Opamp/DC-Offset Problem (Help!)

MWP · 8th September 2004, 10:45 AM

Hi all,

My new DAC+AMP is based around this simple circuit:

The DAC outputs the audio on a 1.6V bias volatge.

The design was meant to use the pot to trim out this voltage on the first opamp.
But no matter what its set to, i always get a -0.7V dc-offset on the OPA627 ouput.
This then leads to a +12V dc-offset on the LM3886's output.

Can anyone see what is wrong here?

I really dont want to put DC blocking caps after the OPA627 to fix this.

Thanks for any help.

Nuuk · 8th September 2004, 11:15 AM

Shouldn't that 10K from non-inverting to ground be 180K?

analog_sa · 8th September 2004, 11:34 AM

Circuit looks fine to me. Are you measuring the voltage at the positive input?

Your speakers will enjoy life close to the edge

MWP · 8th September 2004, 11:46 AM

Quote:

Originally posted by Nuuk
Shouldn't that 10K from non-inverting to ground be 180K?

I dont think so.
It wouldnt really make a difference anyway.

Quote:

Originally posted by analog_sa
Circuit looks fine to me. Are you measuring the voltage at the positive input?
Your speakers will enjoy life close to the edge

The positive input?

analog_sa · 8th September 2004, 12:06 PM

Just to make sure that the pot/bias source are correctly connected. The negative input then will have little choice but follow.

Yoghourt · 8th September 2004, 12:07 PM

What are supply voltages for your OPA627 and your LM3886?

MWP · 8th September 2004, 12:13 PM

Quote:

Originally posted by analog_sa
Just to make sure that the pot/bias source are correctly connected. The negative input then will have little choice but follow.

So my design is correct then?
If its built correctly (which im 99% sure it is as 4 of these circuits do the same thing) what could it be?

Quote:

Originally posted by Yoghourt
What are supply voltages for your OPA627 and your LM3886?

OPA627 is 12-0-12.
LM3886 is 35-0-35.

MWP · 8th September 2004, 12:45 PM

Strange... it needed 2.4V on the OPA627's + pin.
All fixed, can now get ~5mV on the LM3886 output.

Thanks.

drdagor · 9th September 2004, 05:00 AM

The op amp bibles say to use a servo to control offset. There are some good notes on designing servos in these notes.

Essentially, provide negative feedback through a low pass filter -- very low pass (like 8 Hz for f0) and it will lock the output of the first stage to ground.

Be sure to use a low noise opamp in the feedback circuit, because you are applying the output to one of the inputs of your gain stage.

drdagor · 9th September 2004, 05:13 AM

The op amp bibles say to use a servo to control offset. There are some good notes on designing servos in these notes.

Essentially, provide negative feedback through a low pass filter -- very low pass (like 8 Hz for f0) and it will lock the output of the first stage to ground.

Be sure to use a low noise opamp in the feedback circuit, because you are applying the output to one of the inputs of your gain stage.

8th September 2004, 10:45 AM	#1
MWP diyAudio Member Join Date: Oct 2002 Location: Adelaide, South Australia	Opamp/DC-Offset Problem (Help!) Hi all, My new DAC+AMP is based around this simple circuit: The DAC outputs the audio on a 1.6V bias volatge. The design was meant to use the pot to trim out this voltage on the first opamp. But no matter what its set to, i always get a -0.7V dc-offset on the OPA627 ouput. This then leads to a +12V dc-offset on the LM3886's output. Can anyone see what is wrong here? I really dont want to put DC blocking caps after the OPA627 to fix this. Thanks for any help.

8th September 2004, 11:15 AM	#2
Nuuk diyAudio Member Join Date: Feb 2003 Location: Somerset, SW England	Shouldn't that 10K from non-inverting to ground be 180K? __________________ The truth need not be veiled, for it veils itself from the eyes of the ignorant.

8th September 2004, 12:07 PM	#6
Yoghourt diyAudio Member Join Date: Jun 2004 Location: Alpes	What are supply voltages for your OPA627 and your LM3886? __________________ Dzharis: raaaaaah lovelyyyy!

9th September 2004, 05:00 AM	#9
drdagor diyAudio Member Join Date: Mar 2004 Location: Seattle	Servos might help The op amp bibles say to use a servo to control offset. There are some good notes on designing servos in these notes. Essentially, provide negative feedback through a low pass filter -- very low pass (like 8 Hz for f0) and it will lock the output of the first stage to ground. Be sure to use a low noise opamp in the feedback circuit, because you are applying the output to one of the inputs of your gain stage. __________________ Dr. Dagor

9th September 2004, 05:13 AM	#10
drdagor diyAudio Member Join Date: Mar 2004 Location: Seattle	Servos might help The op amp bibles say to use a servo to control offset. There are some good notes on designing servos in these notes. Essentially, provide negative feedback through a low pass filter -- very low pass (like 8 Hz for f0) and it will lock the output of the first stage to ground. Be sure to use a low noise opamp in the feedback circuit, because you are applying the output to one of the inputs of your gain stage. __________________ Dr. Dagor


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8th September 2004, 11:34 AM	#3
analog_sa diyAudio Member Join Date: Aug 2002 Location: Sofia	Circuit looks fine to me. Are you measuring the voltage at the positive input? Your speakers will enjoy life close to the edge

8th September 2004, 12:06 PM	#5
analog_sa diyAudio Member Join Date: Aug 2002 Location: Sofia	Just to make sure that the pot/bias source are correctly connected. The negative input then will have little choice but follow.

8th September 2004, 12:45 PM	#8
MWP diyAudio Member Join Date: Oct 2002 Location: Adelaide, South Australia	Strange... it needed 2.4V on the OPA627's + pin. All fixed, can now get ~5mV on the LM3886 output. Thanks.

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