Cmoy volume control

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lagoausente

Well-known member
Joined
Oct 2, 2006
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288
Location
Spain
Hello,
  I have the impression that the volume control on my CMOY headphones amp, has some quality loose. When the pot is at middle position, the amp doesn´t sound as clear as when is to max. Of course, when I put to max, I decrease the level on the pc to compensate that.
   I have thought if could be used a rotary switch like used in preamps, to adjust gain,  but have read that the gain of the op-amp circuit of the CMOY, has some relation with impedance, and the impedance requirements of headhones.  Anyone can give an opinion?   Maybe could I have  better result if replace the pot  by a rotary switch switch, maybe?
 

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I don't know the circuit, but it may not have much to do with the quality of the potentiometer as it does with the loading on the wiper.  Therefore replacement of the pot with a step attenuator might give little improvement.
 
bcarso said:
I don't know the circuit, but it may not have much to do with the quality of the potentiometer as it does with the loading on the wiper.  Therefore replacement of the pot with a step attenuator might give little improvement.

What loading do you refer?

The step attenuator would be just resistor replacing the pot?
 
lagoausente said:
bcarso said:
I don't know the circuit, but it may not have much to do with the quality of the potentiometer as it does with the loading on the wiper.  Therefore replacement of the pot with a step attenuator might give little improvement.

What loading do you refer?

The step attenuator would be just resistor replacing the pot?

Oh good you have the correct schematic now!

I mean the loading by IC1, and C2, R2. 
 
PRR said:
Reduce R4 to 1K. How does it sound?

Conductor said:
Increase C2 to 1uF or 2,2uF.

  I´ll try it tomorrow. Do you mean that the value of R4 and C2 does affect to the loading that Bcarso refers to?
  I would liket o understand the reasons, if is not too much work for you explain that, about the relation about the pot position, and R4, and C2.
 
Only problem i ever had with the volume control is that the dual pot doesn´t track very well (i used 4.7k instead 10k, could be lower)
 
Silvas said:
Only problem i ever had with the volume control is that the dual pot doesn´t track very well (i used 4.7k instead 10k, could be lower)


  4,7 pot instead 10k would decrease  the loading problem that tells Bcarso?
 
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.
 
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.
 
 
Is your Cmoy headphones amp relative to this project:
http://www.headwize.com/projects/cmoy2_prj.htm

If yes, there are comments in this page about some trouble shooting noise issues for this amp
(For example it is suggested to connect amp input to line out jack from PC audio card, rather than the headphone out jack)
 
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 lf 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.

 
sahib said:
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.


    Thanks a lot sahib for your explanation.  I find  both  theorical and iron necesary, to can understand what I´m doing and so to  evaluate the result of the iron changes. I´ll come with results soon. 
 
My apologies about a typo on the 4th line. I corrected it but it has ultimate effect on the LF of the amplifier. And that is why increase in value was suggested also by Conductor.
 
  I have one question regarding R2 and pot interaction.  It´s supposed that moving the wiper to the mid position would give less low frequencies??  If I understand it ok,  high frequencies would give more voltage at the op-amp circuit than low frequencies?

  I have to check it again, but I had in mind that I had less high frequencies, I remember the feeling of less clear sound, maybe I´m wrong?  Can you confirm me that?  Are low frequencies what are supposed to get lower?
 
> I remember the feeling of less clear sound, maybe I´m wrong?

You are unclear. "Less clear sound" is a VERY unspecific complaint.

> Diferent tips make me confused.

Everybody is confused by "feeling of less clear", and throwing darts all around it.

> When the pot is at middle position, the amp doesn´t sound as clear as when is to max. Of course, when I put to max, I decrease the level on the pc to compensate that.

I see two explanations:

1) The pot sucks, worse when turned down.

Yes, pots are imperfect, and some audiophiles use very elaborate variations such as switched attenuators. But plain pots are WIDELY used in audio. Most recordings have been through dozens of pots. If your one pot caused "less clear", then most recordings would be utterly UN-clear.

2) The PC output sucks, less when its level is decreased.

The CMoy is really intended for WalkMan outputs, under 1 Volt. Even so, it does not need a gain of 11, which is what your version has. Most PCs can make 2 Volts output. You have to turn the CMoy volume pot WAY down. But then you may set the PC level way-up. If it is set to give 2.5V peaks, but can only make 2V peaks, it will lose clarity, perhaps without gross obvious distortion.

My suggestion is to set the CMoy gain near 2 (1K and 1K gain resistors). With this lower gain, you can keep the CMoy volume control higher, even while the PC level is not-high.

I don't see a problem with the stock input values, nor any of the suggested alternates. The PC can probably drive pot and R2 as low as 1K. The chip can be happy with R2 of 100K or maybe more (you have NOT said what chip you are using!). C2 of 0.1uFd against R2 of 100K gives 17Hz bass-cut. Putting 1.0uFd in from of 10K or 20K pot also gives bass-cut of 17Hz or lower. In either/any case, so far below the musical range that it would not cause "un-clear", you would have to A/B-test pipe organ or bass-synth recordings to verify any difference.

Another possibility. Ground layout is rarely discussed in the CMoy community. But Doug Self teaches that in loudspeaker amplifiers, high speaker load currents contaminate ground. If your headphone currents must pass "under" or "past" the volume pot ground end, you could have ground-garbage which is not passed-up when VOL is at max, but contaminate the signal when VOL is set lower.

You also have not said what power supply you use. Many folks try to power a CMoy from the PC power supply, and that just does not work (however it won't work at any volume setting).

I still feel, without a "clear" understanding of "un-clear", that the main design issue is the quite high gain due to using R3 R4 values scaled for porta-player output rather than PC outputs. And the way to check that is super-easy. If I'm wrong, I own you $0.20 of resistors. Try that before you throw $2 for big caps or $2-$20 for different pots.
 
> moving the wiper to the mid position would give less low frequencies??

Plan shown in top post this thread:

Full-up, assuming low source impedance: bass cutoff is 0.1uFd against 100K or 15.92Hz.

way-down: bass cutoff is again 0.1uFd against 100K or 15.92Hz.

At "half loudness": bass cutoff is 0.1uFd against 100K+2K5 or 15.54Hz.

No difference except with careful measurement.

There is a different issue at the high end. A 1Meg pot against a tube with 100pFd grid capacitance, pot at half-loud, will have high cut-off at 6.4KHz, much better at any other setting. But with 10K pot this would be 640KHz worst-case! And the chip has less than 100pFd input capacitance.

So low-end and high-end variations with these values (or any of the suggestions) are way beyond the audio range, and not causing any obvious change of sound.
 
PRR said:
> moving the wiper to the mid position would give less low frequencies??

Plan shown in top post this thread:

Full-up, assuming low source impedance: bass cutoff is 0.1uFd against 100K or 15.92Hz.

way-down: bass cutoff is again 0.1uFd against 100K or 15.92Hz.

At "half loudness": bass cutoff is 0.1uFd against 100K+2K5 or 15.54Hz.

No difference except with careful measurement.

There is a different issue at the high end. A 1Meg pot against a tube with 100pFd grid capacitance, pot at half-loud, will have high cut-off at 6.4KHz, much better at any other setting. But with 10K pot this would be 640KHz worst-case! And the chip has less than 100pFd input capacitance.

So low-end and high-end variations with these values (or any of the suggestions) are way beyond the audio range, and not causing any obvious change of sound.

  Hello PRR. Sorry for being "un-clear".  It loose some high frequencies. It´s not exage´rated, but that´s why I told "less-clear".
  The op-amp is OPA2132. Power supply are two 9V batteries.
  The Cut-off frequency you mention at 16hz would not affect as you tell.
  Haven´t had time yet to try changes, but the gain changing seems the way to go. But doesn´t affect the output impedance?  I think have read something about that and the headphones impedance.
  Output was Scope Home PCI souncard, what has aceptable quality, but yesterday I tried with the Uwe Beis DA kit that sounds excellent http://www.beis.de/Elektronik/ADDA24QS/DA24QS.html
    One question, the pot itself, doesn´t affect at all at the frequency resnponse?  Is not that reason why usually rotary switches are used in preamps? 
 

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