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Old 20th January 2009, 11:57 PM   #1
scraph is offline scraph  United States
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Default Blown amp pre-amp in or HU pre-amp out?

To begin with, I have a DC pop that has appeared. It is always on power off, not always on power on. I have two amplifiers powering on with the HU. One is a subwoofer amp w/o a speaker currently connected. The other is a two channel. Both amps are connected via RCAs to the appropriate HU pre-amp outs.

When the HU is powered on there is no measurable DC offset on the speaker terminals or on the pre-amp terminals. When the HU is powered off ... an 8.8Vdc potential appears across the pre-amp terminals (w/ RCA still connected to HU and amp). When all RCAs are removed the 8.8V remains on the Amp pre-amps but can be discharged, apparently capacitively. When any RCAs remain connected to either amplifier the 8.8V persists even on disconnected RCAs (ie still connected to HU). When all RCAs are disconnected the 8.8V vanishes. The 8.8V cannot be produced when main fuse is removed from the HU. The DC pop is still present on power-off with RCAs removed from amp but at a very low level. DC pop volume much louder when RCAs connected.

Any clue? I have never seen this 8.8V before... is this the rail voltage of the HU pre-amplifier? Or is the fault in the amplifier pre-amp section?

Thanks for any insight, explanation, or suggestion you can offer.
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Old 21st January 2009, 03:07 AM   #2
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Does it have horrific engine noise? Do you have a Pioneer radio?
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Old 21st January 2009, 03:50 AM   #3
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A class D output will appear on a DC meter to be hovering around Vcc/2. In fact it's switching between 0V and 12V at 100s of KHz but your meter can't see that.

The system is working OK right? I mean, it plays music without distortion on all channels?

It's probably a timing thing. You want the last amp stage to power down first while the preamp maintains a sane input signal. Vice versa on power up.

Just a thought.
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Old 22nd January 2009, 04:42 PM   #4
scraph is offline scraph  United States
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Yes, there is engine noise. Yes, it is a Pioneer receiver ... possibly DEH-7300MP? Not certain of that, at work right now. It appears to be the pre-amp section of the amp ... I had to pull the fuse on the amp to prevent the burning component smell... I had the same receiver on another amp in another truck ... with no noise problems but in this truck I could not find a grounding point that worked for me. The DC pop started coming out of the amp when I was searching for a noise free ground for the HU. I got complacent and was just bringing the ground lead around the truck holding it to points ... and sometimes it would slip off and shut down the HU gracelessly.
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Old 22nd January 2009, 10:40 PM   #5
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I will post a link tonight that will help you fix your problem.
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Old 23rd January 2009, 12:58 AM   #6
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http://bcae1.com/images/rca/temporar...eldrepair.html
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