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Old 30th April 2004, 08:21 AM   #1
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Default High DC-Offset - Why?

OK, I know there are solutions etc already availible through searching, but my question is why would this be happening.


The story:
First, I had my amp all built(identicle to BrianGT's NIGC) with LM3875T's, then after forgetting about insulating the chips, I killed them. OK, now all my components seem fine (although I don't know how to test the 1000uf Caps).

So I've now replaced the LM3875T's with LM3875TF's, and I'm getting DC-Offset anywhere from 60mv to 900mv and its consistenly boucing back and forth between various values. My previous setup did no produce anywhere near this amount of dc-offset, it was actually less than 60mv constantly.

So why would this be the case? Everything else has stayed the same, my power supply still produces 25.5v DC nicely, and all my resitors have been replaced to try and improve the problem, but to no avail.

Any ideas?
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Old 30th April 2004, 09:16 AM   #2
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The LM3875 has in fact (worse case) rather much offset but you can also create offset by choosing not optimal resistor values.

You can do two things:

Measure the voltage across the + - inputs. Should be 1-10 mV plus or minus

Measure the input bias currents. Check the datasheet how much they should be (rather much).

All solder joints are OK? The inverting input is connected to ground? The input coupling cap is OK? You have no oscillations?
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