Powered monitors. Every time I turn each one off there is a very loud and disturbing thump. I don't mind pulling the amps to replace the on/off switches if that is the source of the problem, but I'm not certain so I wanted to ask first.
You are probably picking up the arc from the power switch as it opens up. The best way to cure that is to install an MOV across the power transformer primary rated for about 150 VAC with a high energy rating. I have forgotten how to rate these since it has been years since I last bought any. Doing this will also greatly extend the life of your power switches.
Note that this is to be connected right across the primary so that it clamps the arc, and if it shorts due to an AC mains event (voltage spike), it will blow the fuse.
As it happens to all your monitors it will be a feature of the design.
Or to be more precise, the monitors have not been designed to eliminate turn on / turn off
Turning down the volume before power off may or may not reduce the thump.
A muting circuit would solve the issue.
Yes, it is probably due to the amplifier design during turn-off knowing that there is no "crack" or "snap" component involved. When I hear of a large noise "at turnoff", that usually means the switch arced which the still active amplifier picked up. If the noise occurs as the power decays, then it is a design feature.
Having said this, it is a great idea to install an MOV across the primary of the transformer in anything you would like to keep. This quenchs the turn-off arc which most often occurs, depending on which part of the AC cycle the contacts open. The MOV absorbs the voltage spike due to inductance and therefore saves the switch or relay contacts that are interrupting the current flow. So, an MOV rated at 150V these days is a worthwhile investment. If you get a big electrical spike, or the line goes high,the MOV will conduct and maybe blow the fuse. If the MOV fails, it will blow the fuse. If the MOV fails open, then nothing matters anyhow. Chances are everything plugged in, on or not, has been zapped to death.
My apologies for the confusion of terms. The sound is most definitely a pop, and it happens despite amp or source volume, or when plugged into a surge protector, suppressor, conditioner, or noise filter.
These drivers aren't meant for the kind of excursion I see when they are turned off, and if the voice coils aren't already damaged, they will be after more on/off cycles. I know they're entry-level garbage, but for my purposes they perform just fine.
Thanks for the suggestion of a varistor. That looks to be the way I'll go, after reading up and gaining a better understanding of course.
I think you have the problem identified correctly. It's a "feature" of that design given the more complete report on what happens when he turns it off from Sintek. I have no doubt at all that it would behave the same way if the power cord was unplugged.