2nd opinion on car audio hum - AVS Forum
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post #1 of 6 Old 12-14-2010, 03:01 PM - Thread Starter
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I recently had my stock head unit replaced with a Kenwood KDC-MP445U. One of the first things I noticed is that has an annoying hum, roughly 130 Hz to my ear. The hum does not scale with the volume, it's most audible when the volume is at zero, or if there's a relatively quiet segment in the music. It's not apparent if the music is high, but it's really annoying between songs and if the music hits a low. It doesn't appear to change with engine usage.

The installer claims it's not a ground loop because it's constant in volume. His best theory is that it's because the KMP445U's output is high compared to what the stock amplifier is expecting. His solution is to re-wire the car speakers so they're directly connected to the head unit, which is somewhat expensive in labor. My problem is I'm having trouble is I think the hum is consistent with what I know about ground loops, which would only require a ground loop isolator to fix. Of course, I could be mistaken.

Can anyone shed some light on this?
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post #2 of 6 Old 12-15-2010, 05:00 AM
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Ground loops change in both frequency and amplitude so I would doubt this to be a ground loop.

It sounds like an unshielded line is running too close to an electronic clocking device of some kind. Maybe there is a line inside the HU that wasn't run properly or a defective output filter on the HU. Either way I would suspect a faulty HU of some kind because what you're hearing is not normal.
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post #3 of 6 Old 12-15-2010, 06:36 AM - Thread Starter
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Thanks for the response. Now it seems I've got a different problem - the guy who runs the audio shop isn't even considering that it might be a faulty head unit. He's convinced it's due to a voltage mismatch between the head unit and the amplifier. I don't particularly want to have my speakers re-wired, less pay for that, if it's not going to fix the problem.

I suppose I could contact Kenwood directly about a warranty repair / replacement. That would mean I'd have to learn how to pull the existing head unit myself.
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post #4 of 6 Old 12-15-2010, 09:54 AM
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Quote:
Originally Posted by GusSmed View Post

I recently had my stock head unit replaced with a Kenwood KDC-MP445U. One of the first things I noticed is that has an annoying hum, roughly 130 Hz to my ear. The hum does not scale with the volume, it's most audible when the volume is at zero, or if there's a relatively quiet segment in the music. It's not apparent if the music is high, but it's really annoying between songs and if the music hits a low. It doesn't appear to change with engine usage.

The installer claims it's not a ground loop because it's constant in volume. His best theory is that it's because the KMP445U's output is high compared to what the stock amplifier is expecting. His solution is to re-wire the car speakers so they're directly connected to the head unit, which is somewhat expensive in labor. My problem is I'm having trouble is I think the hum is consistent with what I know about ground loops, which would only require a ground loop isolator to fix. Of course, I could be mistaken.

Can anyone shed some light on this?

Can't see how this will be that expensive labor-wise, but not sure what your car would entail. I also agree that it isn't a ground loop. I also don't think you are overdriving the amp or at low volumes it wouldn't be there and at high volumes you would get clipping/distortion. Not sure it is a faulty head, but you can try swapping it for a new one. I agree with Barney that something is picking up noise and it could even be coming through where a traditional ground loop gets picked up. System noise can be a PIA to track down and eliminate. Good luck!

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post #5 of 6 Old 12-15-2010, 09:55 AM
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I remember awhile ago an installer on here mentioning that they often run a shared ground wire between the headunit and amplifer. You could try doing this. Find the ground from the headunit and run a wire to the ground on the amplifier temporarily to see if it helps.
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post #6 of 6 Old 01-17-2011, 04:13 PM
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Quote:
Originally Posted by razel View Post

I remember awhile ago an installer on here mentioning that they often run a shared ground wire between the headunit and amplifer. You could try doing this. Find the ground from the headunit and run a wire to the ground on the amplifier temporarily to see if it helps.

Agreed...
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