LM3875 Gainclone Not Working and Stuck to Rail ... - diyAudio
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Old 24th February 2013, 11:52 AM   #1
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Default LM3875 Gainclone Not Working and Stuck to Rail ...

Hi ...

Today just build a gainclone LM3875 amplifier and found that it is not working ... output stuck to rail ... the circuit i used is i followed from the NS datasheet ...

BTW , i found that in the feedback path there is a capacitor in the schematic By NS. But i refer to some of the DIY projects schematics they are not included ... is it the cause of my problem ... ?

I need serious help here ... any feedback and suggestions are much appreciated and welcomed ...
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Old 24th February 2013, 01:06 PM   #2
Bone is offline Bone  United Kingdom
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So you used Fig 1.? What were your DC supplies?
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Old 24th February 2013, 01:42 PM   #3
JMFahey is offline JMFahey  Argentina
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Please post the schematic here so we all talk about the same.
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Old 24th February 2013, 02:41 PM   #4
DUG is offline DUG  Canada
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Fig 1 or Fig 3?
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File Type: jpg LM3875-FIG-1-SPLIT-SUPPLIES.JPG (35.0 KB, 172 views)
File Type: jpg LM3875-FIG-3-SINGLE-SUPPLY.JPG (42.5 KB, 169 views)
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Old 24th February 2013, 02:45 PM   #5
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Fig 1 dual supply version ...
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Old 24th February 2013, 03:00 PM   #6
JMFahey is offline JMFahey  Argentina
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Ok.
Measure and post voltage on all 5 legs shown (1/3/4/7/8)
Measure from PS ground to each leg, straight at the chip.
Be careful not to let the multimeter tip slip and short to another leg.

By the way: I hope the chip is mounted to a heatsink and that you insulated it with mica+grease (and a screw-insulating nipple if needed).
Also, with power off, measure resistance to ground from leg 4 .
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Old 25th February 2013, 05:18 AM   #7
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ok ... get it work perfectly already ... caused by bad grounding issue ...
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Old 25th February 2013, 10:46 AM   #8
JMFahey is offline JMFahey  Argentina
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Quote:
caused by bad grounding issue ...
Which was .....
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Old 25th February 2013, 10:50 AM   #9
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1st ... i didnt use the proper star grounding
2nd ... bad soldering joint from the ground to the 1k feedback resistor (which suppose connect to ground because of bad contact it didnt ...)
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Old 25th February 2013, 10:56 AM   #10
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completed amplifier
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