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Old 21st January 2006, 02:51 PM   #1
Buhl is offline Buhl  Denmark
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Default Problems with DC offset

I have build a PA 100 - more or less as shown in the 1192 app note - but I have serious problems with DC offset. I measure 0.17 volts on two of the chips, and 1.18 and 11.9 volts on the two others !!
I am not using matched resistors or anything as mentioned in the app note, and the amp is DC coupled. I am using the schematic on the madaboutsound site.
Does anybody have any clou what can be giving me this huge offset - I dont find any errors in my implementation.

Cheers !
Buhl
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Old 21st January 2006, 03:16 PM   #2
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Hi!

There are many here that would be able to give you an advice, but who are not familiar with this particular amp (out of probably several 1000 floating around in internet space). It would be helpfull (and also considerd courteous) to post a schematic.

Jan Didden
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Old 21st January 2006, 03:29 PM   #3
Buhl is offline Buhl  Denmark
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Hello Jan
Thnx for the hint - should have thougth of it, as it is obvious that people cant help one if they dont know what to help with.
Here goes the schematic:

Thanks !
Buhl
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Old 21st January 2006, 03:48 PM   #4
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Default Re: Problems with DC offset

Quote:
Originally posted by Buhl
...I am not using matched resistors or anything as mentioned in the app note...
That is most likely your problem. Try following the instructions and see if that sorts out the problem.
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Old 21st January 2006, 03:52 PM   #5
Buhl is offline Buhl  Denmark
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Ok, thank you pink mouse - I will try, but really, almost 12 volts of offset due to unmatched resistors ??
How do I match them - I mean, better then my ohm-meter, can I apply some sort of voltage, and then measure the voltage drop ?


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Buhl
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Old 21st January 2006, 04:05 PM   #6
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Eek,I missed the decimal point on the 11.9V.!

2V or so is quite possibly due to component mismatch, but the 12V suggests a dead chip or wiring error. For sensitive matching of components, a Wheatstone Bridge is the way to go, quite simply made, and google will give you all the info you need rather than going into lots of detail here.
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Old 21st January 2006, 04:09 PM   #7
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Do you measure offset with source connected or disconnected?
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Old 21st January 2006, 04:12 PM   #8
Buhl is offline Buhl  Denmark
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Hi peter
I measured without source.
I have found a dealer near me who sells 0.1 % resistors, so I guess this is the way to go. The two "crazy" offset values must be an error though..... i will go through the whole circuit to nigth.....


Thanks for helping !
Hans
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Old 21st January 2006, 04:14 PM   #9
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Which chips are you using?
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Old 21st January 2006, 04:17 PM   #10
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Thanks for pointing out the first thing I should have thought of Peter!
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