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Power to rear amp in A3 8L

Discussion in 'In-Car Entertainment' started by beanoir, Oct 30, 2011.

  1. beanoir
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    beanoir Sharpest Tongue in the West

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    Right i'm tearing my hair out on this one, I'm hoping somebody can help.

    I'm fitting an aftermarket Parrot headunit in place of the standard one, and I have a non-Bose system in my S3.

    I have installed using the correct adaptors taking low level signal to the amp etc and it all works, apart from an annoying high pitched hum in the rear speakers without playing music or even switching the head unit on.

    Now i've fitted a ground loop isolater and tried all sorts of stuff, and still does it even if I remove the head unit, all of the Autoleads adaptors back to the standard Audi connectors and then just bridge the power from the brown plug straight to the large black amp plug it still does it, but if I connect all the plugs back in to the Audi Concert head unit it stops....

    I think i've effectively eliminated all of the gear i'm putting in from the cause of it, so thats one thing.

    I really can't figure this out unless it's something to do with the Audi head unit stepping the current/voltage down or something??

    Any ideas?

    Cheers

    Nick
    Last edited: Oct 30, 2011
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  2. AndyMac
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    AndyMac Moderator Staff Member Moderator

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    Depends which adapter you are using as there are 2 types.
    Is it the RCA input adapter or the speaker input adapter?
    Also there is no power on the brown plug, it is the front speaker harness.
    TBH you'd be far better just rewiring the rears direct to the HU and bypassing the puny Nokia rear amp.
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  3. beanoir
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    beanoir Sharpest Tongue in the West

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    Thanks Andy,

    I'm using the RCA adapter, so not taking speaker level signal to the amp.

    I think I got the plug colour mxed up, it is in fact the other black plug that feeds power to the headunit. I've also tried tapping in to the fuse box for the power and that produces the same issue. The amp seems to produce feedback through the rear speakers as soon as it is powered up even with no headunit or sources attached. It is only when the power is routed back through the original Concert headunit that it seems to stop.

    I have ordered a Boss power line filter to see if that makes a difference. If not then i'll go with your suggestion of wiring the speakers directly to the headunit. I assume if I do this then the sub will no longer function?
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  4. AndyMac
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    AndyMac Moderator Staff Member Moderator

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    Well you can leave the sub wired in as you have it now as the interference won't be heard through the sub.
    The interference is caused by an earth loop i.e. the earth at the HU is slightly different to the earth at the rear amp. I would also try re earthing the new HU and see if that makes a difference as the Audi earths are particularly poor
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  5. beanoir
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    beanoir Sharpest Tongue in the West

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    Thanks Andy, I'll have another go at it sometime this week and try a few ideas including the earth.

    Cheers

    Nick
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  6. beanoir
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    beanoir Sharpest Tongue in the West

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    Right, I think i'm about to give up on this.

    I tried the earth today, wired from the car chassis at a painless clean point and still no joy or even any affect whatsoever.

    Looks like i'll be re-wiring the speakers to the head unit. What annoys me about this option is it's not a problem fixed it's just masking it, and that bugs me!!
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  7. AndyMac
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    AndyMac Moderator Staff Member Moderator

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    Not at all, it's by far the better option. You really don't want the powerful signal from the new HU to be watered down by the puny Audi rear amp. You will now have twice the power going to the rears as well as the fronts.
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  8. beanoir
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    beanoir Sharpest Tongue in the West

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    Yeah I know....i'll get there Andy, in the end i'll get there!

    I'm very tempted to rip out the Focal speakers and JL amp in my MX5 now and just stick all that in whilst i'm ripping door cards off and re-wiring. This was supposed to be a simple plug and play with the Audi....but it's never simple is it.

    Either way, It WILL be sorted out this weekend.
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  9. AndyMac
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    AndyMac Moderator Staff Member Moderator

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    You don't need to rip the door cards off, you can pick up the feeds to the rears from the connector on the sub
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  10. beanoir
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    beanoir Sharpest Tongue in the West

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    thats interesting...

    Is the connector to the amp/speakers the one I can see attached to the inside of the rear wing in the gap between the sub and the rear lights?
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  11. AndyMac
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    AndyMac Moderator Staff Member Moderator

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    Yes the red one, it is actually clipped to the sub enclosure, the pinouts are here:
    [​IMG]
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  12. beanoir
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    beanoir Sharpest Tongue in the West

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    Andy you're a star, thanks for the help.

    I've spent the morning re-wiring the rear speakers from the head unit to the plug at the sub like you've shown. No more noise through the speakers and it actually sounds so much better. The sub is now running from the sub pre-outs on the head unit which is better because I now have more control over the bass with all the built in filters etc.

    Job done....for now ;)
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