Opamp/DC-Offset Problem (Help!) - diyAudio
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Old 8th September 2004, 09:45 AM   #1
MWP is offline MWP  Australia
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Unhappy Opamp/DC-Offset Problem (Help!)

Hi all,

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

Click the image to open in full size.

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.
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Old 8th September 2004, 10:15 AM   #2
Nuuk is offline Nuuk  United Kingdom
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Shouldn't that 10K from non-inverting to ground be 180K?
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Old 8th September 2004, 10:34 AM   #3
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Circuit looks fine to me. Are you measuring the voltage at the positive input?

Your speakers will enjoy life close to the edge
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Old 8th September 2004, 10:46 AM   #4
MWP is offline MWP  Australia
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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?
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Old 8th September 2004, 11:06 AM   #5
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Just to make sure that the pot/bias source are correctly connected. The negative input then will have little choice but follow.
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Old 8th September 2004, 11:07 AM   #6
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What are supply voltages for your OPA627 and your LM3886?
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Old 8th September 2004, 11:13 AM   #7
MWP is offline MWP  Australia
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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.
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Old 8th September 2004, 11:45 AM   #8
MWP is offline MWP  Australia
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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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Old 9th September 2004, 04:00 AM   #9
drdagor is offline drdagor  United States
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Default 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.
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Old 9th September 2004, 04:13 AM   #10
drdagor is offline drdagor  United States
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Default 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.
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