[akgfan]

Hi.
 
I have a strange problem with my volume potentiometer on my headamp. I have used this amp for 2,5 years now but volume potentiometer drives me crazy.
 
When I turn on my headamp and turn up volume it makes cracking sound. This sound lasts for about one second when I drive it with mp3 discman. However, this case is not my problem. I use this headamp with PC only and soundblaster audigy se sound card. And with PC is this cracking sound far more present and lasts for long time when I turn volume up and down. Finaly it disapears (sometimes randomly presented).
 
And my question is, what's wrong? Is that a problem with shared line out/headphone output on soundcard? And is there something I can do with it?
 
Here's my schematics. It's modified Richard Murdey's amp. (http://gilmore2.chem.northwestern.edu/projects/showfile.php?file=murdey_prj.htm)

 

[akgfan]

I made yesterday one channel of this amp for testing purposes and came up with interesting findings.
 
When I put 10k or 25k pot as volume there was no more cracking problem. Then I put another 100k as a volume and cracking problem again. Why is it that? Strage.
 
I also wanted add 220n bypass film capacitor on output but there was no improvement at all. Maybe I will put that capacitor as a placebo efect though. I dunno.
 
And question about output headroom - I have 3,33V. Is it enough? According R. Murdey it is OK for headphones with 60 Ohm and less.

 

[power911]

probably the potentiometer problem?
grease the inside of the pot with some vaseline or wd-40 and give it some turns to smooth out the lube
 
it works for me

 

[akgfan]

No it's not the potentiometer problem. I tested it with another pot with the same result. I'll buy some quality 10k pot. It's probably some impedance problem.

 

[nikongod]

Do you have a voltage mismatch somewhere between the source and the amp? Are all the grounds at the same voltage? Are both signals at the same DC voltage? 
 
After that, Id bet its just an annoying dirty pot. You can buy chemicals specifically to clean pots although I have not had consistently great luck with them.

 

[adamus]

its ac coupled at the input, so my first guess has to be the pot, seems unlikely to have two bad ones though

 

[akgfan]

I measured dc voltage on input - 40mV.

I guess we found the problem. When I unplug signal from amp, dc output from soundcard is 29mV.
 
Edit: Uff. I connected earth wire from outlet with my amp ground. I guess it would be a grounding problem then. Am I right? I've read (if I remember correctly) that you can connect amp's ground to earth via some resistor, capacitor or even diodes(?). I definitely messed up something.
 
Edit 2: I will try this: http://sound.westhost.com/earthing.htm .

 

[nikongod]

A ground loop breaker may help. 
 
The DC offset from your source is not really good. Id sort that out first. As odd as it may sound, try putting a cap between the source and the amp as a temporary test. You will always need the cap between the pot and the input jfet, dont take that out

 

[akgfan]

Edit: I thought that I solved it, but obviously not.

 

[akgfan]

This thing is driving me crazy! I thought that I fixed it and I actually did, but I found out that I could be mistaken.
 
I was playing a game and I noticed that menu music was causing not very audible pot cracking. There is normaly no cracking at all. This was the first time I heard it after my mod.
 
I was messing with 50Hz tone in audacity and guess what? That pot cracking was loud as hell. I tried 1uF input cap instead of 220nF (because I thought that this was causing that problem) and no difference.
 
I understand that my soundcard is not very good but what should I do with it? And there are probably no output caps. Or are there?
 
Edit: It looks like there is 4556 opamp on the headphone/line out output.

 

[nikongod]

It sounded like you had it beat

Are you sure its not the source overloading/clipping this time? Try turning the main volume control down about half way.

 

[akgfan]

I made yesterday proper star grounding and looks like there is finaly no more cracking. And there is only small cracking when playing 50hz tone. Looks good so far. I hope it will be OK this time...

 
And i tried three pots with my other modules which are not in case and only one is absolutely OK. So that small cracking with 50hz tone or so is just faulty pot. Pots are Alps and Piher.