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Interferance from altenator?

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tblo163

New Member
The stereo amp that I built (one of my earier threads)works perfectly,it plays the MP3 with good sound quality and volume response,however there's a very faint whine (you can only just hear it) when the engine RPM increases,I assume this is coming from the charging system,can anyone suggest a suitable choke or similar device,would a 2.2uF condensor(the type fitted to the back of a car radio)be any use?If I can't get rid of the whine I don't think it will do any harm? Thanks to all,Happy easter.
 

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Hayato

Member
The noise is not from the alternator. It comes from your HV system. You should use supressive/resistive spark plugs and cables.

Maybe your AMP's PCB is not well drawed/etched. And yes, a 2.2uF capacitor could help.
 

tblo163

New Member
Supresion?

Thanks for that,the Volvo leads are just about the best available,so I'll try the condensor,(can't do any harm) the noise doesn't bother me,it's only just audible,so will see what I can do .Anyway,thanks for the quick response,I'm
impressed.& will add to your reputation. Terry.
 
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tblo163

New Member
Thanks Willbe,looks like I've got a bit prodding to do,will get there.I'm going to try the condensor first,and go from there.
 

crutschow

Well-Known Member
Most Helpful Member
The noise is not from the alternator. It comes from your HV system. You should use supressive/resistive spark plugs and cables.
If it's a whine, it is from the alternator. Rectifying the high frequency AC in the alternator generates harmonics that can be picked up by an amplifier.

If the noise is from the ignition system you will hear impulse noise, not a whine.
 

MikeMl

Well-Known Member
Most Helpful Member
Alternator whine in car (and airplane) audio is caused by a classic ground loop.

The car body is used as the return path for every power consuming appliance in the car, because each appliance is "grounded" locally to the car body. In some parts of the car, the current that is flowing through the car body adds up to several amps. The steel from which the car is fabricated is not a perfect conductor, so several amps of current in the body parts causes voltage drops of tens of mV.

Now, say you have an audio source like an MP3 player powered from a cigar lighter plug. It is connected to an audio amp mounted under the seat, which happens to be "grounded" to a screw to the floor under seat. A shielded cable with an 1/8" plug connects between the MP3 player and the stereo amp.

Most likely, the sleeve of the stereo jack on the MP3 player is internally tied to the negative wire of the cigar plug powering the MP3. Most likely the shield on the cable running to the audio amp is tied to the ground inside the amplifier, maybe through the shell of a RCA phono jack. The power ground wire from the amplifier is internally tied to the same PC trace as where the shell of the RCA jack, and as we have already said, it is connected to a screw under the seat...

So, the MP3 is "grounded" via the cigar plug. The amplifier is "grounded" to the screw under the seat. We get alternator whine. How is it getting into the system?

As I said above, there are sizable voltage drops between various parts of the car body. Most of the time, the alternator carries the total electrical load. It uses a three phase full-wave rectifier system which puts out mostly DC, but there is about 10% ripple at an integer multiple of the rotation rate of the alternator, and it happens to be plainly audible (about 1-3kHz). The ripple rides on all currents flowing to and from various appliances in the car.

So suppose that there is 30mV of alternator ripple between where the cigar plug is "grounded" to the car body (under the dash), and the part of the car under the seat where the amp is "grounded". That 30mV will appear effectively to be in series (common-mode signal) with the audio output from the MP3 player.

But wait a minute, isn't the output audio from the MP3 at "line level", something like 2Vp-p? 30mV compared to 2V shouldn't be a problem, however, 20log(.03/2) is only -36db, and what is the range of human hearing?

No amount of "filtering" with capacitors (even inductors) will cure this. There are only a couple of things you can do. First, suppose instead of operating the MP3 player using a cigar plug power cord, you ran it on its internal batteries. If there is no connection to the car body at the MP3 player, and the only connection form it to the amplifer is via the shielded audio cable, then that might cure the problem.

If you powered the MP3 from inside the amplifier by back-feeding DC power from amp to MP3 while at the same time feeding the audio from MP3 to the amp, you have created a "single point ground" locally near the amplifier, where no part of the audio system touches the car body anywhere else in the car except at this single point, and that usually works. Bear in mind, that this is sometimes hard to do, because maybe the "amp" is really a stereo receiver, which also has an AM-FM antenna mounted elsewhere on the car body, so is already part of a different ground-loop.

Another strategy to break ground-loops involves transformer coupling. Suppose the MP3 player is powered via the cigar plug. There is a large common-mode ground-drop between the cigar plug ground and the amp ground. If you insert an audio coupling transformer in the audio path between MP3 and amp, the isolation from transformer primary to secondary effectively breaks the ground-loop.

I know this game. I have spend years of my life tracking down ground-loops in industrial and robotics instrumentation, and more recently getting alternator whine out of aircraft audio entertainment systems.
 

tblo163

New Member
Thanks for the info I will let you know the result ,when I digest all the data you have sent me.TERRY
 

tblo163

New Member
Reply to your post

I've had a good look at the info,you sent.I understand what you say about the ground loops,& the difficulties solving this problem,the MP3 has it's own supply,(1.5volt AAA Battery)the Amp is powered from the ACC supply of the car.I've not tried anything yet,I'm still looking,which got me thinking(logicly)
There is no problem with interference with the HU Radio/cd player,so there must be something in this to make it imune from the whine.I've got a HU I don't use any more,so I looked to see if there is anything in it that could be used.The only thing different with the power supply is a diode(suitably connected between the -&+ supplies,& a transformer in series with the + input.the transformer has just 2 pins & when tested shows no voltage drop ,there is 0.5 ohm resistance between the pins,it is marked as 16F001,and
on the diagram of the HU as coil #L720,GOA151J00004,Can't find it on any data sheets,but it must be there for a reason?the comments you made re-
audio transformers made me look at this prospect.I've sent a pdf of the HU
that I'm savaging for parts,it's a very usefull file,which is worth saving to your collection, regards Terry
 

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tblo163

New Member
Just a quick update on progress,re;interferance from altenator.MP3 Player is powered by 1.5volt AAA battery,I fitted the 2.2µF condensor & grounded the heat sinks & enclosure,Amp powered by Acc + ,whine still there.Connected Amp to seperate power supply,& whine is GONE. So your conclusion re-
Ground loops is obviously the answer,I will try giving the Amp it's own ground
directly from the car's battery,to see if there is any change,if not then I think the best solution would be the seperate power supply.I am still looking inside the spare HU to see what makes it imune to this type of interferance.
Like you said it's a very complex problem.Terry
 

MikeMl

Well-Known Member
Most Helpful Member
Question: are your speakers grounded to the car body (or are they driven differentially)?

Even if your speakers are driven single-ended, connect the other side of the speaker back to the same ground point where the amplifier gets it DC power ground using a twisted pair. Yet another step toward a "single Point ground".
 
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microtexan

New Member
"the transformer has just 2 pins" That is not a transformer, but a filter/choke and is designed to eliminate part of your noise.
 

tblo163

New Member
O.K. Thanks.will try it. The speakers are in fact run independently and are not connected to the vehicle ground.
 

tblo163

New Member
Thanks,that's good news,I've been looking for some kind of filter,so this might work,if not,it wont do any harm fitting it ,Terry.
 
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