Problems with DC offset

Buhl · 21st January 2006, 03:51 PM

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

janneman · 21st January 2006, 04:16 PM

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

Buhl · 21st January 2006, 04:29 PM

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

pinkmouse · 21st January 2006, 04:48 PM

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.

Buhl · 21st January 2006, 04:52 PM

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 ?

Cheers !
Buhl

pinkmouse · 21st January 2006, 05:05 PM

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.

Peter Daniel · 21st January 2006, 05:09 PM

Do you measure offset with source connected or disconnected?

Buhl · 21st January 2006, 05:12 PM

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

Peter Daniel · 21st January 2006, 05:14 PM

Which chips are you using?

pinkmouse · 21st January 2006, 05:17 PM

Thanks for pointing out the first thing I should have thought of Peter!

21st January 2006, 03:51 PM	#1
Buhl diyAudio Member Join Date: May 2002 Location: Aalborg, Denmark	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 __________________ "Calculations? We don't need no stinkin calculations! We just make them really big!" Quote: Nelson Pass

21st January 2006, 04:16 PM	#2
janneman diyAudio Member Join Date: May 2002 Location: Where Germany, The Netherlands and Belgium meet Blog Entries: 6	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 __________________ /New Linear Audio publication: Baxandall & Self on Audio Power!

21st January 2006, 04:52 PM	#5
Buhl diyAudio Member Join Date: May 2002 Location: Aalborg, Denmark	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 ? Cheers ! Buhl __________________ "Calculations? We don't need no stinkin calculations! We just make them really big!" Quote: Nelson Pass

21st January 2006, 05:05 PM	#6
pinkmouse diyAudio Moderator Join Date: Apr 2002 Location: Chatham, England	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. __________________ Al I conceive of nothing, in religion, science or philosophy, that is more than the proper thing to wear, for a while. Charles Fort

21st January 2006, 05:09 PM	#7
Peter Daniel diyAudio Member Join Date: Jul 2002 Location: Toronto, Canada	Do you measure offset with source connected or disconnected? __________________ www.audiosector.com “Do something really well. See how much time it takes. It might be a product, a work of art, who knows? Then give it away cheaply, just because you feel that it should not cost so much, even if it took a lot of time and expensive materials to make it.” - JC


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21st January 2006, 05:12 PM	#8
Buhl diyAudio Member Join Date: May 2002 Location: Aalborg, Denmark	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 __________________ "Calculations? We don't need no stinkin calculations! We just make them really big!" Quote: Nelson Pass

21st January 2006, 05:14 PM	#9
Peter Daniel diyAudio Member Join Date: Jul 2002 Location: Toronto, Canada	Which chips are you using? __________________ www.audiosector.com “Do something really well. See how much time it takes. It might be a product, a work of art, who knows? Then give it away cheaply, just because you feel that it should not cost so much, even if it took a lot of time and expensive materials to make it.” - JC

21st January 2006, 05:17 PM	#10
pinkmouse diyAudio Moderator Join Date: Apr 2002 Location: Chatham, England	Thanks for pointing out the first thing I should have thought of Peter! __________________ Al I conceive of nothing, in religion, science or philosophy, that is more than the proper thing to wear, for a while. Charles Fort

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