And yet such impressions conflict with how power supply circuits work and how THD measurement works. Wall power is incredibly noisy and there’s nothing much an

And yet such impressions conflict with how power supply circuits work and how THD measurement works. Wall power is incredibly noisy and there’s nothing much an electrician can do to change that. If this noise had any meaningful effect on the sound—it doesn’t—you would be able to measure it as a variance in THD. (Now of course that might be less true for some poorly made vinyl/phono level equipment but if you actually cared about audible distortions you wouldn't be listening to the sound of a needle scratching its way across a fragile plastic groove in the first place...)