LM3886 Chip Amp Getting VERY HOt - diyAudio
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Old 30th December 2013, 02:19 PM   #1
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Default LM3886 Chip Amp Getting VERY HOt

I recently "finished" building an LM3886 amplifier from Chipamp.com. I have it currently hooked up using a 22v transformer, so after it is rectified to DC power it is about +/-30v. One of the boards is working great, I believe. I can only leave it on for about 5 seconds, so I couldn't do much testing. But on to my problem, the other board.
On the second board, the chip gets hot. It gets noticeably hot in about 5 seconds, and too hot to touch in about 10. This happens exactly the same regardless of if there is input, no input, speakers connected, or no speakers. If I connect speakers, it plays music, but with a terrible buzzing/crackling noise (sounds much worse and different than I imagine ground loop hum would be). For a while you could hear a hum from the board itself, although I could not tell which component specifically, and it has since stopped. One of the rectifiers gets a little warm, and the transformer is heating up too, more than what I think is normal. The chips are attached to a heat sink, but my thermal paste has not come yet. I understand that these chips can get hot when not cooled properly, but this seems excessive. Does anybody have any idea what the problem could be, or what I should check next?

Thanks for all your help! Remember, this is my first build, and I'm a beginner, so please keep the replies somewhat simple, stuff I can understand.
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Old 30th December 2013, 04:01 PM   #2
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A heatsink without some sort of thermal interface material (ie paste) isn't going to do much.
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Old 30th December 2013, 04:08 PM   #3
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what kind of insulator did you use between the chips and the heatsink ?

check your solders, post a picture...
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Old 30th December 2013, 04:14 PM   #4
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Originally Posted by peufeu View Post
what kind of insulator did you use between the chips and the heatsink ?

check your solders, post a picture...
None, yet. But it still shouldn't get that hot in a matter of seconds, I don't think. I will post pics tonight, when I'm home from work.
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Old 30th December 2013, 04:37 PM   #5
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Refer to the section of the data sheet "Determining the Correct Heat Sink"

Second issue is oscillation which will cause the chip to heat up correctly. Proper dressing of leads and an output zobel are mandatory on the TI Overture series chips.
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Old 30th December 2013, 04:40 PM   #6
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You should not run the LM3886 without heatsink. Approx. 50 mA * 60 volt = 3 Watt = hot!
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Old 30th December 2013, 04:46 PM   #7
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Originally Posted by jackinnj View Post
Refer to the section of the data sheet "Determining the Correct Heat Sink"

Second issue is oscillation which will cause the chip to heat up correctly. Proper dressing of leads and an output zobel are mandatory on the TI Overture series chips.
What is an output zobel? And what do you mean by "proper dressing of leads"?
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Old 31st December 2013, 03:26 PM   #8
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Guys the chips are the insulated ones so there was no need for insulators between the chips and heat sink.

I helped bryce put this amp together and we DO have a heat sink in it, but we were out of thermal paste so there is none between the chip and heat sink which I under stand is not good. We have a parts express order coming in this week with paste and will add it right away. The heat sink was however filled flat so there is good contact with the chip and aluminum. I wouldn't think the chips would heat up that fast when not powering anything. My chip amp runs really cool at low volume and only gets warm at full volume.

The transformer he is using is 22 + 22 volts but only 25va which will get up graded in the future to 200va.

He said both the bridge rectifiers are getting hot as well as the transformer which makes me thing you guys are onto something with the oscillation. We put the amp together at my house and I tested it for just a moment (since we didn't have thermal paste) and all was well. The amp was sent home with Bryce so he could add proper input and output jacks and then the problems showed up. Ether something changed after it left my house, or I didn't test it long enough to see the problems which is very possible because I didn't test it for very long.

Perhaps Bryce can post some pictures of his amp so you guys can get a better look. Please keep in mind he was going for a "steam punk" look and the case is a little rough : )

Dale P.
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Old 31st December 2013, 03:59 PM   #9
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Looking at some posts about oscillation I wonder about the length of our speaker wires. They are quite long and run from one side of the case to the other.

You guys mentioned an output zobel net work. would something like this picture help or it this already built into the chip amp board?


Dale P.
Attached Images
File Type: gif 1229794706_5870_FT15493_zobel.gif (2.4 KB, 230 views)
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Old 31st December 2013, 04:19 PM   #10
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Speaker cables can be quite long in a room, so doesn't matter how long it is in the amp.
If the 2 sides are identical, then check the connections, maybe bad soldering somewhere. Everything is from chipamp.com? Board, parts?
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