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Old 10th April 2010, 07:26 AM   #21
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Originally Posted by luis.jimenez View Post
the other channel is still exhibiting some of the same symptoms, and it gets extremely hot right off the bat - the other channel doesn't. I checked everything and can't figure out what's causing it to get so hot. Could the chip be shorted inside? It amplifies the music, but very noisy and with a cadence, not like the motorboating earlier but like the protection ckt is kicking in and out, in and out, every half a second.
The amp could be picking up RF. Funny that you should use an RF filter before the transformer, but none at the amp's in- and output. If RF is coming in through the output, then the components *Rsn, *Csn, *R and *L shown on page 6 of the datasheet should help. If RF is picked up through the input, Cc directly at the IC pins could do the trick. If your amp is oscillating due to layout issues, *Cf and *Rf2 could marginally improve the situation.
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Old 10th April 2010, 10:28 AM   #22
AndrewT is offline AndrewT  Scotland
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sounds like the remaining channel is oscillating. This is causing the various chipamp protection systems to activate and affect the sound output in addition to the distortion of mixing the audio with the oscillation.
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Old 10th April 2010, 11:34 AM   #23
Piggy is offline Piggy  Hong Kong
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Originally Posted by luis.jimenez View Post
Hello again,

I corrected the ckt and got one channel to work, pretty quiet too. But, the other channel is still exhibiting some of the same symptoms, and it gets extremely hot right off the bat - the other channel doesn't. I checked everything and can't figure out what's causing it to get so hot. Could the chip be shorted inside? It amplifies the music, but very noisy and with a cadence, not like the motorboating earlier but like the protection ckt is kicking in and out, in and out, every half a second.

Any help is appreciated.

Thanks,

Luis
It happen to me a few times......

My LM3886 prototype is on the test bench. The PCB is homemade. The circuit I am using is from the LM4780 data sheet page7, Fig5. The only different is a 4.7uF cap and 22K resistor at the input. I use 1K/22K at the feedback network. Each time I power on the amp, the DC offset at the output is -0.2mV for both channels and I can feel just a little bit of temperature on the chips. The heat sink is cool. No AC voltage will be detected at the output terminal. No hum buzz or hiss. The is the working condition of my amp.

Because it is a prototype, sometimes I will modify the component. Before I turn on the amp again, I will remove the power of the output relay and measure the DC offset. If the DC offset is not -0.2mV ( ~0.2 V or something like that), I will just power off the amp immediately . When I touch the chip, it is hot.

The only thing I can do is to reconnect all the cables. (I use screw terminals for the amp PCB), the DC offset will be back to normal. The DMM will not show me anything......

If I got the motor boating sound, when I touch the input cables, the motor boating sound will changed, it could be the the bad contact of the cables as well...or a faulty volume pot. I just replaced my faulty pot yesterday. This is what I observed.

Because I am no expert, this is all I can tell you. Good luck.
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