Turntables thump when turning off?

Try the above suggestions, you can do 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.

Well known issue with VPI and Rega tables. A cap across the power switch of the right value is the answer. I had that issue years ago when I owned a Rega. 

.01uF HV ceramic cap across the switch.

.01uF HV ceramic cap across the switch.

+1 @lewm 

@ejlif 

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!