Oh, yes, there's RFI from phones and modems and even microwave ovens, plus EMI from cables overlaying others, refrigerators, and fans on the circuit. This equals a bunch of potential noisemakers that can affect the signal (unless you are using lightpipe or S/PDIF digital signals).

Then there is transistor hiss, at a low level in all systems.

And don't forget gain staging. The basic principle is to put the gain as high as you can at each stage and only vary the final stage volume.

The problem a headphone jack presents is that the headphone output does not follow this principle; it's very easy to overdrive and create distortion by turning up the headphone output beyond the voltage from a line-out. And there's more problems that I cited above.

BIAB 2025 Win Audiophile. Software: Studio One 7 Pro, Swam horns, Acoustica-7, Notion 6, Song Master Pro, Win 11 Home. Hardware: Intel i9, 32 Gb; Roland Integra-7, Presonus 192 & Faderport 8, Royer 121, Adam Sub8 & Neumann 120 monitors.