How to convert 12v dc to smooth, as if coming from a battery, 6v dc (EMI – RFI free)
The easiest way is to use a series pass regulator, like a 7806, to drop the 12V down to 6.

You'll waste about P = (Vin-Vout) x Iout = (12V - 6V) x 0.4A = 2.4 watts, so you'll need a heat sink on the regulator.