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Old 19th September 2005, 04:17 PM   #11
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Quote:
Originally posted by GhettoSQ
Thanks guys. I guess I will try the isolator first. I remember reading on Perry's site that ground loops can occur when the cable is too long -- mine (from a kit) is 3ft. and I read that 2ft. or less is better. I mounted the amp up front where the old ones were, and rn the ground out thru the firewall to a ground point in the engine bay (pre-sanded, yay). If that's the problem, will an isolator help or should I ground elsewhere?

So, the other thing is the gain setting -- if the RCAs are fine, is it just that a factory HU uses such low-vltage pre-outs that I do in fact need to turn the gain all the way up?

thanks again!
I would strongly recommend against using a isolator. The people that sell these are like the same people that sell dehydrated water to parched people. The underlying problem still remains, you are just "masking" it. Fix the problem, where it originates, and not only will your equipment be happier, you will also sleep better at night knowing your fixed it instead of covering it up.
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Old 20th September 2005, 11:32 AM   #12
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Well, the isolator helped a very little bit but not nearly enough. I also seem to have messed something up since now the car won't start Once I get that figured out it's back to the drawing board. I wonder if my homebrew RCAs are just crummy since the amp grounding point seems fine...
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Old 20th September 2005, 12:02 PM   #13
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On a system where a standard head unit is driving a standard amplifier input circuit, an isolator is generally a bad idea but sometimes they are required especially if two different input circuits are fighting each other. I know of at least 2 very well known brands that pass a small biasing current (for the input circuits) through the shields of the RCAs. This type of input circuit can cause problems for other amplifiers. In those situations, an isolator is sometimes the only solution.

In his situation, there may well be ~5 volts of DC on both of the signal lines going to the OEM amplifier. Some OEM head units don't have DC blocking capacitors on the output signal and the 1/2 regulated power supply biasing could be on both of the signal leads. If this is the case, he's trying to feed that 5 volts into the signal ground of the amplifier. The isolator will block the DC. It will also create a signal that's free of ground loops. Noise may still be present since the the shield ground of the amp's input circuit may be floating. To correct this, you could ground the RCA shield input to the amplifier at the amplifier.

The isolator should be connected to the two signal leads of the OEM signal lines. There will be no connection to any shield wires of the OEM signal lines. The output of the isolator will go to the amp. Try grounding the shield ground at the amp (where the isolator plugs into the amp). I would suggest making the connection with the amplifier off.

Are you 100% sure that the amplifier is in good working order. Some amplifiers have problems with open shield grounds (RCA connectors shield ground breaks from the board). If the amp has an open shield internally, there will be no way to get rid of the noise until the amp is repaired.
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Old 20th September 2005, 09:42 PM   #14
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Well, spoke to MTX and they seem to think that A) the signal wires are too close to the power wires (it's all under the driver's kick panel where the factory stuff was, but the power and ground run right out a grommet, and the signal cables are on the other side of the amp) and B) 3' of ground cable to the factory bolt under the hood is too much and a bad connection. They said I couldn't really shield the wires, the best bet is to re-ground the HU and the amp. What do you guys think?
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Old 20th September 2005, 09:57 PM   #15
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I doubt that any of that will solve your problem but solutions to engine noise are not always what you'd expect.

How far from the head unit is the amplifier?

How hard would it be to connect the shield ground for the RCAs to the chassis (casing) of the head unit? If it's not too hard, make that connection and connect one of the signal wires to the center conductor of the RCA jacks to feed the amp. It would be a good idea to insert a 4.7 microfarad capacitor in series with the wire going to the center conductor. If it's a polarized cap, the positive lead will go to the signal wire. This type of connection may cause a pop if the head unit is switched on/off while the amp is on but the muting circuit should prevent the pop if the amp and head unit are switched together.

Out of curiosity, have you measured the DC voltage on the signal wires?
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Old 20th September 2005, 10:05 PM   #16
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I guess the amp is about 2-3 feet from the HU. The shield wires come from the HU -- do you mean I should loop them back to the HU chassis? Might be possible.

I haven't checked the DC voltage -- what range am I looking for?
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Old 20th September 2005, 10:26 PM   #17
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On a standard head unit, the RCA shield ground is connected directly to the chassis of the head unit. If you run a wire from the chassis of the head unit to the RCA shield ground of the amp, you'd have the same connection as a standard head unit. Here, the term 'shield' has nothing to do with any shield of the OEM wiring.

I wanted to know the voltage to determine whether the signal line has DC on it. If you measure it, leave the meter connected for a few seconds to be sure that you're not seeing leakage across a capacitor. The test should be done with the head unit on and the volume at zero. The voltage will either be near 5 volts DC or 0 volts DC.
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Old 20th September 2005, 11:03 PM   #18
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OK, makes sense...I will give a try soon.
Perry, I hate to ask, but any idea what I could've done to make the car not start? I only tapped 2 wires trying to find the remote lead, doesn't seem like that would do it. All the accessories turn on but it doesn't crank, just clicks once. I checked all the fuses, they are fine, and tried to jump-start with no luck.
Here I am hijacking my own thread...
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Old 20th September 2005, 11:25 PM   #19
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I don't think you could have done anything by probing wires behind the head unit.

Connect your meter leads to the battery terminals (the part that is directly connected to the wire, not the actual battery terminal) and try to start it. Does the voltage drop while the key is in the start position. Watch it closely at the instant that you switch it to the start position.
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Old 20th September 2005, 11:31 PM   #20
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OK, I'll try that when my wife comes back I'm thinking I somehow tapped the anti-theft system...ugh.
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