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Blown sub pre-out

Discussion in 'In-Car Entertainment' started by Naresh, Oct 30, 2003.

  1. Naresh
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    Naresh Member

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    Hi can anyone help me as I have just had my Kenny repaired after the sub pre-out was diagnosed as being blown, plus a crossover was damaged.

    The repair centre told me that it was apparently due to a wiring fault so before I re-install it can someone please suggest some probable causes and solutions as well??

    It was the sub and sub amp that was affected so it would be cool if someone could point me in the right direction before I start ripping out my whole system and trim panels again! Both my amps are located in a false floor and earthed to seat belt bolts.


    Thanks in advance.

    Naresh

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  2. AndyMac
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    AndyMac Moderator Staff Member Moderator

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    Hi Naresh
    The most obvious cause is bad earthing on the sub amp. This would cause the amp to try and earth itself via the RCA's back to the HU, and thus the blown pre-out stage. The RCA's and subsequent circuitry in the HU are not designed to take any current at all, the earthing is purely for screening against engine noise. Bearing in mind your sub will be pulling 30amps+ it doesn't take much of a leak to put heavy current back to through the RCA screens. What amp are you using for the sub and has it been tested? It could be a fault on the sub amp itself rather, or in addition to, a wiring fault. If you've got a multi-meter, check you're getting zero resistance between the battery earth and the chassis of the amp. If there is any resistance, even 1 or 2 ohms, then the current will take an easier route back to the HU. Also check you've got an open circuit between the RCA signal inputs to the amp and the chassis of the amp. This current could have shorted out the RCA's on the amp, therefore reconnecting the HU could cause further damage. If in doubt I would pay someone to have a look at your install, as you don't want the same thing happening again.
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  3. Naresh
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    Naresh Member

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    Thanks Andy, that was a very useful reply. I will test the amp stand alone, and with the battery earth to see if its damaged before re=connecting. It was an Intimidator XS so not really a good brand in my opinion, just something powerful enough to drive my sub!

    What do you think about earthing the HU as well - any real benefits to be gained from it?

    Another thing - my amps are earthed to the rear seat belt bolts, which go into the chassis, but via a layer of material which looks like metal but not necessarily the chassis itself. Do you know what I am referring to? I just wanted to know if this arrangement was sufficient in earthing my amps!

    Naresh
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  4. AndyMac
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    AndyMac Moderator Staff Member Moderator

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    Yeah I was gonna say that a quality amp (like Alpine, or Rockford)is designed with protection circuits to stop this happening. I worked on a friends Sony HU once and blew up the internal amp on it just by touching the pre-outs to the chassis while the unit was on. Sh*t design.
    Earthing the HU is only really necessary if you're getting engine whine/interference.
    The rear seat belt bolts should provide a good solid earth. Just checked mine at they give a big fat zero ohms to the earth point near the OEM sub, but it is the connection you need to check, so from the end of your earth lead to a known good earth. It could be the earth strap is not making a good connection, or is too long or not thick enough. Remember it only needs a small amount of resistance to cause a problem and this can be introduced purely by the earth strap itself.
    My guess is you may have ruptured the sub amp RCA input's and they are now shorting to earth. You need to check these first. Go and get a multimeter and check for any continuity between the inner & outer connection on the RCA's. Have a good smell round the amp as well as burnt out electronics have a horrible stink.
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