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Old 2nd April 2009, 08:05 PM   #1
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Default Advice needed on engine whine in speakers

My sound system sounds fine when the van is off, but when it is running, I can hear all kinds of engine noise in my rear speakers. It is only the rear speakers that make the noise.


Setup: '92 Mazda MVP. Pioneer DEH-P640 head unit drives 4 speakers in the front just fine. A stereo RCA cable runs between the head unit and a little pyramid amp. The Pyramid amp drives two big cabs in the back. They are just woofers with lots of bass, not subs. A cable runs from the positive terminal on the battery, through the firewall behind/under the steering wheel, and there it joins the power leads for the amp and the head unit. The amp and the head unit share the same ground point, which is a screw below the head unit. The cigarette lighter wire is connected directing to both the amp and the head unit.

Thanks for any advice.
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Old 2nd April 2009, 09:46 PM   #2
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Check the shield ground of the head unit's RCA output jacks:
http://bcae1.com/images/rca/temporar...eldrepair.html
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Old 2nd April 2009, 11:03 PM   #3
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I did the test. I measured 1 ohm between the shields of the front jacks and the case. I also measured the subwoofer jacks and they are 2 ohms. Should it be zero in order to stop the whining?
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Old 2nd April 2009, 11:27 PM   #4
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2 ohms is OK.

Disconnect the RCA cables from the amp and measure the resistance from the amp's RCA jack shield to chassis ground. It should not read anything less than ~100 ohms.

Disconnect the RCA cables on both ends and measure the resistance from both the left and right shield of the cable to ground. It should read infinite resistance.
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Old 3rd April 2009, 12:54 AM   #5
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Ok, those tested out well. Is it possible that the point I'm using for ground isn't close enough to the negative terminal on the battery?
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Old 3rd April 2009, 01:01 AM   #6
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The ground point isn't important as long as there's a good connection to the chassis ground.

With no RCAs plugged into the amp, do you have ~0 ohms between the shield of the amp's RCA input jacks and the non-bridging speaker terminals?

If so, the amp is likely defective.
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Old 3rd April 2009, 01:37 AM   #7
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By "non-bridging speaker terminals" do you mean the actual wires that connect to the speakers?

Edit: How can the amp be defective if it plays the output from a portable CD player just fine?
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Old 3rd April 2009, 01:55 AM   #8
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If the input circuit is damaged, it could produce engine noise when driven by a signal source that's grounded to the vehicle (the head unit) but not when driven by an isolated signal source.

It 'could' be the head unit but generally, if the shield ground is OK, the head unit is OK.

Yes, from the shield to the non-bridging speaker terminals (the ones NOT used to bridge the amp).
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Old 3rd April 2009, 03:20 AM   #9
ppia600 is offline ppia600  United States
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Which model pyramid is it? I would ground the amp where its located unless its something small that uses a 10amp or less fuse. I wouldn't run anything from the cigarette lighter jack. The radio remote wire (blue or blue/white) should be connected to the remote turn on of the amp. If all else fails, you can buy a "ground loop isolator" and plug it in between the deck and the amp's rca cables.
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Old 3rd April 2009, 03:44 AM   #10
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http://www.parts-express.com/pe/show...number=265-079

Could the cigarette lighter cable be related to my problem?
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