Pioneer GM-40s.....again! - diyAudio
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Old 23rd July 2007, 02:35 PM   #1
diyAudio Member
Join Date: Jul 2007
Default Pioneer GM-40s.....again!

Ok, I have installed two Pioneer GM-40's one for the front speakers and one for the rear. They have a big 12v cable direct to the battery, and a big ground cable to the negative battery terminal. They switch on using two SPST relays that I have put between the 12v out from the amp PCB, and the BTB wire, which is requires a signal from the 12v out to turn the amps on. These relays use the same ground as the amps, and a switched 12v from the head unit to throw them. the head unit uses the original radio ground in the dash.
With the engine off and the ignition on, everything is great, save from a few audible whirrings from the cd player in the head unit. With the engine on I get a VERY audible alternator whirring, increasing with engine revs. I can also hear the indicator relays through the speakers. After three days of soldering and chopping wires about this is totally annoying, could this be being picked up through my line in? Any ideas welcome, thanks.
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Old 23rd July 2007, 04:15 PM   #2
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Join Date: Nov 2003
Location: Louisiana
It sounds like a ground loop. I don't have a schematic for the GM40 but in the GM120, there are several grounds in the DIN plug. Pin 2 is the audio ground and should not be connected to the other grounds in the plug.

If that's how you have it, you may need to use a ground loop isolator to break the ground loop.

In most amplifiers, there is some sort of isolation between the audio ground and the chassis ground. In the GM120, there doesn't seem to be any isolation. If that's the case with the GM40, the isolator is likely to be the only solution.
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Old 27th July 2007, 12:10 AM   #3
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Thanks, I thought that may be the case. I identified the audio ground on the PCB, this indeed used to be connected to pin 2. Strangely, referencing this to the source ground does not make much diffence, but referencing the amps main ground to the source cuts about 80% of the noise. There is absolutely no noise when the RCA's are disconnected, leading me to agree that this is a ground loop. One other question though, could the RCA sockets on the head unit be badly earthed? I instantly questioned the integrity of some 1980's amps, but not the head unit. The head unit in question is a sony CDX-L450, does anyone know if this has any line out problems? Thanks.
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Old 27th July 2007, 12:37 AM   #4
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Join Date: Nov 2003
Location: Louisiana
Open shield grounds are a common problem. Disconnect all RCA cables from the head unit and measure the resistance from the shield of the head unit RCA to the outer casing of the head unit. It should read very near 0 ohms.
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Old 27th July 2007, 05:09 AM   #5
ppia600 is offline ppia600  United States
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Join Date: Jun 2007
Location: Louis y ana
/\ What he said. People fry the rca grounds on decks all the time, they like to use tiny ground screws on their amps with huge wire terminals that like to get loose.
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