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Old 2nd October 2006, 01:10 PM   #1
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Default problem with lm 3886 application circuit

hi,

i´m having the following problem with my circuit using the lm 3886:
if the speaker is connected to the output of the amp, it plays for 3 or 4 secs after power up and then it stops, the current sinks to about 20 mA and then nothing changes until a new power up. my circuit is nearly the application circuit from the datasheet.
a few more infos on my circuit:
-power supply ist quite low at the moment, +- 15V
-blocking capacitors are soldered dirctly to the pins of the chip.
-there are two 1000uF bypass caps on the chip board
-the mute pin (pin 8) ist connected to Vs- over a 22k resistor, so mute mode should be off
i know that for a detailed analysis i had to give more infos, but i was
hoping that this is perhaps a "standard" problem with well known reasons.
thank in advance,

jonas
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Old 2nd October 2006, 01:15 PM   #2
djQUAN is offline djQUAN  Philippines
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do you have a heatsink? if you do, is it big enough? if you don't have any, get one.
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Old 2nd October 2006, 01:29 PM   #3
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yes, the chip is mounted upon a heatsink. it should be big enough at least for testing, its about 12X7X3 cm.
ive measured the voltage drop over the resistor which connects the mute pin to gnd. it decreases continuously after power up, until the chip is in mute mode. the decrease is much faster when a speaker is connected, but also with open output the chip goes tu mute mode, after a few minutes
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Old 2nd October 2006, 01:33 PM   #4
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sorry the mute pin is connected over a resistor to vs- of course, not to gnd
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Old 2nd October 2006, 01:39 PM   #5
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About the mute and undervoltage protection.

Under 12 V DC you have a muting function. Are you aware of that? 15 VDC is very low, why not use a higher voltage?
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Old 2nd October 2006, 01:47 PM   #6
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i used the 15VDC because that is the limit of my DC Power Supply.
i have also the components for a power supply from a 18V~ transformator, but i have to finish it yet. Then i will try it again and post the result.
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Old 2nd October 2006, 02:42 PM   #7
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ok, that solved it! with about 25 VDC the problem described above has
vanished. but here´s the next:
without input signal(input is open), the dc voltage across the output is about 9V without speaker. when connecting an 8ohm speaker, it rises to 10V. that means the current drawn is more than 1 amp .
accordingly the chip gets quite hot...
what could be the reason?
thx for every answer!
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Old 2nd October 2006, 02:56 PM   #8
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Check to make sure the capacitors are reversed.

Check all wiring that goes to ground.

Have you checked for cold solder points?
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Old 2nd October 2006, 03:27 PM   #9
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i checked every connection, seems to be ok.
after power up, the dc voltage at the output rises continuously. i will post the circuit diagram
later, and perhaps take a photo.
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Old 2nd October 2006, 05:18 PM   #10
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I bet your first problem was the Mute resistor was too high. Change it to 15K for the 15V. The good way to remember what value to use is to simply take the supply voltage and put a K on it. So if +/-25V then use something around 25K, round down to a standard value. Having more current pulled out of the Mute pin will not hurt anything.

If getting DC then something is wrong with the circuit. Check to be sure you are not getting DC from the input, is there an input cap? If not is there any DC from the source? Check the caps. in the circuit as the most likely problem.

-SL
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