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Old 29th January 2013, 05:12 AM   #1
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Red face LM3886 with some problems

I had an old transformer and got a pair of free lm3886 so I decided to build a chipamp. After doing a little of reading I decided to build the following amp:
Click the image to open in full size.
And as Im quite cheap, I decided to do point to point soldering. I finished 1 channel and decided to test it. To test I used ~+-20VDC.
Shorted the input and looked for DC in the output, my cheap meter had moving values and the max was ~4mV.
So I connected a crappy speaker and a source, got music. Then I realized that the lead of C4 was not connected to ground, so I disconnected the speaker and the source, shorted the input and connected the lead of C4 to ground (all grounds together) and after a few seconds I see smoke and the joint of C4 and R6 melts!

I tried again and got the same result. If I do not connect the lead of C4 to ground it "works", but if I connect it, it gets real hot, real fast. I rechecked the solders/wires and see no shorts.

Does any of you have any idea on what could cause this?
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Old 29th January 2013, 06:27 AM   #2
JMFahey is offline JMFahey  Argentina
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Probably your amp is oscillating big way.
When you connect the capacitor, the series resistor becomes a load and overheats.
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Old 29th January 2013, 06:33 AM   #3
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It is possible that the LM3886 goes into oscillation. Also you are missing a capacitor 220 pf between pin 9 and 10. It is use to prevent oscillation. Did you check the cap to be good?
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Old 29th January 2013, 06:41 AM   #4
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Thanks for the replies, I did test with another cap in C4 and saw the same problem. The amp is project 19 in esp's web site: Single Chip 50W Stereo Amplifier
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Old 29th January 2013, 07:09 AM   #5
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I think you should double check all the connections since you are wiring the amplifier point to point. Ii is easy to miss a problem. I already built one LM3886 and didn't have any problem. I am using my own board. My advice it is to make a copy of the schematic and check it by marking the schematic as you go checking the wiring.
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Old 29th January 2013, 07:18 AM   #6
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I'm suspecting my grounding. I need to check how to separate the signal and power grounding properly.
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Old 29th January 2013, 07:21 AM   #7
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Can you post the PS schematic?
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Old 29th January 2013, 07:24 AM   #8
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It's most likely oscillating because you "connected all grounds together." The low signal (input, feed back network and pin 7 ground) and high current grounds (power by-pass caps and speaker return) must be connected to the power supply common ground separately to prevent the large currents from causing a voltage drop variation in the small signal paths.

Mike
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Old 29th January 2013, 12:21 PM   #9
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If you keep it "working" like that you will likely destroy the speaker.
So welcome to the "separating grounds club" ). Separate signal and power grounds and you should be ok. (not completely! they will still meet in the power supply probably)
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Old 29th January 2013, 01:30 PM   #10
sraudio is offline sraudio  Lithuania
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It seems that you get an oscillations.Best to check with the scope. Get rid of R6 and C4. These can be the reason for oscillations.
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