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Car amp fading hum

I bought my amp (Alpine MRP-F550) over a month ago, but finally got it installed only 2 weeks ago. At turn-on, I've noticed it has a LF hum (70hz?) that slowly damps out (10-40 secs). It happens whether I turn the key to start or accessory and is not dependent on volume. I also found that it doesn't happen every time I turn it on, only if it's been sitting off for a few minutes.

Has anyone experienced this or have any theories as to why it's happening?


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  • Tygh Duval
    Tygh Duval Mar 10, 2008

    Although I did not personally install the amp, I know from talking to the installers that the preamp lines are run on the opposite side of the truck from the power wire.
    I also suspect the issue isn't the RCA jacks touching something metal, as I would expect the hum to be present constantly or at least intermittently. As it is, the amp only hums at turn-on and only after it's been off for at least a few minutes. It's never hummed while on nor immediately after being turned off.
    As for "making sure my frequencies aren't too high", I'm not sure what you are referring to. The amp is running door speakers that are almost full range (using the amp's built-in high-pass crossover at 60 Hz).

  • Tygh Duval
    Tygh Duval Mar 11, 2008

    If you'll read my initial description again, you'll notice that the problem:

    1. Fades after 10-20 seconds, so I can't tweak and adjust to my hearts content

    2. Still occurs even with no RCA preouts (aka the stereo) connected, so the volume of the stereo has no bearing on this issue.
    Plus, the gain is set to nominal, so I doubt that's the issue.

  • Tygh Duval
    Tygh Duval Mar 13, 2008

    I'll be the first one to admit I'm wrong, so here goes. When I mentioned that the gains were set to nominal, I hadn't actually checked them recently. When I went to tweak something on the amp today, I notice one gain (ch1&2 or 3&4, not sure which) was set slightly above nominal. A quick tweak back down, and it seems the hum has gone! So while I'm not sure why a gain adjustment fixes an issue that occurs even when the stereo is not connected, I'm glad its gone!



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Make sure your rca jacks arent touching any metal and your hot (+) cable is not ran on top of your rca jacks and check and make sure your frequencies arent too high

Posted on Mar 08, 2008

  • tony sanders Mar 11, 2008

    turn your gain level down and turn your radio up slowly add the gain level if it stops your amp is too high turn down enough till sound stops



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