Need help fixing amp in my car; amp has power, but sub wont work

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Need help fixing amp in my car; amp has power, but sub wont work

  • Just under two weeks ago i went to Bestbuy to have them look at my sub/amp.  It just wasn't working anymore.  After they were all done my sub started working again for about two weeks, but still would cut out and sometimes not work correctly.  Eventually it just stopped working again.... which really has ticked me off.  The guy at Bestbuy said he re-wired everything and I even checked to make sure and all the wiring is perfectly fine.  He also said that my amp was "dying" and that I'd need to purchase a new one. I personally know several people whose amps have last over 5 years and I'm not even that harsh with blasting my subs and what not.  I figured there's gotta be something I can do to fix this and that's why I came here for help.  My amp shows that it has power but currently has no output.  I'm pretty much a noob when it comes to this stuff, but I've been trying to do my research lately because it's really been irritating me,  Help would be very appreciated!  


  • Amp and sub make and model #'s ?

    GLH Geeked

  • Alpine MRP-M500 is the amp

    and i bought these used i know its a Kicker CompVR 12" im not sure if its the 2 or 4 DVC.....

  • Used equipment? That's probably the first error.

    Need to know for sure the model of the sub and it's wiring configuration. Could be that the amplifier has been asked to play at a lower impedance that it's designed to play at and this damaged it.

    Tell me, have you a Digital Multi-Meter?

    If so, measure sub's Impedance, is it presenting a 1 ohm, 2 ohm or 4 ohm load?

    Second, unhook the sub and then measure the amplifier's speaker terminals for an AC voltage with the stereo playing.

  • Yeah he only had it for two months and he's a friend so i figured nothing would be wrong.  

    I learned how to use the DMM today and the impedance of my sub is 4 ohms.  my amp im almost positive is 2 ohms so tha could be it, but could you explain how to measure the amplifier's speaker terminals for an AC voltage with the stereo playing?? I tried to put the DMM on 200 V-ac and I left the wires screwed in with the cord, but unplugged the cords from the sub.   And then I went and played the music with the red probe on the positive speaker output and the black probe on the negative speaker output but it read nothing?  and i took note that the only wired going from my sub to my amp was one wire (black and blue) and it connected straight to the speaker output section of my amp, but nowhere else.  I wasn't sure if that was correct?  


  • Sub presents a 4 ohm load, so that is what the amplifier would run at, even though it is stable down to 2 ohms.

    The CompVR is a DVC sub, so the model must be a DVC 2 ohm wired in series for a 4 ohm final load. That or it's a DVC 4 ohm model with just one voice coil being used (which is BAD).

    Sub is likely rated for 400RMS, which the amp is rated to produce 300RMS@4 ohms, so it could be that the gain were set too high and that could have damaged the sub, but if it's reading 4 ohms, that's unlikely.

    Sounds like you have measured the amps terminals for and output signal properly, so that "could" indicate a faulty amp. Though you lost me a little when you said the cords were unplugged from the sub, but then say,

    and i took note that the only wired going from my sub to my amp was one wire (black and blue) and it connected straight to the speaker output section of my amp, but nowhere else.

    So the sub "was" still wired to the amplifier, but it was just one of the wires (positive or negative)?

    Still, no output was measure at the amp, so now we need to figure out why.

    Do you have a 3.5mm to RCA cable you can use to plug a CD player, iPod, phone, MP3 player, etc into the amplifier's RCA input jacks directly as an alternate input source? If so, this would let us know if the preamp inputs are likely bad on the amplifier.

    Also, stepping back a little, you said the amplifier shows power, but the little blue light doesn't really mean the amplifier is powering up, just that it's receiving power. Sometimes, and amplifier only needs a remote turn-on lead and the ground connected for the LED to light up, after all, the light doesn't demand much power. That said, where is the amplifier grounded? It could be that the ground is poor enough that the amplifier can't fully power up, but the connection is sufficient for the LED to light up.

  • 1. So Tommorow I will open up the box and see if the sub has two DVC or just one.  If it has two should i hook them up in parallel series so that it puts out 2 ohms? to match with my amp?  And if it ends up being only one

    DVC what should I do?

    2. What I am trying to say is that there is only one cord that connects my sub and amp.  That specific cord is blue and white and they break apart into the positive and negative speaker outputs on the amp and then connect to the sub and they break apart into the positive and negative terminals of the sub.  I was just trying to see if that was correct.   When I turned my car on to play music and measure the AC voltage of the amp's speaker outputs (described in last post) I unplugged them from my sub like you said and it read nothing.   So I am still not sure how to read that unless I did do it correctly and it just wasn't putting out anything?

    3. I'm not really sure what a 3.5mm RCA cable looks like but I can look that up and see if I do have one and will test it in my ipod if I do tomorrow.

    4. That makes total sense, but my ground is connected in my trunk.  I can take a picture of it and post it up here tommorow.  ( I can do that right?)

  • If the sub is a CompVR, it's DVC (Dual Voice Coil), but the question is, is it DVC 2 ohm or DVC 4 ohm and weather or not both coils are wired together, either in series or parallel. If only one voice coil is being used, and it's a dual voice coil 4 ohm model, then wire the coils in parallel to drop the impedance to 2 ohms and wire to a functioning amplifier stable to run at 2 ohms that can produce a solid 400RMS@2 ohms.

    Ah, I see. It really does sound like you did things correctly. I mean you set the DMM for AC voltage and with music playing, were measuring the speaker output terminals of the amplifier looking to any sort of voltage. With the sub presenting a 4 ohm impedance, to produce 300RMS you would be getting 34.64VAC, but that voltage should only be measured if you were playing something like a test tone and at the volume levels used when the gain were set. With music, since it's very dynamic, I would expect you to have gotten a continuously changing voltage that may have spiked up around 35VAC, but would regularly read much lower, say the mid to high teens for example.

    This is an example of the cable I'm asking about, not that I expect you to have one, but you could for all I know.

    Yep, you can post a photo of your ground, that's perfectly within the guidelines of what you are permitted to post. Hopefully the spot the for the connection of the ground has been scraped free of paint and has a good electrical connection to the car's chassis. Seat belt or seat bolts are frequently used and often provide a poor electrical ground connection.

  • Ok cool I can easily check the Sub tommorow.

    And since I did everything right and there was no output on the amps speaker terminals what should i do?

    I'll look for an RCA and I'll also post that picture of my grounding tommorow.

    Thanks so much, You're help is greatly appreciated. I can't wait to solve this problem!

  • Awesome, let us know what you find out about the sub.

    Since there was no output from the amplifier measured, we are going to try and find out why. It could be that the RCA inputs are bad on the amp, RCA preamp outputs are bad on the stereo, ground may be too weak for the amplifier to power up, etc, etc. All we know for sure at this point is that the amplifier is getting power and isn't producing an output. The Blue LED doesn't tell us if the amp is going into protect mode or not fully powering up due to a weak ground, just that the amplifier is getting power (either from both the battery and/or remote turn-on lead), as well as there is a ground connection that is adequate enough for the LED to light up (which as I said, doesn't take much).

    I'll await your next update.

  • Well I think I figured out the problem.  The RCA preamp outputs are bad because when i plugged my RCA cord (which i did have!) into the amp and played my iphone nothing came out of the sub.  

    I also learned that my sub is the DVC @ 2ohms, as it was labeled on the back of the amp,  So if the amp needs to be played @ 2 ohms, then how do I fix the sub to put out an impedance of 2 ohms as well? Couldn't this be a problem too??  One of the coils is connected to the sub box , cause the wires are sodered into the speaker terminals, with the wire and the other voice coil has a single blue wire ( the wire usually has blue and black and is adhesed together) running from the positive and negative coil prongs.  Not sure if that was right?

    What should I do next?

  • Maybe.

    Still haven't learned if there's a possible ground issue.

    We are ASSuMEing the amplifier is getting power, but you could use the DMM set for DC voltage and measure the power reaching the power terminal.

    Also need to look into the ground of the amplifier still, as it "could" be a part of the issue. "IF" the amplifier isn't getting enough power (less than 10V or so on the power wire) or the ground isn't sufficient for the amplifier to power up (perhaps due to corrosion at the connection point on the vehicle now), then the amp will not power up, though the LED may light up, giving a false indication that the amplifier is actually powering on.

    If the amplifer isn't powering on, then the amp won't produce power, no matter the source signal.

    So far, we have learned that the issue is not that the amplifier wasn't getting an input signal from the stereo on the RCA inputs, as the amp didn't produce an output with an alternate source.

    We also know that the sub is a DVC 2 ohm model that is presenting a 4 ohm final impedance, so it's coils must be wired in series. This isn't a problem, as the only options would be 4 ohms or .5ohm (which the amplifier isn't stable at). The only issue, which isn't affecting the amplifier's ability to function, is that when it runs at 4 ohms, it is rated to produce 300RMS, which is 100rMS less than the sub is rated for. Issue with this, clipping can become a real possibility, though it's the least of our worries at this point. Once we get this or another amplifier working properly, then we will address how to properly set the gain.

    So, now we are waiting on the update on the ground.

  • how do you upload a photo from the computer on here?

  • As you can see, the sheet metal of the car is painted, which paint is a very good isolator.

    Factory bolts don't often make for ideal grounding locations and with the location being used, there's no only the painted body of the car, but a bracket that goes to (?). While the bracket is unpainted, it's pressed against the painted body of the car. Really, the only thing that is grounding the amp, is the bolt and where it's threads touch bare sheet metal of the car inside the hole of that thin sheet metal. not to mention, that particular panal may only be tack welded to the rest of the car.

    My recommendation is to at least remove the bracket and sand the body of the car where it makes contact, then reinstall everything. Actually, I'd say you'd fair better if you picked a location on the floor of the trunk that you can still reach from under the car and scrape it free of paint, drill a hole, then securely fasten the ground there using a bolt and locking washer/nut.

    So, address the ground and let us know if it makes any change. Weather or not it does, your grounding location needs to be addressed regardless.

    Also, nice mess of wiring. And I see the amplifier appears to not be securely fastened, so it can potentially slide about in the trunk. Are you telling me "THIS" is what BB left you with?!

    Lastly, I see a black box on the power wire, what is that? A fuse holder? Distribution block?