From the output characteristic curves I have seen posted hub dynamos can put out far above 6V at higher speeds with the correct load attached. They will not go above about .6 amps though due to internal impedance. To get maximum power out of a hub dynamo you need a koad with higher than normal dynamo light resistance which is why most homebrew multiple LED dynamo headlight circuits have the LEDs hooked in series. I believe that the Magicshine has the 4 LEDs on the chip connected in parallel. Not dynamo compatible unfortunately.
The more you are trying to get out of a hub dynamo the higher the speed at which the light will reach full luminosity too. Street legal German dynamo lights reach rated light output at about 6 MPH while the Supernova E3 Triple, not German street legal, reaches maximum output at 25 MPH per the manufacturer. Lots more light total but it is using the dynamo to near maximum capability.
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Gear Hubs Owned: Rohloff disc brake, SRAM iM9 disc brake, SRAM P5 freewheel, Sachs Torpedo 3 speed freewheel, NuVinci CVT, Shimano Alfine SG S-501, Sturmey Archer S5-2 Alloy. Other: 83 Colnago Super Record, Univega Via De Oro
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http://groups.yahoo.com/group/Geared_hub_bikes/