Power is conditioned by the amp? Power conditioners which remove any DC current on the line restore the symmetry of the AC sine wave and prevent transformers from making noise. I have never read/heard about an amp that has this capability, though I'm sure there are some extremely expensive units out there that have this feature. This also goes with EM and RF interference on the mains. (I wish I could find the post by the President of PS Audio who said that, "theoretically", DC on the mains feeding even their best power regenerators could cause audible distortion (hum) produced by the transformers in their power regenerator. He suggested that a sine wave balancing device feeding the power regenerator would prevent this.)
What do you imagine gets through? Do you notice your music playing faster if you run on a 60Hz outlet rather than a 50Hz outlet? Do your speaker measurements show all the micro-fluctuation in your mains?
Of course they do not. The power coming in to an amp is adjusted and pooled into capacitors, which are quite good at isolating input from output. Indeed; capacitors are regularly used as filters.
Noise floor is a factor of dynamic range.
As far as I understand noise floor, there are 3 types. The noise floor of the listening environment (dogs barking, HVAC, refrigerator running, etc.),the noise floor produced by the equipment you're listening to, and the noise floor of the recording. The last, obviously, can not be controlled by the end user. The first can be controlled by room treatment, location of the listening room, etc. The second can be controlled by power conditioners which re-balance the AC sine wave and remove EMI and RFI off the mains. The purpose of shielded power cables is to prevent EMI and RFI from re-entering the electrical supply between the power conditioner and the stereo components.
Yes. The room has a noise floor.
The recording doesn't have a noise floor. It has a dynamic range (the difference between its loudest and softest noise). You could argue that whatever the softest noise is on there is "the floor", but that's stretching the issue.
The electronics can add cross-talk, or an electrical signal on an analog connector can add noise. These do become a "noise floor" in that music quieter than these signals will not be heard (at least not correctly). But, as mentioned, that's below human hearing in the bulk of cases, and the rest are either equipment malfunction or problems with interconnects.
digital cables are immune to interference
Optic fiber digital cables are immune to EMI and RFI, but coaxial cables are not. Digital coaxial cables are also capable of transmitting noise (ground loop hum, transformer hum, etc.) from one component to another. I have never seen a good digital coaxial cable that did not include shielding in its design to prevent EM and RF interference. I am assuming it is there because it is needed.
Digital signals either arrive, in which case their reproduction is perfect, or they do not arrive, in which case their reproduction is not perfect. There is an edge-case in audio SPDIF where some level of error can be worked around; but again, this is for a very narrow line between "perfect" and "non-functional".
Unlike you: I've seen literally tons of digital transmission cables with no shielding (other than electrical insulation). You'll see them all the time in PCs, where they transmit data that the loss of even a single 1 or 0 can crash the system.
gear is capable of better-than-human range [
hearing (?)] I know that at Audioholics it is accepted as a given that you can not hear audible differences between cables, DAC's, and bit/sample rates, etc. But it really depends on who the "you" is. Some people have excellent hearing and have trained themselves to listen....well. This video from Grammy winning mastering engineer Bob Ludwig contains a couple of anecdotal stories of how great some people's hearing is as well as one about being able to hear a difference from changing out cables:
Nope.
It's possible to build a cable that alters the sound... it would effectively be a filter. An easy way to do so accidentally would be a very long, very small-gauge cable. In those cases, the difference in sound is both audible and measurable.
The idea of an audible but unmeasurable difference is myth.
