DC on output

udip · 10th January 2005, 12:34 AM

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:

udip · 10th January 2005, 01:01 AM

BTW, I used no transformer on output......

Maybe this is why........

darkfenriz · 10th January 2005, 01:07 AM

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?

udip · 10th January 2005, 01:42 AM

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???!!!

darkfenriz · 10th January 2005, 01:53 AM

no it isn't for sure
try to skip this 10k resistor to the ground and maybe decrease this 10k r at integrator output

udip · 10th January 2005, 02:18 AM

I tried and no change....

Do u think it's because the different primary resistence of the
transformer?

darkfenriz · 10th January 2005, 02:52 AM

no idea then

udip · 10th January 2005, 02:54 AM

Thanks anyway...

udip · 10th January 2005, 05:34 PM

Anyone have a clue how to measure correctly dc on output?
Either my multimeter or oscilloscope have problem with mixed
signal (AC+ DC)?

Thanks...

Eva · 10th January 2005, 06:11 PM

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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10th January 2005, 01:01 AM	#2
udip diyAudio Member Join Date: Oct 2004 Location: Israel	BTW, I used no transformer on output...... Maybe this is why........

10th January 2005, 01:07 AM	#3
darkfenriz diyAudio Member Join Date: Jun 2004 Location: Warsaw	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?

10th January 2005, 01:42 AM	#4
udip diyAudio Member Join Date: Oct 2004 Location: Israel	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???!!!

10th January 2005, 01:53 AM	#5
darkfenriz diyAudio Member Join Date: Jun 2004 Location: Warsaw	no it isn't for sure try to skip this 10k resistor to the ground and maybe decrease this 10k r at integrator output

10th January 2005, 02:18 AM	#6
udip diyAudio Member Join Date: Oct 2004 Location: Israel	I tried and no change.... Do u think it's because the different primary resistence of the transformer?

10th January 2005, 02:52 AM	#7
darkfenriz diyAudio Member Join Date: Jun 2004 Location: Warsaw	no idea then

10th January 2005, 02:54 AM	#8
udip diyAudio Member Join Date: Oct 2004 Location: Israel	Thanks anyway...

10th January 2005, 05:34 PM	#9
udip diyAudio Member Join Date: Oct 2004 Location: Israel	Anyone have a clue how to measure correctly dc on output? Either my multimeter or oscilloscope have problem with mixed signal (AC+ DC)? Thanks...

10th January 2005, 06:11 PM	#10
Eva diyAudio Member Join Date: Oct 2003 Location: Near the sea	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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