Yes, you need to know the impedance. Also, not all TVs are made to spec. LCD TVs may(?) have standardized in the last years, but there is still no universally working schematic.
Except perhaps mine
(see my
last post which addresses this).
It's a real bid for a universal cable. I avoid the resistors and voltages with the penalty that you have to select the AV channel when you turn your Amiga on.