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silver1omo 29th January 2013 04:12 AM

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:
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?

JMFahey 29th January 2013 05:27 AM

Probably your amp is oscillating big way.
When you connect the capacitor, the series resistor becomes a load and overheats.

tauro0221 29th January 2013 05:33 AM

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?

silver1omo 29th January 2013 05:41 AM

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

tauro0221 29th January 2013 06:09 AM

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.

silver1omo 29th January 2013 06:18 AM

I'm suspecting my grounding. I need to check how to separate the signal and power grounding properly.

tauro0221 29th January 2013 06:21 AM

Can you post the PS schematic?

Michael Bean 29th January 2013 06:24 AM

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.


Bloodmist 29th January 2013 11:21 AM

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)

sraudio 29th January 2013 12:30 PM

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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