Fixed Ground, How to fix screech?

Red_Beard · 01-05-2008, 04:40 PM

I just fixed my ground loop problem. Grounded it straight to the chassis. No more alternator whine.

Now, I've had this problem all the while, its a little more peculiar. This sound starts approximately when the mini gets to mac os x. Its a sorta screech. Does NOT change with engine speed. Never changes. Sometimes goes out then comes back. Really unsure what this is.
Any idea?

2k1Toaster · 01-05-2008, 05:00 PM

does it go away if you mute the music, or does it remain?

Red_Beard · 01-05-2008, 05:13 PM

Mute the music on the Mac, yes.

Oh, forgot to mention.
When I detach the monster cable to the mac mini it stops.
If I hook that same cable up to a ipod or mp3 player, sounds better than the mac mini, no noises.

travisroy74 · 01-05-2008, 05:21 PM

try running another cable directly to your amps, outside of the car, then to the mini. This will tell you if its the cable. It could be a bad cable or just bad routing through the car.

Red_Beard · 01-05-2008, 05:35 PM

I don't have any amps hooked up at this point.
Bad routing? What would you say bad routing would be? I didn't do the GREATEST job routing it. It just gets there.

My cable, hm possibly. My positive line is a little iffy now that I think of it.

I couldn't buy any 16ga wire from the Shack so I bought 12ga. Here's the setup basically:

Code:

+     -  positive terminal
--    -  16ga wire
//    -  the fuse
==    -  12ga wire
[ ]   -  the unit


+---//=========-----[   ]

so it goes from 16ga to the fuse, up to 12ga, then when it gets close to the unit, it goes back to 16ga. You think this could be it?

iamgnat · 01-05-2008, 06:43 PM

Is that wiring from the battery to the power supply?

If so, 12ga wire is not nearly enough. You should be running 8ga at a minimum. Anything less is running the risk of a fire hazard. Since you are talking Radio Shack for the wire, I assume you are using from fuse from them to? Nothing from them is fit to be in the engine bay of your car. If this is really the PSU wiring you have set up you need to go disconnect it and go have a good Car Audio shop rewire it for you.

-dave

Red_Beard · 01-05-2008, 06:48 PM

Even for the CarNetix P1900? It comes with 2 16ga positive inputs, so I figured it can only accept that much input.

Red_Beard · 01-05-2008, 06:53 PM

From the CarNetix Manual:

travisroy74 · 01-05-2008, 06:54 PM

I mean the actual monster cable audio cable.

To troubleshoot any audio problem just follow the flow of the system.
1st test the source, hook up a pair of computer speakers to the computer's output. do you have the noise now? If yes, then the problem lies within your computer, either swap the computer or try to use a different audio output. USB audio outs are not that expensive, you could try one of those if the computer is the problem.

2nd. if the noise is gone when you listen through the computer speakers then the computer is good, so test the audio cable. Run a different audio cable from the computer to wherever the other end of that monster cable goes. just run it loose and not tucked in to the car. If you have noise with the new cable then your monster cable is probably not the problem. If the noise goes away, try running the temporary cable alongside the the monster cable and see what happens. If the noise comes back then, you should seek out a different path through the car from the computer to the destination and your monster cable is probably okay.

If the noise is still there in the temporary audio cable, then the sound is entering the system after the Monster Cable. What is the monster cable running to?

The things you describe with your positive cable shouldnt' have anythign to do with this noise if you can still hear the noise using an ipod since the ipod is not powered off of that line, but I agree with Dave on his recommendations.

Let us know.

travisroy74 · 01-05-2008, 06:56 PM

Quote: Originally Posted by Red_Beard

Even for the CarNetix P1900? It comes with 2 16ga positive inputs, so I figured it can only accept that much input.

You could have power cables the size of your thumb, a piece of gear will only pull as much power as it needs. Think of it as if you had different glasses in front of you that were full of water and each had a different size straw in it. The smallest one would probably not get the job done, but having a straw that supplied more water than you need wouldn't drown you, you only drink as much as you want. It's a flawed example but I'm too tired to be creative.

iamgnat · 01-05-2008, 09:00 PM

Actually it's not that flawed of an example. The missing part is the work that it takes to get the water through the straw and the effects that has. In this case he's trying to suck a gallon of water through a 3 foot long coffee stirrer

The wire that comes on the PSU is sufficient for the length that it is, not for the length of the run that you should be making from your battery to the PSU. If you ran that gauge the entire length, as it starts approaching it's max pull the wire will heat up and melt the casing which will allow for a short that could cause a fire.

Think about about what happens when you pull too many amps through a small fuse (the ones you can barely see the element of). Most of the time you get some charring when it pops which means a fire, however small. All the fuse is in it's purest form is nothing but a very fine wire. All wire (no matter how thick) is essentially a fuse and if you run more amps over it than it can handle (determined by length and gauge) the same thing happens as it did to the fuse. Just on a larger scale.

Further more, short of industrial grade wire, wire that small will not have a covering that is made it to stand up to the heat and other bad things running around the engine compartment. They usually also aren't made to withstand the rubbing that is typical in that environment either.

You are of course free to leave it how it is and the chances are that you may never have a problem, but the cost of the damage that may occur far outweighs the cost of using the right materials.

-dave

Red_Beard · 01-05-2008, 09:14 PM

The 12 ga wire I used was 600V capable, 80 degrees celcius, pretty thick stuff in my opinion.
So, you're thinking the PSU isn't getting enough power because I'm only pulling the amount I need from the beginning and it kinda fades? That kinda makes sense. Hm.

Red_Beard · 01-05-2008, 09:18 PM

I also have a 25 amp switch on the power line. You think that could induce this noise? Its a RadioShack 25 amp DC switch.

iamgnat · 01-05-2008, 11:23 PM

No. If you were pulling too much power, you would have had a fire already.

600v doesn't mean much, it's the amperage rating that is important. And your engine is a couple of hundred degrees.

Like I said, the chances are that you will be ok but you are risking a fire. It would probably taking maxing the PSU out to have a problem though (e.g. running your LCD, the P5V under it's full load, and the Mini drawing a full load).

The purpose of a fuse (or switch in your case) is that it should allow less amps to pass through than the wire it is attached to. This is to prevent exactly what i'm trying to warn you about. In your case I would not be surprised if your wiring and your fuse has a very similar failure point. It's been awhile since I installed mine, but I believe 25amps is much more than Mike recommends anyway.

As far as your problem, I doubt this wiring has much/anything to do with it. Since you say that it doesn't happen when you connect up your iPod, it is probably a combination of 2 things (what you describe sounds almost exactly as what i've experienced in the past). You have a cheap (read poorly insulated) audio cable and the Mini/PSU still isn't grounded perfectly. The easiest thing you can do is change the routing of the audio cable to make sure it isn't anywhere near a power line (the more power it carries, the farther it should be). That's all I did to fix my issues and now I only get that noise when the Mini is off or the audio line has been disconnected from the Mini.

-dave

Red_Beard · 01-06-2008, 10:00 AM

Well, I've tried running a 12 gauge line directly to the battery negative, which didn't help.
The best ground I found so far was a body mount in the door jamb. I can still hear a slight whine, but it nots nearly as bad as before.
The audio cable has to be good. Its really thick, hard to bend. Its a Monster one. [LINK] I would think when I attach the ipod, that interference of the power lines should still be there no?
I'm going to study the noise some more.
Some more clues:
When I pump the headunit to full volume, that noise is more enunciated and there's a uniform popping noise. The more volume, the louder it gets.
With the ipod, nothing.

I really appreciate your help and patience.