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Old 29th April 2010, 11:31 PM   #1
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Join Date: Feb 2005
Location: Watertown, NY
Default Got a Problem, Need Some Help

Ok. Here it is. I installed a car audio system. I built the sub myself. The amp is a low end mtx, bridged to give a little over 100 rms (all i need)

Heres the deal: with the engine off, running in accessory, all is great. The sub hits hard and is clean sounding, with no problems at all. As soon as I start the engine, the sub then "pulsates" roughly 1-3Hz varying. You can also hear engine noise in it. And before you say it, I have already confirmed great ground connection (I think). The ground is attached to bare metal, and has perfect continuity with the battery terminal.

Anybody? Oh and I'm using high inputs from the rear speakers hooked up to a factory head unit. The other car speakers do not pulsate with the subwoofer, so it is not coming from the input. I have confirmed all connections on this thing. The remote turn on is coming from a 12v line used to power my stock 6 disc changer located in the trunk. I know a fair share about electronics, I repair them for a living. However, i know nothing of vehicles at least nothing worth noting.

I greatly appreciate the help
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Old 30th April 2010, 12:21 AM   #2
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Join Date: Nov 2003
Location: Louisiana
You first need to confirm that the head unit's shield ground is intact. Unplug the RCA cables from the head unit and measure the resistance from the shield ground of the RCA output on the head unit to the case of the head unit. It should read ~0 ohms. Is that what you have?

If that's OK, the shield ground may be broken on the RCA jack on the amp. Does the amp's RCA jack have a plastic body or a metal frame?
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Old 30th April 2010, 01:12 AM   #3
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Join Date: Feb 2005
Location: Watertown, NY
Hi thanks for the thought, but I am not using rca cables. I am using High input. I have the speaker outputs fom the factory head unit connected to the High Level Inputs on the amp. I also tried not bridging the amp, connecting the sub to only one channel at a time. The symptoms are on both channels individually and bridged. More so with bridged mode because of the nature of bridging an amplifer.
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Old 30th April 2010, 01:21 AM   #4
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You should use a Line Output Converter and use the RCA inputs on the amp. That typically causes fewer problems than using the speaker level inputs.
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Old 30th April 2010, 02:08 AM   #5
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Quote:
Originally Posted by Perry Babin View Post
You should use a Line Output Converter and use the RCA inputs on the amp. That typically causes fewer problems than using the speaker level inputs.
I will give it a try. I don't see how an external one would be any better, a converter is typically only made up of a couple resistors. But hey, I guess I have seen cheap manufacturing enough to say maybe it could be.
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Old 30th April 2010, 02:25 AM   #6
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I no longer install stereo equipment but when I did, the LOC worked better in many cases. A good LOC isolates the ground and blocks DC that's present on many OEM amplifiers. Maybe one of the installers here could offer their opinion.
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Old 30th April 2010, 02:19 PM   #7
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Location: Watertown, NY
I'm also thinking i have crappy factory ground strap. This would introduce noise into anything. I while ago I had to really clean my battery terminals. Car wouldn't start at all, acting funny. I bet there are plenty of other connections in there that need cleanin. Its a 2000 chrysler concorde. Man these things are a pain in the ***. Especially when you have a bad back, its hard to work on cars. All that bending and trying to fit into small spaces, etc. Tonight I'm going to clean every ground connection I can find.
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Old 22nd July 2010, 04:12 PM   #8
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Thought I would update and post my solution as to be somewhat helpful for anybody using the search function and finding my thread.

The fix was a line out converter. While I am not too happy about the low frequency roll off these things introduce into the system, I am happier without power wasting engine rumble.
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