I agree with CS2 mostly, sharp guy. I am however of the opinion that noise should be tackled at the source, having done this many times (also have an engineering background in cellular). The noise fron the alternator is fron 2 sources, ac ripple on the main output since there is no filtering in the alternator. The alternator produces Alternating current and rectifies it into DC, but its bumpy DC without the filtering. A decent 3 wire filter at the alternator will kill it 95% of the time.
CS2 is absolutely correct about the grounding issues, our cars and almost all cars were not designed with adequate ground systems.
The second source of Alternator noise is RF noise caused by the magnetic activity going on inside the alternator and radiating out from the alternator. This is a bit harder to kill because it requires adequate Shielding (good ground shields on the audio cables) Monster is overpriced but they do build a good cable.

just my .02, think I'll go get a cup of coffee now

 

LOL...yeah, you're right GENE...I guess I assumed too much. The amps you're talking about shouldn’t be selling for more than $20 a piece. If ya want quality sound, don't be cheap.

 

Never considered filtering at the alternator. Good Idea. You would need a beefy filter for the nominal current draw and it would have to be able to withstand the engine compartment environment (temp, moisture). I'll have to try it sometime. The other source I think you’re referring to would be from ignition EMI radiation/conduction. Not sure how I would tackle it at the source other than utilizing extensive shielding and grounding techniques…shielded ignition wires, shielded distributor cap if applicable., etc. The newer cars are better designed to reduce the ignition EMI, but our older Vetts are analogous to water in a strainer.

 

first you need to figure out if its ignition noise or alternator noise. then the next step is to isolate the area that the noise is entering the system , then and only then can you deal with it. Filters are a band aid for a bad component or a poor installation. Its very rare that you cant get rid of system noise. Im not a engineer but I was a install manager at a high end shop for years - that and I got a good nights sleep

 

Well, I'll be engineer #3 to toss in his thoughts on automotive electrical systems.


Grounding:
Does not cause noise, but the sequence of grounding can cause you to
pick up noise from another device that's grounded on the same line as you.
Hence the advice to connect directly to the battery is correct.
In a typical rats nest of an automotive harness, you don't know whats
connected to the same ground line as your radio.
Going directly to the battery eliminates that problem.
There is a saying in automotive electronics that everyone wants to be "the first one off the posts".

Now if you've taken the trouble to isolate your power and ground lines to the radio,
you should see if the radio or amp uses its metal housing as ground.
If it does, you could have problems as you pick up noise from currents returning to the battery on the frame ground.

Wire gauge:
Yes, bigger is better, but the law of diminishing returns applies.
10 AWG gauge wire is required if the Amp is a couple hundred watts,
but in a standard 2x20W head unit, its over kill and hard to work with.
The wire should be sized properly for the currents that you expect to pass.
Voltage drops caused by changes in high frequency current is noise.

There was mention of "one very large ground wire", implying that the ground wire has to be bigger than the power.
This is true if we are using the ground to return the radio and a bunch of other stuff like head lamps, tail lamps, etc.
In our case, however, we are going directly to the battery posts.
The current returning should be equal to what went out.
The power wire has to match the size of the ground wire.

Noise:
No one talked much about the noise generated by the ignition system.
The spark is created by switching fairly large currents through a coil.
The ON/OFF switching is creating a square wave with lots of harmonics in the audio range.
I'd figure there would be more noise out of that than the alternator.

 

You didnt waist your money on a Custom Auto Sound crap---did you.
This junk will always give you problems.
I had three of them and will NEVER NEVER buy or be given one again.
NEVER NEVER NEVER NEVER!!!!
Ok Im done,,, sorry.

 

Please let me add my 2 cents worth. I agree with several of the previous posts, especially mrvette. I fought this problem in my '69 for years. I bought cheap to expensive head units, all with the same whine from the alt. I did the most obvious thing, I bought a good am-fm/CD player. Also bought a new instrument bezel. Installed both in an afternoon and have no noise whatsoever. No noise except pure unadulterated loud tunes. At 65 MPH with the tops off and rear window out, I hear Bob Seeger loud and clear. The new CD player even drownds out the Flow masters at all but the highest RPMs(6500).

I know, I know, everyone wants the stock look. You upgrade everything else, why not the radio? I was the last to want to alter the dash area but this looks great, and no damage to the instrument cluster. Pioneer DEH-P4700MP $220.00 New Bezel $249.00

 

I have to disagree with this statement...You're obviously well versed in electronics, but the concept of installing a very large ground wire is not due to current handling. The idea is to create a path of least resistance for the ground current to flow. For instance...the head unit is almost always connected to an antenna, which is grounded. Can't get around it very easily. This secondary ground connection can and will cause a ground loop condition…might not always cause the introduction of engine noise, but it is considered to be a leading culprit by experienced professional installers of high-end units. Another example is with an external amp. It has an inherent secondary ground condition also…the grounding provided by the audio patch cables running from the head unit. Again, it may not always cause a problem, but by adding the “very large” ground wire to every audio unit significantly reduces the ground loop conditions which cause engine noise. We try to eliminate secondary ground connections as best we can by not mounting audio units directly to the metal frame of the vehicle. The large ground wire tends to swamp out the effects of those unavoidable secondary ground connections.

OK, I’ll shut up now.

 

From an amateur radio point of view.....

Follow this lead and you should be golden....

 

As we both are, but with different experience and therefore slightly different perspective.

Two approaches to reduce noise issues on a PCB layout are:

1: Lay down a big fat ground and tie everything to ground.
(What your suggesting)

2: But for analog stuff, we often seprate it from the digital ground
and take power in and out at one point. (What Im suggesting)
With 2, the power and ground can be the same size.