Question about JBL PB12 Subwoofer

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Noise when it turns on

Hi, I have a JBL PB 12 sub and have a problem, when it turns on automatically it produces a humming sound, just in the instant the leds go from red to green, then it runs fine with no audio distortions. I know it has a delay circuit just before the final amplification stage to prevent it to damage the speaker when turned on at very loud volumes and it seems to be a problem with this circuit. My guess is the zener circuitry just before the power stage, but I'm not 100% sure. Does anybody has an idea on where to check?
Thanks, Douglas

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Re: Noise when it turns on

Have you checked your music source? Some systems like computers have a sort of output first before normalizing, which isn't your powered subs or powered amps problem. Try plugging in a regular solid state player like an ipod if the humming is still there when automatically turning on or off...

Posted on Dec 09, 2007

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Are you running the Ipod into its L & R RCA jacks as prescribed?

Have you tried it with something besides the Ipod? A sub isn't supposed to reproduce a full range signal. Come to think of it, is the Ipod even outputting anything within the sub's operational range? I don't expect a lot of real bass comes out of one.

Perhaps you could try adjusting the Low Pass up to 180 to see if anything anywhere near the 'real' bass region is present.

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Loud hum from a loosened cable is normal as hum usually means there's an ungrounded connection somewhere acting as an antenna and the amp is doing its job to amplify what it thinks is a signal, in this case 60hz stray ac current is being sensed nearby. It's a good sign that you get hum from the sub because that means the amp is alive.

Avoid manipulating cable connections with the Sub or Receiver turned on as you could introduce a static spike that could harm any connected equipment.

Are you certain a bass signal is making it into the sub? The complexities of modern AV receivers vary in how you configure them to direct LFE to a sub. Bone up on that end of the chain.

Or, just hang a CD player on the Inputs of the sub and play something with a reassonable amount of Low Frequencies.

>>>> Be advised that this way of testing has NO VOLUME CONTROL because amps without volume controls always operate at maximum gain, relying on your external controls to attentuate it (hence, the loud hum from a loose cable); so choose a track that eases into the loud parts with some quiet parts up front and BE READY TO PAUSE THE PROGRAM AT ANY SIGNS OF STRESS. <<<<

A living sub will produce only muffled rumble in the absence of other speakers producing the higher frequencies which carry the intelligence of the signal. If that works, back to the receiver for settings. If not... sigh.


For hum problems, even those you cause yourself:

Disconnect all inputs to see if that helps. If it goes away start with the signal cables and add in things until it comes back.

Sometimes the reversing the orientation of the AC plug can help with hum. Or it could be something like a loose or high resistance connection internal to the sub. Good luck.

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There is actually some good news.  I had this problem with my PB10s (1 have 2 PB10s and 2 PB12s) It is one of 2 things, the first is that it could be a capacitor and resister combination on the amplifier board (which was the fix for our PB10 with the same problem), or 2nd, the AMP IC, which is not available for the PB12. hmmm

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