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Old 9th November 2012, 07:25 PM   #1
tbj is offline tbj  United Kingdom
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Default Oops. Popped LM3886?

I am building an LM3886-based chip amp. One channel is working, decided to throw the other one together tonight and stupidly managed to connect the V- rail directly to pin 8 (the mute pin) rather than pin 4. I heard a funny noise and the negative rail went to full -ve.

Turned it off straight away and corrected the wiring, but now the chip does nothing but show full negative voltage on the speaker output. Have I popped it? Is it a usual failure mode for these things?

Rather peed off at myself, given that these things cost over a fiver each.
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Old 9th November 2012, 07:59 PM   #2
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Quote:
Have I popped it?
I'm afraid so. The mute pin is designed for 0.5mA current and it's likely you greatly exceeded that.
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Old 10th November 2012, 01:46 PM   #3
AndrewT is offline AndrewT  Scotland
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The mute pin is connected to the input stage.
It's likely you flashed Vcc right through all the components in that input stage.

This may well be the first mis-wiring that a bulb tester cannot protect against !
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Old 12th November 2012, 05:51 AM   #4
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Originally Posted by tbj View Post
. . . now the chip does nothing but show full negative voltage on the speaker output. Is it a usual failure mode for these things?
Yes, that is the typical failure mode for chip amplifiers.
I like both chip amplifiers and speakers, so I like output caps too.
It is good to include speaker protection in chip amplifier builds.
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Old 12th November 2012, 03:10 PM   #5
johnr66 is offline johnr66  United States
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Originally Posted by AndrewT View Post
The mute pin is connected to the input stage.
It's likely you flashed Vcc right through all the components in that input stage.

This may well be the first mis-wiring that a bulb tester cannot protect against !
I bought some amp ICs from ebay. One came with a supply rail on the output pin right from the seller. The bulb tester wouldn't have helped there either.

The bulb tester would have helped save the speaker though.
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Old 12th November 2012, 03:19 PM   #6
tbj is offline tbj  United Kingdom
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Quote:
Originally Posted by johnr66 View Post
The bulb tester would have helped save the speaker though.
Ouch!

That is why one should test into a dummy load, monitoring the output using an oscilliscope. Resistors are cheap, speakers are not.
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