Q. How does QED calculate the loop resistance of their cables?
It is important to note that loop resistance per metre means that the conductor is actually two metres
long (there and back again), so the quoted figure will always be at
least twice that of the resistance per metre of a single conductor
That being said, the quoted figure for resistance will
always be slightly higher than the theoretical one because we take a
real world measurement which includes the effects of stranding and the
resistance of the connection to the speaker binding posts and banana
plugs. So although the theoretical loop resistance of 2.5 mm2 of solid
copper at 20C should be 6.74 mOhm/m in the real world the 79 strands
combine to give a conductor with a physical CSA of about 3.3 mm2
(because of gaps between the strands) which includes roughly 2.48 mm2 of
copper plus all the tiny additional resistances of the strands as they
join and separate along the length of the cable; plus the distance is
doubled because the speaker signal has to travel there and back again.
In addition, there is the resistance of the contacts at each end of the
measuring device - so we quote a measured loop resistance of 16 mOhm/m, using the QED Profile 79 Strand as an example.