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Old 10th January 2005, 01:34 AM   #1
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Default DC on output

Hello everybody!!

I have a dc problem on my preamp, and I hope u can help me
find a solution:

I have built a mic preamp, something like the one in linear tech
lt1115 datasheet, with a dc servo instead of caps in the signal line

For the transformer I used SOWTER 9820, with
5k1 and 220pf for termination.

Instead of the LT1115 I used the OPA627 from TI.

For the integrator I used OPA134, 470n capacitors and
220k resistor (instead the 100k and 1uf).

Now here is the thing, I checked for oscillations and other errors
with my oscilloscope, and everything is fine except that I have
50-200mv on the output!! which changes with gain and frequency.

I couldn't find why, does anyone know??

Thanks!!

The schematic:
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File Type: jpg dc.jpg (67.9 KB, 331 views)
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Old 10th January 2005, 02:01 AM   #2
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BTW, I used no transformer on output......

Maybe this is why........
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Old 10th January 2005, 02:07 AM   #3
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there still shouldn't so huge dc on output
what's the point of this 2ma current source?

what is the dc at integrator uotput?
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Old 10th January 2005, 02:42 AM   #4
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Hi,

The 2ma current source is to bias the opamp in class a.....

The dc on the output of the integrator is changing from -15
to 15 VOLTS depending on the gain setting....

Is this ok???!!!
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Old 10th January 2005, 02:53 AM   #5
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no it isn't for sure
try to skip this 10k resistor to the ground and maybe decrease this 10k r at integrator output
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Old 10th January 2005, 03:18 AM   #6
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I tried and no change....

Do u think it's because the different primary resistence of the
transformer?
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Old 10th January 2005, 03:52 AM   #7
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no idea then
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Old 10th January 2005, 03:54 AM   #8
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Thanks anyway...
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Old 10th January 2005, 06:34 PM   #9
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Anyone have a clue how to measure correctly dc on output?
Either my multimeter or oscilloscope have problem with mixed
signal (AC+ DC)?

Thanks...
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Old 10th January 2005, 07:11 PM   #10
Eva is offline Eva  Spain
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It appears that the +-15V output from the DC servo is not enough to correct the offset of the gain devices through the 10k/10ohm attenuator. Try 100 ohm instead of 10 ohm. It may be also a wiring mistake causing the DC servo to produce positive feedback

To measure the DC component from a waveform with a dominant of AC component just insert a lowpass filter before the multimeter [ie: some resistors and capacitors]
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