You can try a spark suppressor across the switch terminals.
0.1uF 600V film cap in series with a 250R 1 watt resistor should work.
The thump is loud, very loud if it's at the full volume I would play an LP at. I guess you guys are saying it's normal and I just have to live with. It's there no matter the table or phono stage and even does basically the same thing in a whole different system. It's a VPI classic 3 and Rega RP8. I had Nottinghams before and those don't turn off ever so maybe that is why I didn't ever notice until now. You have to stop the VPI motor after every LP side since it uses the center weight and periphery ring. The RP8 I can just snag the LP off without stopping it.
Hard to imagine that a snubber cap across the power switch would be missing or broken. But I would definitely check.
More distance between turntable power switch and phono pre might already be the solution. Is the power cable to the turntable very close/parallel to the interconnects from the cartridge to the phone-pre? After all the snubber only minimizes the problem and doesn't neutralize it totally.
If nothing works and you are really “desperate” you can build in an extra switch that only turns off the low voltage connection to the motor. You would lose the warranty but the problem would be fixed. But I would first try disconnecting the motor manually (unplug the cable if possible) instead of using the switch and see if the problem is gone.
This is a common problem- if the 0.01uf 1000v disk cap across the switch fails, it will result in a thump or pop no matter where that switch is in the system and it will show up more on the phono preamp.
Its caused by the magnetic field of the motor or power transformer collapsing and causing an enormous high voltage spike (the same principle that energizes spark plugs in a car BTW). This is picked up by the phono section because it has the most gain. The cap shorts out the high frequencies that are otherwise jumping across the switch contact as its opened. For this reason it also prevents the switch from being damaged- the switch contacts erode a bit every time this happens.
So you do want to get this fixed rather than use the mute!
You know what the Doctor says to patient?
It hurts when I do "x" Doctor-then don't do "x"
Yeah no sheet. I was looking for a solution to having to turn down the volume or mute each time I change the record. Just pointing out that if left at normal volume that I play a record at the thump is dang loud. Obviously I don't do that and I turn it down.
Sounds like plenty of suggestions. I was thinking it's probably not a fault of one of the tables. I have a VPI classic 3, a Rega RP 8 that is brand new and an old Dunlop Systemdek IIX and they all do it and with two different phono preamps and various amps and preamps. I'll try some of the more careful wire routing see if that helps. Thanks for the many useful suggestions.
Now that's an interesting bit of information! And that causes me to think its not a failed power switch bypass cap. You have something more basic. I would investigate your AC wiring- have an electrician test the outlet to see if its properly grounded and the AC wiring is correct. This could also be caused by an unshielded tonearm cable being routed very near the turntable power supply, but this latter idea is a bit sketchy.