FIG. 1 is an isometric view of the adjustable dumbbell base in accordance with the present invention;

FIG. 2 is a front elevation view thereof;

FIG. 3 is a rear elevation view thereof;

FIG. 4 is a top plan view thereof;

FIG. 5 is a right side elevation view thereof;

FIG. 6 is a left side elevation view thereof;

FIG. 7 is a bottom plan view thereof; and,

FIG. 8 is an isometric view of the adjustable dumbbell base, wherein the broken line showing of the adjustable dumbbell is for illustrative purposes only and forms no part of the claimed design.