Ok, I called the guy at Sturmey-Archer "Sun Race" USA in California about this and he said he'd forward the question to the engineers but I've not gotten an answer on this yet, so I thought I'd see if someone else with experience with dynamo hubs can answer it.
I have on order a new X-FDD hub, this is the SA hub with a dynamo plus a 70mm drum break. It is, of course, NOT the old S-A Dynohub.
I am designing a lighting system using superbright (like 18000mcd) LEDs. All the information I could find on the hub says that it produces "6V, 3W". Ok, so a half an amp at six volts, presumably AC like the old hub. I've heard that the hub, unlike the old dynohub, was regulated to not exceed that rating. But I was talking to an engineer friend about the project and when I said that he asked "Do you just think that or do you know that it's power-regulated AC?" He said he found it unlikely that they'd do that because it takes a lot more components than either (1 rectifying it into DC and power regulating it or (2 leaving it in AC and not bothering to regulate at all.
So well, if they turn it into DC, it's easy, just dump it right into the system. If it's AC I need to rectify it myself and if (and only if) they don't regulate it at all I need to set up some sort of z-diode/resister pressure valve system. I, of course, don't want to build my circult with a rectifier and z-diode built in if I don't need it, but at the same time I don't want to turn the system into a chunk of expensive plastic if the hub isn't that friendly (18000 mcd LEDs aren't cheap!). Does anyone know about this hub?