LM3886 fried? How did i manage? - diyAudio
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Old 6th August 2014, 05:06 PM   #1
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Question LM3886 fried? How did i manage?

Like the title says, i may have burned out a couple of LM3886 chips and i'm not really sure why.
At first when i got them almost a year ago i was using a SMPS to power them so i was using the single supply application.
After a while i decided they can do better directly connected to the speakers so i rewired them for the split supply mode. They worked like that flawlessly for many months, with +/- 38V (measured) and 8 ohm speakers, until 2 days ago i realised the mute caps were left backwards, like in the single supply mode. I reversed them and powered up the circuit. After a few seconds delay (as it should normally be due to the time constant) the transformer started buzzing louder and before i could switch the thing off i heard a loud pop sound and a bad smell appeared. I couldn't really tell where it came from but i suspect it was the IC's. The fuses did not blow not a single one of them (frustrating) and no sound whatsoever came from the speakers as this was happening. Now testing again the IC's are slightly warm as they always were, the voltage is 35V per rail but they don't seem to output anything, not even if i turn the potentiometer to maximum and touch the input pin, then they should make a loud buzz noise.
I have several scenarios in mind for what happened but i'd appreciate an opinion:
1. the supply being not regulated, the voltage had risen way too high during mute condition, then fried them when they turned on
2. the mute capacitors shorted somehow even if they were 50V rated (both of them???)
3. the transient somehow triggered an oscillation
4. the test speakers... yes, they were 4 ohm speakers i noticed afterwards (but there was no signal going through the IC's ?!)
5. the IC's are counterfeit and when they entered their normal regime where they should run they jsut couldn't take it...

Really dying to find the answer because i don't want to cause this again (if it was my fault in the first place) or else i'll probably just go with different IC's instead. I got a chinese stereo "IC" that's made with discrete components that i use now and then but that one can't take the voltage and shuts down after 15 minutes so at the moment i'm confined more or less to headphones.
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Old 6th August 2014, 05:09 PM   #2
AndrewT is offline AndrewT  Scotland
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Did you use a mains bulb tester feeding the transformer to test your assembly wiring?
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Old 6th August 2014, 05:12 PM   #3
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No... not really. So you're basically talking about a large bulb in series with the transformer right? I was hoping the fuses would help but obviously not, not even the main fuse which is 500mA very small for the transformer size.
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Old 6th August 2014, 09:33 PM   #4
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sorry for double post. Here is how i will do it next - no mute cap, it's not needed anyway because the LM doesn't plop at power on. And before you say it, there is no hum, do not worry. But i should add some resistors onto the psu caps to prevent excess voltage build up at transients.
And i have also put fresh mica washers (unfractured) and been generous enough with the thermal paste.
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Old 6th August 2014, 09:46 PM   #5
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1. the supply being not regulated, the voltage had risen way too high during mute condition, then fried them when they turned on
2. the mute capacitors shorted somehow even if they were 50V rated (both of them???)
3. the transient somehow triggered an oscillation
4. the test speakers... yes, they were 4 ohm speakers i noticed afterwards (but there was no signal going through the IC's ?!)
5. the IC's are counterfeit and when they entered their normal regime where they should run they jsut couldn't take it...

1/ Doesn't matter.
2/ Replace them probably faulty/shorted if reversed.
3/ I wouldn't have thought so.
4/ Should be ok unless you hit them with more than half power.
5/ If you got them off ebay then they probably are fake.

Are the two 220uf close to the LM3886 ?
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Old 7th August 2014, 07:12 AM   #6
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Yes they are soldered onto the pins themselves. I would have used larger values but they would have been phisycally too large to be supported by the pins.
I did not get them off e-bay, but worse, from a cheap store that is rumoured to get parts by the pound from an even cheaper supplier that has only parts of unknown origin or quality. Believe it or not this is the first time i had trouble with their stuff, and it was my fault probably.
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Old 7th August 2014, 08:28 AM   #7
godfrey is online now godfrey  South Africa
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IMHO, the relevant parts of the story are:
a) The amp worked flawlessly for many months with the split supply.
b) You reversed the polarity of the mute caps because you thought they were the wrong way around.
c) The amplifier promptly died.

I think it's safe to assume that reversing the caps is what killed the amp.
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Old 7th August 2014, 09:12 AM   #8
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If you have the T version and the heatsink is connected to ground. The insulation between chip and heatsink is the only thing preventing total destruction of the chip.
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Old 7th August 2014, 09:21 AM   #9
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@godfrey they actually *were* the wrong way around all the time, at least according to the datasheet... but as nigelwright said, they were probably damaged from having reverse voltage applied. funny thing is they're not even needed.
@Mark Whitney yes the T version, but i measured and there was no short. I replaced the washers anyway like i said, the old ones weren't very thermal conductive.
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Old 7th August 2014, 10:07 AM   #10
AndrewT is offline AndrewT  Scotland
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post4,
PSU is wrong.
That is not the way to wire up a bridge rectifier.
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