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Old 20th September 2003, 02:09 AM   #1
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Default LM4766 motorboating / static

I printed a board and assembled a bridged amplifier using the LM4766 from National, and plan to use it for a subwoofer amp. Then, one day, I ran it too hard, and burnt out the input, so depending on where the volume control was, I would get a repeating thump through the speakers (I have to use 2 speakers, since they are 4-ohm each). If the volume control was between 0 and 1/3, or 2/3 and max, it would make the thump sound. When it was between 1/3 and 2/3, there would be no sound at all. I replaced the chip, and the sound was quiet and distorted. After plaing with it for a while, I found that the wire giving pin 15 power had broken, so only half the amp was on. I fixed the wire, and it worked properly. I did notice some motorboating and static in the speaker sometimes. To fix it, I have to physically move the board. What causes this, and how do I fix it?

Thanks for All of your help, Mike
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Old 20th September 2003, 05:34 AM   #2
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Should be a soldering problem somewhere...try wriggling the pins one by one to see which one...or to take the guess work out...re-solder everything...including the PSU...
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Old 20th September 2003, 01:28 PM   #3
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I will try soldering everything again, but there is one other problem I noticed. When the volume is turned up high (as in very hign), the sound will cut out, the speaker will move out about 1/2 inch, then in about 1/2 inch, then back to normal. Then, there will be no sound until I bring the volume down, which is when it begins plaing again. Is this the protrection kicking on, or a different problem?

-mike
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Old 26th September 2003, 09:01 AM   #4
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Have a look at the datasheet.

Specially check out the protection graphs.
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