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Old 20th May 2008, 06:50 PM   #1
Dan2 is offline Dan2  South Africa
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Default engine noise - bad amp??

i know there have been many threads on this topic, but i went through them and couldn't find a solution to my problem.

i want to hook up my mids to my amp (located in the trunk). it is a 4 ch and 2 run my sub, so i just hooked up the back mids, switched to high pass filter, started the car and got alternator niose coming through.

first i thought it was the rca cable, cos it runs with the power cable. so i hooked another cable up to the head unit (temperary, over the seats etc) and still got the noise.

i checked the ground wire, from gnd on amp to battery gnd was about 1ohm (or 10 ohm, cant remember) between them. so i hooked up a ground wire straight from amp to battery, still got noise. but i did use a thin wire, which i thought was ok cos i didn't crank it up.

i also checked the rca jacks on the head unit and the amp. the negative terminals are shorted to the chassis of both the amp and head unit.

so now i wonder if i just have a crappy amp with bad (or no) noise filters in the signal line. oh yes that reminds me, if you switch the lpf on the noise goes away - but then so does the music!!!

i dont have too much time or money to play around with - i am hoping its something simple i am missing. or else i will just stick to using the head unit's amps.
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Old 20th May 2008, 07:15 PM   #2
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You need to re-check the resistance from the RCA shield on the amp to chassis ground with no RCAs plugged into them. It can't be 0 ohms to ground. If it is, the amp is defective.
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Old 20th May 2008, 09:18 PM   #3
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Typically, the chassis of the head unit is tied to the car frame, just by nature of the mounting. If the RCA shield is tied to the frame (which you measured to be the case), there's a likely source of noise. Same goes for the amp.

Also verify, at least for testing purposes, that the amp frame is isolated from the car frame. As Perry mentioned, disconnect the RCA leads before measuring any further.
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Old 20th May 2008, 09:57 PM   #4
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The head unit's shields are virtually always connected directly to the case of the head unit (and therefore chassis ground). The amp's shield can't be tied to chassis ground. If they are, there will be a ground loop and noise. The amp input shields need to be able to float.

On Rockford and MTX amps, there is a diode that sometimes shorts (when someone allows a 12v source to contact the shields). This causes the RCA shields to become shorted to chassis ground and will cause noise.
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Old 20th May 2008, 10:22 PM   #5
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Quote:
Originally posted by Perry Babin
The head unit's shields are virtually always connected directly to the case of the head unit (and therefore chassis ground). The amp's shield can't be tied to chassis ground. If they are, there will be a ground loop and noise. The amp input shields need to be able to float.

On Rockford and MTX amps, there is a diode that sometimes shorts (when someone allows a 12v source to contact the shields). This causes the RCA shields to become shorted to chassis ground and will cause noise.
Perry, don't most pioneers heads make an exception to this rule? Could swear I remember their low level outputs no being tied to chassis ground.

- Matt
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Old 20th May 2008, 10:34 PM   #6
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In virtually all head units, the shields are supposed to be tied to the case of the head unit. Pioneer uses a fuse to protect the vehicle if someone allows a 12v source to contact the shield of the RCA cable. This is a common mistake and the fuse opens. The following page covers it briefly.

http://bcae1.com/images/rca/temporar...eldrepair.html
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Old 21st May 2008, 03:26 AM   #7
ppia600 is offline ppia600  United States
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Quote:
Originally posted by MatthewS


Perry, don't most pioneers heads make an exception to this rule? Could swear I remember their low level outputs no being tied to chassis ground.

- Matt

Haha, the pioneers are most DEFINATELY tied to ground if working properly. What amp are you using? And yes on a good amp that actually uses a seperate power supply instead of just some 12v ic amp, the rca inputs shouldn't be connected to ground. (of course make sure the rca cables aren't connected when testing)
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Old 21st May 2008, 05:15 AM   #8
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Quote:
Originally posted by ppia600



Haha, the pioneers are most DEFINATELY tied to ground if working properly. What amp are you using? And yes on a good amp that actually uses a seperate power supply instead of just some 12v ic amp, the rca inputs shouldn't be connected to ground. (of course make sure the rca cables aren't connected when testing)

Are you laughing at me to be rude? or do you just think I'm that funny?

It was a legitimate question. I had a very old pioneer CD deck that one day, out of the blue, during normal operation began to develop an 'echo' effect. When I took it apart to look for ground damage I didn't find a direct tie to ground, and I didn't find an inline SMT fuse either. What I found was each ground line running to a SMT transistor.

My assumption at the time was that pioneer was either attempting to 'filter' the ground to reject ripple and noise, or that it was some sort of balanced arrangement. I was 17 the last time I saw it, and that was a long and inexperienced time ago. Either way I don't think it was an outrageous thing to mention here. It' wasn't directed at you, and I'm not the one in need of help here.

But thank you for attempting to marginalize my intelligence. Whatever makes you feel superior I guess?

- Matt
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Old 21st May 2008, 05:35 AM   #9
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The transistors were probably muting transistors. They clamp the audio to prevent pops. The muting transistors are not active when audio is present at the RCA jacks.
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Old 21st May 2008, 06:49 AM   #10
ppia600 is offline ppia600  United States
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"and I'm not the one in need of help here" ???? Chill out dude, you are jumping to conclusions WAY too quickly. First, do a search and you will understand why I mentioned them being DEFINATELY grounded. Second, I wasn't poking fun at you. The pioneer rca grounds are supposedly known for being delicate, its been mentioned many times. But since you obviously measured them to ground the deck should be ok. Just in case you noticed I also tried to give some helpful info. You still haven't mentioned what amp it is by the way. That would be helpful, some of these guys (me included) might be able to help diagnose the problem if you shared that piece of info. This is one of the few good sites where you don't have to worry about snobby responses. Everyone tries to be helpful.

Quote:
Originally posted by MatthewS



Are you laughing at me to be rude? or do you just think I'm that funny?

It was a legitimate question. I had a very old pioneer CD deck that one day, out of the blue, during normal operation began to develop an 'echo' effect. When I took it apart to look for ground damage I didn't find a direct tie to ground, and I didn't find an inline SMT fuse either. What I found was each ground line running to a SMT transistor.

My assumption at the time was that pioneer was either attempting to 'filter' the ground to reject ripple and noise, or that it was some sort of balanced arrangement. I was 17 the last time I saw it, and that was a long and inexperienced time ago. Either way I don't think it was an outrageous thing to mention here. It' wasn't directed at you, and I'm not the one in need of help here.

But thank you for attempting to marginalize my intelligence. Whatever makes you feel superior I guess?

- Matt
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