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Discussion Starter #1
I have been trying for a while to track down a slight engine noise that has been present ever since I installed my stereo system about a year ago. The engine noise is very faint, but gets worse when the air conditioning is on.


First the system:

Stock head unit
Line out converter
Rockford Fosgate P8004 powers four speakers
Rockford Fosgate P3001 powers two subs
Polk DB6500 6.5” component system installed in doors/dash
Polk DB650 6.5” coaxial speakers in rear shelf
Rockford Fosgate P18S4 8” subs in custom boxes in the side panels running in parallel for a 2 ohm load on the P3001 amp.

The wiring is done as follows:

To keep from cutting the stock wiring harness, I tapped into the stock wiring in the doors and in the rear side panels. I used male spade connectors pushed into the stock wiring harness that plugs into the speakers. The wiring then runs along the driver side of the car back to the trunk, and into a line out converter. I used shielded RCA cables to go from the line out converter to the amps.

All of the connections are made on the back of the amp board that mounts to the gas tank cage. I used screw-type terminal blocks from Home Depot and ring terminals to make the connections. You can see the back of the amp board in the picture. The outputs to the speakers are at the far left at the top. The inputs from the stock stereo are at the bottom right. You can see the line out converter at the bottom right.



This is what the board looks like in place. The power cable has not yet been run down the passenger side in this view.



I grounded the two amps to a block mounted directly to the floor of the trunk. The metal was sanded clean. To try to get rid of the noise, I tried moving the ground to the gas tank cage mount, which did nothing to get rid of the noise.

Later, I installed a low voltage trigger to turn on the amps only when the stereo was on. Previously, I had the amps wired to be on whenever the ignition was on. This didn’t get rid of the noise either. The noise is present whether the amps are on or off, and whether they are connected or not.

So, I started looking at the wiring. If I disconnect the front speakers at the terminal blocks in the trunk, the noise goes away.

My question is for those who have done similar setups... How did you wire things, and do you get any noise from it. I can’t figure out where the noise is coming from...
 

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llafro said:
So, I started looking at the wiring. If I disconnect the front speakers at the terminal blocks in the trunk, the noise goes away.
The speakers are connected to a terminal block?

Is your line out converter powered?
 

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Discussion Starter #3
The speakers are connected to a terminal block?

Is your line out converter powered?
The speakers are connected via terminal blocks on the back of the board. That's the only way I could do it and still have the board be semi-removable. Basically, the wires run from the amps to the terminal blocks, and then out to the car. There are eight sets of connections for the speakers.

The line out converter is a Scosche SLC4 and is not powered.

Disconnecting the RCA cables at the amps does not get rid of the noise, so I doubt the problem is with the line out converter.
 

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Well those RCAs are really short, and they don't have a good path to ground anyway since they go through the converter. I wouldn't have guessed that they would be the source of the noise.

So when you disconnect the front speakers, there's no whine coming from the rear coax speakers even though they're connected to the same amp?
 

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Discussion Starter #5
So when you disconnect the front speakers, there's no whine coming from the rear coax speakers even though they're connected to the same amp?
Correct.
 

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Unplug the front speakers, then switch the RCA cables so the front output of the converter is going to the rear input of the amp, and see if there's any noise from the rear speakers then.
 

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llafro said:
The engine noise is very faint, but gets worse when the air conditioning is on.

The noise is present whether the amps are on or off, and whether they are connected or not.

If I disconnect the front speakers at the terminal blocks in the trunk, the noise goes away.
So you're saying that with the amplifiers powered off, you are still getting noise from the front speakers. Disconnecting the front speakers causes the noise to go away.
Those two conditions imply that there is a source of audio power which is being coupled through the amplifer (even when it is off) to the speakers.


If I were a betting man, I'd say the problem is at the line level converter. Your line-level converter has 4 small transformers inside (you can see them through the translucent cover...), one for each audio channel. The audio coming out of the factory head unit has a DC signal applied which is used to turn on the factory amplifier. This signal can cause problems with the transformer coupling inside the converter. Try turning down the factory amp gain adjustments, and turn up the line out gain adjustments.

On the side of the converter is a switch which can be used to select the mode of operation (factory amp, common ground, or line out). You'll need to check the manual for the proper position, but just as a guess I'd say to use Factory Amp. I tried to find a manual or schematic online, but didn't have any luck.

There's also a black wire near the switch. This wire is intended to be grounded in order to reduce the engine noise. Again, check the manual for directions on this.


For a premium quality converter which easily handles the signal from the factory head unit, take a look at the NE-684V here: http://www.autosound2000.com/adaptor_products.htm
(no, I'm not a salesman, just a satisfied customer).
 

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Discussion Starter #8
So you're saying that with the amplifiers powered off, you are still getting noise from the front speakers. Disconnecting the front speakers causes the noise to go away.
Those two conditions imply that there is a source of audio power which is being coupled through the amplifer (even when it is off) to the speakers.
Last night, I got to checking some more to see if I could find the cause of the noise.

1. Disconnected the RCA cables at the amplifier - Noise still there.
2. Removed the fuse to the amplifier - Noise still there.
3. Removed ground wire to the amplifier - Noise still there.
4. Removed remote activation wire to amplifier - Noise still there.
5. Disconnected all the speaker wires from the amplifier - Noise still there.

So at this point, the amplifier was totally disconnected, and the noise was still there. That would seem to rule out the line out converter and the amp as sources of the noise.

6. I disconnected the four sets of speaker wires from the terminal blocks on the back of the amplifier board. - The noise was gone.
7. Reconnecting any of the four speaker wires caused the noise to return.
8. I tried swapping the right and left channels, and that did not get rid of the noise.
9. I reconnected the speaker wires at the terminal blocks, and disconnected them at the crossovers, which are mounted on the back of the amp board. you can see them in the picture above. That gets rid of the noise.
10. Thinking the crossovers might be the source of the noise, I bypassed them temporarily by connecting the outputs from the amp directly to the door speakers. The noise was back.

So, my guess is that the wiring is somehow picking up interference from something in the car. When I installed the wiring, I ran the wires all together down the driver side of the car. Could this be a problem?

For example, I ran the wire from the driver side door down the driver side into the trunk. It goes through the line out converter, into the amp, and back out into the crossover. From there, I ran two sets of speaker wire back to the front to the dash tweeter and the door. Adding in the wires from the passenger side, there are six pairs of speaker wire running down the driver side under the door sill. Is it a problem to run the wires together like that? (I know that you aren't supposed to run the speaker wires next to the power wires - the power wire is on the other side of the car)

Where did most people mount their crossovers? I have mine mounted in the trunk on the back of the amp board. Could it be a problem to have the wiring running forward from there to the speakers?
 

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Can you try putting everything back together the way it was, but without the terminal blocks (run the speaker wires straight to the crossover)?

I think the location of your crossovers is fine. Do any of the input wires into the high/low converter run by or around the wiring harness for the gas tank or the large power wire for the amp?
 

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Discussion Starter #10
Can you try putting everything back together the way it was, but without the terminal blocks (run the speaker wires straight to the crossover)?

I think the location of your crossovers is fine. Do any of the input wires into the high/low converter run by or around the wiring harness for the gas tank or the large power wire for the amp?
The speaker wires already run directly from the amplifier to the crossovers. From there, they run to the terminal blocks.

I added some callouts to the picture of the wiring on the back of the board to help you see where everything is.



Could the fact that the power wire passes right next to the crossover be inducing some interference?

I doubt right now that the line out converter is the source of the noise. Last night, it was not physically connected to the amplifier, but the noise was still there.
 

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llafro said:
The speaker wires already run directly from the amplifier to the crossovers. From there, they run to the terminal blocks.
I meant on the output side of the crossover.


Could the fact that the power wire passes right next to the crossover be inducing some interference?
It's possible. That'd be another thing to try - remove the crossovers from the board and set them a few feet away from the amp and see what happens.

You may end up needing to get a few of these:
http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=119-035
 

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Lothar34 said:
I meant on the output side of the crossover.



It's possible. That'd be another thing to try - remove the crossovers from the board and set them a few feet away from the amp and see what happens.

You may end up needing to get a few of these:
http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=119-035

Yes, move your x-overs. They are picking up a radiated noise from somewhere. Since the whine was still there with the fuse pulled from the main power line I would guess against that being the culprit. Most likely something in the car. I have run into this exact issue w/ x-overs place in a certain spot of the kicks as well. Moved the x-overs, viola, noise gone.
 

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Try to ground your headunit to something more solid rather than depending on the ground through the harness. I found a spot under the drivers side footwell.
 

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Discussion Starter #14
The noise is finally gone! :judge:

I moved the crossovers from the back of the amp board to beside the gas tank and the driver-side shock mount. It would appear that our guesses about the interference from the power cable were right.

Thanks to everyone for the help in diagnosing this.

I realized how much I used to drive with the stereo on just so I wouldn't hear that little noise.
 
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