LM3886, 20v at output

ryan750 · 1st November 2008, 03:29 PM

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

and the pcb layout

and a couple of pics

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

http://i171.photobucket.com/albums/u...z/DSC02735.jpg

ttan98 · 1st November 2008, 03:37 PM

Check

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

the LM3886 maybe busted.

overall check your components inserted again.

impsick · 2nd November 2008, 03:03 AM

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.

jaycee · 2nd November 2008, 03:58 PM

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.

AndrewT · 2nd November 2008, 06:09 PM

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.

llmobll · 2nd November 2008, 08:06 PM

I second the grounding option, double check the mechanical connection on the input terminal.

ryan750 · 3rd November 2008, 11:02 AM

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??

AndrewT · 3rd November 2008, 02:30 PM

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.

ryan750 · 3rd November 2008, 02:53 PM

thanks Andrew, maybe I'll be much more careful next time, So for now i'll just replace the ic..

ryan750 · 14th November 2008, 04:34 PM

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?

1st November 2008, 03:29 PM	#1
ryan750 diyAudio Member Join Date: Oct 2007	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 and the pcb layout and a couple of pics http://i171.photobucket.com/albums/u...z/DSC02734.jpg http://i171.photobucket.com/albums/u...z/DSC02735.jpg

2nd November 2008, 03:03 AM	#3
impsick diyAudio Member Join Date: Dec 2006 Location: Huntington Beach, Ca	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. __________________ "Nothing happens for a reason. There are just reasons why things happen." Quote me on that. http://stores.ebay.com/AudioGrade?_rdc=1

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1st November 2008, 03:37 PM	#2
ttan98 diyAudio Member Join Date: Apr 2006 Location: Melb	Check 1. feedback resistor not properly connected. 2. input terminal not properly grounded. the LM3886 maybe busted. overall check your components inserted again.

2nd November 2008, 06:09 PM	#5
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	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.

2nd November 2008, 08:06 PM	#6
llmobll diyAudio Member Join Date: Feb 2005 Location: UCSB	I second the grounding option, double check the mechanical connection on the input terminal.

3rd November 2008, 11:02 AM	#7
ryan750 diyAudio Member Join Date: Oct 2007	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??

3rd November 2008, 02:53 PM	#9
ryan750 diyAudio Member Join Date: Oct 2007	thanks Andrew, maybe I'll be much more careful next time, So for now i'll just replace the ic..

14th November 2008, 04:34 PM	#10
ryan750 diyAudio Member Join Date: Oct 2007	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?