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Old 5th September 2007, 07:48 AM   #1
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Default Need HELP with alt whine

I'm at my wits end. I've checked and rechecked connections and even took my vehicle into a reputable audio shop and they could not find the problem

I have a 34 year old truck with a two wire style alternator. I recently bought a HU, eq, and two amps. All have clean grounds. I have unplugged the RCA connectors at each unit working my way back to the HU and no noise (alt whine). I have just turned on the HU, bypassed the eq. and I still get noise. I have turned the HU off and just turned the eq. power on and I get noise.

I unplugged the two wire pig tail out of the alt and the noise was greatly reduced. I assume this is for the charging circuit. I really have no idea where to go from here.

The shop said that old style alts can be the problem. Should I install an alt suppressor (filter) on the output of the alt? I have experimented with RCA filters and they work pretty good except they completely cut the frequency and the system sounds like crap. Thanks....
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Old 5th September 2007, 08:40 AM   #2
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Measure the resistance from the shield ground of the head unit's RCA jacks to the case of the head unit. The resistance should be near zero ohms. Do this with no RCAs plugged into the jacks.
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Old 6th September 2007, 08:13 AM   #3
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The HU has wires for speakers along with a pre-amp output. I wired the speaker wires to rca cables. Everything plugs into the EQ. Anyway, I checked the rca jacks on the EQ since it makes a noise as well, and I wasn't sure what to check on the HU since there is only one set RCA jacks. The ohm reading for the EQ was .5 ohms. Not sure where to go from here. Thanks!!!!
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Old 6th September 2007, 01:35 PM   #4
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Quote:
Originally posted by pityocamptes
I wired the speaker wires to rca cables.
Could you clarify this statement?
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Old 6th September 2007, 09:29 PM   #5
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The HU had regular speaker wires coming out of it. The EQ only has RCA plugs. I connected the speaker wires out of the HU to an RCA plug (basically cut one end off of an RCA wire and soldered the shield to ground and the hot to hot).
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Old 6th September 2007, 09:59 PM   #6
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That would be your problem, then. The speaker outputs are not suitable for plugging into a line level input (EQ), nor are they referenced to ground in a low-noise fashion.

You mentioned your HU has a pre-amp output. Why not just use that? Disconnect the speaker wires on the head unit; they are useful for absolutely nothing. Connect the RCA output of the HU to the EQ input. Then the EQ output(s) to the amplifier inputs.

I don't buy the noisy alternator bit; it takes a LOT of noise to mess up today's head units. As well, a vehicle that old has a FANTASTIC grounding plane; it's actually made of steel, not plastic.
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Old 6th September 2007, 11:17 PM   #7
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If for some reason you HAVE to use the speaker outputs on the HU and not the pre-outs, it is possible to use a circuit like the one below to get the correct output levels.

Click the image to open in full size.

HOWEVER, this will only work with single ended outputs. Newer HUs will have bridged outputs and you cannot under any circumstance connect the negative speaker lead to ground.

You could however try to use the positive speaker lead and the power ground as the signal ground, leaving the negative speaker wire disconnected(I haven't actually tried this, but the theory is sound).
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Old 7th September 2007, 12:28 AM   #8
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I am sure he has a new head unit. Do not use the picture above if you have a new head unit. You really need to try using the RCA jacks of the radio.
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Old 7th September 2007, 02:21 AM   #9
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Quote:
Originally posted by I Am An Idiot
...Do not use the picture above if you have a new head unit...
I said that?

I also said it "SHOULD" be possible to replace "SPEAKER GND" in the pic with the power ground to acheive a single ended output(DC blocking cap required). It might work it might not; the only way to find out is to try. But all that is be pointless if there are working pre-outs.
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Old 7th September 2007, 05:33 PM   #10
shagone is offline shagone  United States
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if there are no available pre-outs why not just get a suitable line out converter to convert speaker level to rca outs. they are cheap and easy to find these days for around 20 bucks.
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