What is the correct input voltage for the 9v Lego train speed regulator? I wanted to buy a transformer for the speed regulator, so based on this image, I bought a transformer with 12V DC output. It works.

However, discussing this with a fellow AFOL, he pointed out that his (original from LEGO) transformer has a 10V AC output, as seen in this image.

So... which one is correct? Or they both work? Or there are different versions of the speed regulator with different input requirements?
My speed regulator is clearly labeled as being 9-12V AC. I have an official 12VAC transformer.



I was curious about the internals of this device, so I opened it up:





The circuit begins with 4 diodes in standard bridge rectifier configuration. This converts the AC input into a rippled DC voltage. The large capacitor (2200uF, 25V) then cleans up the ripples. There is also an LM317 adjustable voltage regulator configured to output various voltages depending on the dial setting.

This means that the input can be 9-12 voltes AC or DC because the rectifier simply passes DC current.