Op-amp DC offset question...

Thread Starter

foxfire3

Joined Jan 29, 2013
34
OK thanks for the clarification. The potentiometer was always in circuit.

I am still at a loss then to explain why you could test various resistor values from the non-inverting input to ground (or at least in parallel with the pot wiper to ground ?) and obtain a change in output offset whilst adjusting the volume control setting had no effect at all. The latter variation in potentiometer setting is in principle equivalent to placing the various resistors across the potentiometer. If that's not the case then you seem to have discovered an interesting phenomenon that appears to be at variance with expected circuit behavior.

Unless your circuit was actually as shown in the attachment with the potentiometer orientated the other way around ....

I shan't bother you any further but leave you with that conundrum to consider.

Good luck with your project!
You're not bothering me.

No, I drew the circuit correctly. The volume control is oriented just like the one in the headphone amp above...not like you posted.

Just for clarification, when trying different resistor from the inputs to ground I left the volume control near its minimum(as for volume level) position. So this would be close to 10k ohms.

Initially, when I only had the 1K resistor in series with the pot, I did vary the volume control while measuring the offset as you or someone here suggested.
Varying the pot had no effect on the offset as it was 1.6 volts anywhere I set the pot.

Based on what I see and everyone's recommendation here, I see no way to use the AD828 for amplifying audio.
I think it could be made to work as a buffer, but not much else in the audio field.
I have taken it out of my circuit and reinstalled a FET input op-amp.
Now the offset is only a few millivolts...just where it should be.:)

Thanks again guys!
 

Ron H

Joined Apr 14, 2005
7,063
OK thanks for the clarification. The potentiometer was always in circuit.

I am still at a loss then to explain why you could test various resistor values from the non-inverting input to ground (or at least in parallel with the pot wiper to ground ?) and obtain a change in output offset whilst adjusting the volume control setting had no effect at all. The latter variation in potentiometer setting is in principle equivalent to placing the various resistors across the potentiometer. If that's not the case then you seem to have discovered an interesting phenomenon that appears to be at variance with expected circuit behavior.

Unless your circuit was actually as shown in the attachment with the potentiometer orientated the other way around ....

I shan't bother you any further but leave you with that conundrum to consider.

Good luck with your project!
You and I had the same idea. My drawing isn't as pretty as yours.:eek:
 

Attachments

Ron H

Joined Apr 14, 2005
7,063
Just for clarification, when trying different resistor from the inputs to ground I left the volume control near its minimum(as for volume level) position. So this would be close to 10k ohms.
The resistance from the +input to ground would be 1k ohms with this pot setting.
 

Thread Starter

foxfire3

Joined Jan 29, 2013
34
The resistance from the +input to ground would be 1k ohms with this pot setting.
Because of the 1K resistor I later installed in parallel with the pot?

Otherwise without any resistor going to ground I would think it would be 11k ohms as in my initial drawing.
 

t_n_k

Joined Mar 6, 2009
5,455
Because of the 1K resistor I later installed in parallel with the pot?

Otherwise without any resistor going to ground I would think it would be 11k ohms as in my initial drawing.
No with the pot set at minimum position the resistance to ground from non inverting (+) input would be 1k.
 
Last edited:

Thread Starter

foxfire3

Joined Jan 29, 2013
34
No with the pot set at minimum position the resistance to ground from non inverting (+) input would be 1k.
I think we must be getting "minimum position" mixed up.

By minimum position I mean it's turned down where the sound would be the lowest in level and where the pot would near its maximum resistance.
That would be 10k plus the 1k series resistor or 11k ohms, right?

Or am I mixed up on that as well?:confused:
 

t_n_k

Joined Mar 6, 2009
5,455
For the volume to be turned fully down the pot wiper would be at ground potential. This would correspond to the fully counterclockwise position for a typical volume control.
 
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