lagoausente said:
sahib said:
It's just the way that configuration works out. There is not much you can do about it. As bcarso said by using a step attenuator you may get some improvements at best.
I would replace the volume pot with around 20K.
Remove 100K.
Remove C2 and by pass it.
Add 1uF in series with the input.
This is a cheaper option because of its lower part count and the opamp still gets its DC path. It may work better. Worth a try.
Diferent tips make me confused.
One suggesting lower Z pot, other 20k pot. What win higher Z on the pot in that configuration?
What advatage have using a cap at the input instead C2?
What´s the mision of 100k resistor so can be omitted?
What about PRR sugestion about changin R4 to 1k?
Sorry for if I´m tiring. I can understand the basic of how an operational works, but I miss knoledge about impedances and the behaviour of this changes. Does this loading effect occur in every headphone amp based on a op-amp?
I appreciate your suggestions, but I have doubts regarding what really I´m doing.
First I should clarify your concerns in terms of the operation.
R2 is there to set the input impedance and to provide a DC path to the ground. Without it the stability will be all over the place. We need it.
C2 is also there to stop any DC entering into the opamp that may come from the source. We need that too.
C2 also have another function. With R2 it forms a high pass filter and has an ultimate effect on the hf response of the amp. It also interacts with the potentiometer. The wiper of the potentiometer being variable, the resistance C2 sees also changes. So things get a bit out of hand here. Long story. But the important thing is that these all have their own share in terms of the problem you are encountering.
As far as R2 is concerned the source impedance is the value between the input pin and the wiper of the potentiometer and the output impedance of the previous stage. Although when the wiper is in the middle, the impedance ratio between the source and the load is still within prescribed 1:10. Bcarso's thinking is to reduce the potentiometer value so that the loading effect on R2 is also reduced.
My thinking is that, I get rid of R2 altogether, by pass C2 and connect the input of the opamp directly to the wiper of the potentiometer. I increase the pot value to say 22K. Now through the low side and the wiper of the potentiometer the opamp gets its DC path to the ground. Although that varies, it does not matter very much. One thing important though, I am betting on the opamp being standard consumer grade opamp. I increase the value of C2 and move it to the input and the opamp is protected against DC that may come from the source. As far as the source is concerned, it sees 22K potentiometer as the load and more than sufficiently high but the previous interaction between the potentiometer and R2 is eliminated.
We are all trying to find a solution on paper but the result is only achieved with the solder and iron. Try them.