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!
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.
So, I just got my first amp, and I've been using my headphones with it. My setup is Macbook Pro 2007 > Nuforce uDAC > AV123 X-Head.
I've been trying my headphones out with it, and I noticed that I'm getting some hissing that wasn't there before from some of my headphones (in my signature). I get hissing from most of my headphones, except the AKG K240 Monitors, which are 600ohm.
The funny thing is, I had regretted getting the AKG's because they never sounded good, but now they sound great with the amp, and everything else has an annoying hiss.
Should be the other way around. Low impedance and high sensitivity tend to hiss not the high impedance ones. So not surprising that the 600Ohm can has no hiss and the others do.
This is normal, but you can reduce the hiss by using better ICs. You need a clean signal path. Be sure to use the uDAC as a DAC, then its Lineout use good RCA cables to your amp. Clean the tips and ensure no corrosion and dirty/muck on the contacts.
The XP-7 with the stock op-amps hiss with many headphones, even full size ones from Grado and Shure. However, if you change to the AD797, the hiss goes away for most headphones except sensitive IEMs. Just to help set your expectations straight!
The only amp that I've tried where I can't detect any hiss whatsoever with any headphone when set to normal listening volumes, is the Ray Samuels Tomahawk.
I guess I mean audible noise. With my Burson amp there is no noise or hiss (audible to me) even at the highest volume level with no music playing (with any of my headphones)
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