does this cancel out the noise? Also: is a shielded/twisted cable for the input enough, or should I do input & output?
The output of the Amp wires carry strong large amplitude current and so little noise from alternator is not thing to compare. However the noise is NOT very small comparing to the small input channel signals of the Amp. No worry on the output cables.

Why twisted pair? The idea is that if those two wires of each channel picked up the exact same noise level then the Amp would easily cancel the noise out because the Amp works on the differential signal between the 2 wires.
Differential means wire 1 signal minus wire 2 signal, or
Differential = noise1 - noise2 = zero volt.

if the 2 wires are not twisted then each could pick up the noise a little difference from each other, resulting a net non-zero noise signal in the Amp and it got amplified up.

Last edited by Sapote; 11-08-2019 at 06:28 PM.
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Old 11-09-2019, 03:06 AM   #13
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Understood. Thanks for this explanation Still 1 more thing: the input wires could be the problem, but not the output, as the input wires have a way smaller signal to which the alternator noise then seems fairly large, if you would compare it to the output wires, you said. But the actual input is still a certain current from the headunit, and not a really low signal to begin with as I need a high/low converter. Is what I just said relevant?

Nonetheless: I'll give the twisted/shielded wires a shot. What I'm not looking forward to, is to test my amp with only a few inches of wiring, as I would have to start doing my wiring all over again. Yes, if I need to replace wiring I would need to anyways, but then it's final, and not just to test it to rewire it all over again afterwards But it looks like I have not much of a choice if I want to eliminate this noise.

Thanks for the input!


edit: I just read about someone with a similar problem on this forum. He tried disconnecting the HU and connect hit ipod directly to the low signal cables, and his noise was gone. So I might see if the HU ground or something similar might be the problem. But then again: why is this problem not there when I leave the amp out of it and use the HU only?


edit2: https://www.ebay.com/itm/NOISE-FILTE...-/400348357758 would something like this work?

Last edited by mega_stihl; 11-09-2019 at 04:01 AM.
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Old 11-09-2019, 08:16 AM   #14
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Alternator whine --> It's starting to piss me off now

The head unit has a single grounding point in the main connector at the back of the head unit. Have you pulled the head unit? Thereís a metal stud at the back of the head unit. Itís supposed to be covered by a plastic sleeve. The stud is there to hold the head unit in place as it fits into a hole in the either dash frame or a support bracket. If that plastic sleeve is missing the head unit will be grounded in 2 places, thus resulting in a ground loop.

Itís item #4 in the link.

https://www.realoem.com/bmw/enUS/sho...diagId=65_0461

Last edited by markusmarkus; 11-09-2019 at 08:17 AM.
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Old 11-09-2019, 08:55 AM   #15
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Some answers are getting at your problem... what you have done is inadvertently created a ground loop on your filtered audio. I'm not an engineer but I am an audiophile and I have dealt with the problem before outside of cars.

The same thing happens when people try to power their mobile audio equipment through a power inverter. The background hum happens and is always there when audio equipment is sharing inputs with equipment that run off the same power supply. Normally, we don't hear it because the audio equipment filters out the noise by comparing to the ground reference. If however we create a "ground loop" where the ground path runs over the filtered audio path then the ground referencing process causes the hum.

There are two general ways to deal with this: 1) rejigger the wiring and power sources to remove the ground loop, 2) install a ground loop isolator before the amp. Different ones are available based on the input jack. I have no idea if there is one that would work for an e46 setup.

Hope that helps.
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Old 11-09-2019, 03:18 PM   #16
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I very much thank all of you for your input. I'll try some things next week, and I will come back at this once I have results.
@marcus: Yes, that sleeve is still on. Well, I remember it being on when I installed it a year or 2 ago.
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Old 11-10-2019, 09:15 AM   #17
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I very much thank all of you for your input. I'll try some things next week, and I will come back at this once I have results.
@marcus: Yes, that sleeve is still on. Well, I remember it being on when I installed it a year or 2 ago.


Ok, so that potential solution is out the window.
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Old 11-11-2019, 07:15 AM   #18
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Try this:
- pull the head unit
- disconnect all of the connectors
- reinstall the head unit
- check the resistance to ground of the head units metal cover.
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Old 11-11-2019, 07:19 AM   #19
mega_stihl
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Originally Posted by markusmarkus View Post
Try this:
- pull the head unit
- disconnect all of the connectors
- reinstall the head unit
- check the resistance to ground of the head units metal cover.
Already done that. With everything disconnected its 0 ohm. With everything connected but the ground wire: 0 ohm. Connecting ground wire: 60 ohm
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Old 11-11-2019, 09:36 PM   #20
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Possible failed diode in the alternator?
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