Is the Kab Speedstrobe necessary with the VPI SDS?

No, but it sures makes the job easier and repeatable. The KAB is the best accessory I've ever bought.

I'll second that Narrod. Although $80.00 for a 10 inch plastic disc and a cheap box with a strobe light seemed pretty hard to swallow at first sight, now that I've got it I don't know how I ever lived without it.

Piece of mind. That's what it is. Piece of mind.

You still have to calibrate the SDS. Same with Walker Audio's motor controller. As indicated above, it's for that calibration step that you use the KAB disc and strobe. Then it's simply a question of how long the motor controller will retain that calibration before drifting.
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If you live outside of the USA where you have 50hz, the VPI supplied strobe is USELESS. No mention of that in the manual thou.
The KAB and strobe is very easy to use.

I agree Shane. Its got me beat why Harry doesn't supply a strobe disc that includes 50Hz markings - at least for export to England/Australia etc. The strobe disc that came with my SDS is the same one supplied 14yrs previously with my TNT - totally useless with 50hz mains.
The KAB strobe is brilliant. Maybe Harry should offer it as an option?