Most will use a relay to switch the mains through. As to the wireless connectivity to control the device this will always generate some amount of RF noise. The relay should not in and of itself generate any noise, but I guess like anything in audiophile circles someone will have a different take on the matter.
Bottom line is to try it and if you can perceive a difference that bothers you don’t use it.
I use a Sonoff switch for my main system amp, DAC, and Pi. It feeds a decent non-audiophile surge-protected power board (~$50). I A/B/C’d with the amp straight from the socket, just through the power board, and through the switch plus board. Zero difference.
And now I can say “Hey Siri, play something excellent” and my amp switches on, volume is set, TV turned on and switched to Chromecast, and Roon queues up an album tagged as “excellent”. Which is pretty great.
i’m using Fibaro Z-Wave power plugs across the home and have not noticed any negative effect on audio.
However I am not using it for powering audio, but just other components i do not want to be powered when having critical listening session (like TV, HDMI switch, DVD player etc).
I have explicitly chosen Z-Wave as it seemed to me the least RF disturbing technology (e.g. when there is no activity, it send out short signal only once per hour) - unlike WiFi based plugs which usually constantly communicates …
However this is true only for battery powered Z-wave devices - e.g. not the power plugs which are powered from wall But even those are not very chatty - they transmit mostly when power consumption changes by predefined threshold. But again i can’t hear any negative impact on audio.
I have however observed interesting behavior of those Fibaro smart plugs - even when should be powered off, they probably still pass some current thru, as i can see that some low-power devices are turning on for fraction of second (power led) maybe every few minutes. Ideally i would like that smart plug would galvanicaly disconnect all conductors (e.g. cut off the noise “antenna” from power line)
Most LED lighting is electrically noisy, some of it alarmingly so. Whether or not that noise couples with your HiFi and affects the sound is another matter, and every domestic installation will be unique in this respect.
Given the large number of light fittings in many homes, I regard it as a matter if good housekeeping to choose electrically quiet devices wherever possible, but that may limit your choice of lighting quite severely.
If one is using a smart plug to turn their equipment on/off then I think a worry greater than noise is what does this might do the circuits, etc.
Is it worse to let current flow to a device whose normal switch is already in the on position, i.e is there some mechanism that prevents a damaging spike from occurring when the device’s switch is turned on and is there an absence of such a safeguard when the switch is always left on and the device is activated by closing a smart switch?
I thinking that 1) not many people use a smart switch to control their relatively delicate HiFi equipment and 2) that the things that are controlled by such switches are, by virtue of being relatively simple, possibly more immune to such effects.
I hope my HiFi gear isn’t delicate - most of it was bloomin expensive
But seriously; power dips, outages, idiot users (I include myself in that demographic) are a consideration of regs and designers on both the consume and supply side of any bits of kit
Technically I’m a mechanical engineer, but I’ve had quite a few EE classes and have some electrical design experience. But there’s got to be more knowledgable folks on here than me when it comes to circuit design.
That said, I can’t think of an example that the internal power switch in a component is doing something fancy that the smart plug wouldn’t. I’m thinking of old-school gear here where the power switch is switching the mains supply, not something computerized where the power switch is really a low voltage logic switch that kicks off the show with a microprocessor (think your computer power switch).
One thing that may be an issue is that a lot of our high end amplifiers have significant power supplies and capacitor banks, that can have significant inrush when the switch is flipped (many tube amps with solid state HV supplies use an inrush limiter, but that seems less common on solid state amps). Designers will typically put a little capacitor across the switch contacts as a snubber to keep arcing across the switch contacts when the switch is opened up (the power transformer is a big inductor and can have significant flyback when the switch is opened at the wrong time in the power line phase). The smart switch may not be expecting such an inductive load and the on-off cycles may degrade the little relay more than was planned by the smart plug designer. But that wouldn’t damage your audio components, you’d just have a dead smart plug sooner than you’d like.