Did you ground it? basicly you are making a shielded cable fromm your apparently unshielded cables. Shielding power cables prevents them from being a noise generator. Shielding the interconnects is often not recommended and I do not use shielded interconnects since it can limit dynamics because of inductive effects. that said, your shielding may be far enough from the conductors that inductive effects are minimized or even eliminated. Inductive coupling goes as 1/d.
So most people buy shielded power cables for this but your solution seems to work. curious if you grounded it.
Jerry