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Old 1st November 2008, 03:29 PM   #1
ryan750 is offline ryan750  Philippines
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Default LM3886, 20v at output

well, here's my problem, i've recently built a lm3886 single ended configuration, power supply seems to be ok, i'm getting 25 and -25 volts at the power supply, when i test it for the first time, i'm having a 20v dc at the speaker terminal, help me guys, what do you think is the problem?

here is the schematic

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and the pcb layout

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and a couple of pics

http://i171.photobucket.com/albums/u...z/DSC02734.jpg

http://i171.photobucket.com/albums/u...z/DSC02735.jpg
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Old 1st November 2008, 03:37 PM   #2
ttan98 is online now ttan98  Australia
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Check

1. feedback resistor not properly connected.
2. input terminal not properly grounded.


the LM3886 maybe busted.

overall check your components inserted again.
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Old 2nd November 2008, 03:03 AM   #3
impsick is offline impsick  United States
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whats your negative input doing? im not the best to answer this but there may be a problem somewhere in that area. I dont know is the 2.7r needed? Dont quote me but i think there may have been a short from the input area.
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Old 2nd November 2008, 03:58 PM   #4
jaycee is offline jaycee  United Kingdom
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Quote:
Originally posted by impsick
whats your negative input doing? im not the best to answer this but there may be a problem somewhere in that area. I dont know is the 2.7r needed? Dont quote me but i think there may have been a short from the input area.

THe 2.7R between grounds is just to isolate signal ground and power ground to prevent ground noise. The 220pF between + and - is just to suppress RF noise at the input causing a problem.

Really, the +ve input resistor to ground (47K) should be the same as the feedback resistor (20K) but it wouldn't cause 20V of output offset. I use 22K for both in my own design. My -ve feedback capacitor is 22uF compared to your 68uF but again, that shouldn't cause a big problem.

Your soldering is a little messy. At a guess, the problem is going to be there. Is the chip getting hot with input shorted to ground? If so it's probably oscillating, or dead.
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Old 2nd November 2008, 06:09 PM   #5
AndrewT is offline AndrewT  Scotland
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what are the voltages on pins 9 & 10 and across the 68uF?

The schematic is OK.
It must be the soldering. Either a bad connection or heat damage.

That heatsink looks too small, but that is not the offset problem.
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Old 2nd November 2008, 08:06 PM   #6
llmobll is offline llmobll  United States
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I second the grounding option, double check the mechanical connection on the input terminal.
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Old 3rd November 2008, 11:02 AM   #7
ryan750 is offline ryan750  Philippines
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thanks for the reply guys,

@andrew thanks, later if have time im going to measure the voltages on that area.


i have a thought of the chip being dead,
the first time i plug this chip to the power supply, the power supply is unplugged but the capacitors is charged, and i think that the output speaker wires is touching one another. could this kill the chip??
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Old 3rd November 2008, 02:30 PM   #8
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by ryan750
......... the power supply is unplugged but the capacitors is charged, and i think that the output speaker wires is touching one another. could this kill the chip??
Yes, the output leads would discharge the smoothing caps.
Remember that the current for the amplifier output comes from the smoothing capacitors.
The transformer is there to recharge the capacitors.
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Old 3rd November 2008, 02:53 PM   #9
ryan750 is offline ryan750  Philippines
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thanks Andrew, maybe I'll be much more careful next time, So for now i'll just replace the ic..
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Old 14th November 2008, 04:34 PM   #10
ryan750 is offline ryan750  Philippines
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i've finally be able to replace the chip, 20v at output is gone, but now, there is no audio at the output, any ideas?
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