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Old 7th July 2008, 09:48 PM   #1
mudihan is offline mudihan  United States
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Location: Texas
Default LM3886 -- outputing 30vdc

I build my chipamp.com LM3886 amp before the summer and it sounded great. Yesterday, I had some shorting problem with my passive preamp (just a volume control) and, with the amp and speakers connected and powered, I likely burned the voice coils in my speakers (smell and smoke). My mistake.

Now, when I reconnect the speakers to the amp, the woofer produce a constant hum and the cone got pushed all the way out. Both the amp outputs measure around 10ma of DC, which is fine. But the DC voltage at those outputs is around 30V. I don't have another pair of speakers to test the amp with and really I dare not to buy/borrow one.

Are the speakers the only bad parts or is my amp also broken?
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Old 7th July 2008, 09:58 PM   #2
AndrewT is offline AndrewT  Scotland
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I think both the amp and the speaker is damaged.
Is the other channel OK?
Or is that both channels with the same problem?
regards Andrew T.
Sent from my desktop computer using a keyboard
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Old 7th July 2008, 10:27 PM   #3
mudihan is offline mudihan  United States
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Location: Texas
Both channels measure the same and produce the same results on my broken speakers (the humming and the pushing of the cones). Is the 30vdc at the outputs that indicates a broken amp? Is so, is there a way I can find out whether it's LM3886 chips or other components (which are easier to replace) that have been damaged? Thanks!
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Old 9th July 2008, 09:47 AM   #4
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Join Date: May 2008
DC output at approximately rail level is a typical sign for a broken output transistor. So you will most probably have to replace the LM3886s. Your 10mA DC measurement seems to be wrong. With an intact speaker connected you should get several amperes (30V/8Ohm=3,75A, 30V/4Ohm=7,5A). If the speakers were broken, they would not move outward with DC or produce hum. They may however break, if you keep using them with the broken amplifiers.

Another reason for that amount of DC output would be a corresponding DC signal at the input, which you could trace easily.
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Old 10th July 2008, 05:14 AM   #5
mudihan is offline mudihan  United States
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Join Date: Jan 2004
Location: Texas
Yes, the current must be over the limit of my meter. I just noticed that the meter was getting really hot within seconds.
Thank you for all of your help. I have de-soldered the chips and ordered new ones.
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Old 10th July 2008, 06:20 AM   #6
YFW is offline YFW  United States
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It seems the chip was killed.
Please remove the loudspeakers as soon as possible and measure it's impedance.
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Old 10th July 2008, 09:32 AM   #7
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Originally posted by mudihan
[B]..... I have de-soldered the chips and ordered new ones ..... /B]
I suggest ordering in the meantime secondary fuses for the LM3886's too. So, for each board two of them I do not care if there's a protection inside the chip allready. Your case is a good example for it that it doesn't help for all conditions ...

the truth is somewere
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