CH> KP> A sound card sounds far noisier on some headphones than others.
CH>Each headphone / speaker system has it's own frequency response
CH>curve. Because of this, some will accentuate the "hiss" more than
CH>others.
I wonder if that explains the big hiss from the small Sony in-ear phones
versus the minimal hiss from the Optimus sealing type Pro-40s which are
now broken (plastic driver support piece cracked in I think normal use,
but a friend has a friend with access to a plastics welding lab, so
maybe they can be nicely repaired (nice compared to my plan to wrap a
piece of metal around the break and epoxy it)). Turning it up or down
seems to affect hiss little. 
CH> KP> I don't understand how adding series resistance will help,
CH>        though a friend says to try it.
CH>The theory here could be that if the amplifiers in the sound card
CH>have a high degree of noise, the series resistance would REDUCE the
CH>amount of signal fed to the headphone.  Because of the reduced
CH>level, the audio output would have to be RAISED to compensate for
CH>the loss, thereby pushing down the apparent noise level.
Noise from optoisolators, do you suppose?
CH>The level needed to drive the headphones to the previously CH>enjoyed
level could drive the sound card into clipping.
Awful solid state clipping, too.  
CH>Note that headphones vary wildly in impedance and sensitivy.  Try
CH>several brands.  A subtle change in frequency response would CH>
probably do more then knocking the level back. :-)
Noted.  
Still, I think something wierd is going down, man.  
* POW 1.2  On Trial * And Liberty, she pirouettes.   -- Solsbury Hill, Peter 
G.
--- Alexi/Mail 2.02b (#10000)
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