Yup, I mean hiss. The reason I got a DAC in the first place was to try to get rid of the hiss, and it worked! All my headphones (with the exception of the K240) work like a charm with the uDAC, so I use all but the K240 with the uDAC.
I was quite disappointed with the Macbook Pro's audio out - hisses like no other!
Thanks for the good info, Skylab!
I think we need to differentiate "hiss", from "hum". If we are talking about "hiss", then this is not a grounding issue. Let's assume that is the case.
Some amplifiers have enough intrinsic noise that if you have a very high-sensitivity headphone, you might be able to notice it with no music playing. With almost any amp, if you crank the volume all the way up to the top, you will hear some hiss through almost any headphone (again with no music playing).
Further, some SOURCES will have enough noise that this will be audible though the amp - and this is not the amp's fault! Also, some amps will hiss when there is no source connected, but won't when the source is connected but turned off (this is an amp topology issue).
Finally, a very small amount of hiss may actually be a good thing, in that the way this is supressed sometimes is by using global negative feedback in the amplifier circuit, which many people (me included) believe had have a negative impact on the sound.
Bottom line - it's pretty common to have some level of hiss.
