We claim the ornamental design for a synthesizer, as shown and described.
FIG. 1 is a top perspective view of the synthesizer;
FIG. 2 is a bottom plan view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a top plan view thereof;
FIG. 5 is a front elevational view thereof;
FIG. 6 is a left-side elevational view thereof; and,
FIG. 7 is a right-side elevational view thereof.
The broken lines shown in the drawings are included for the purpose of illustrating unclaimed portions of the synthesizer and form no part of the claimed design.