The ornamental design for a synthesizer, as shown and described.
FIG. 1 is a front perspective view of a synthesizer showing my new design;
FIG. 2 is a rear perspective view of the synthesizer of FIG. 1;
FIG. 3 is a front view of the synthesizer of FIG. 1;
FIG. 4 is a rear view of the synthesizer of FIG. 1;
FIG. 5 is a side view of the synthesizer of FIG. 1;
FIG. 6 is a first end view of the synthesizer of FIG. 1; and,
FIG. 7 is a second end view of the synthesizer of FIG. 1.
The broken line showing of portions of a synthesizer is included for the purpose of illustrating environmental structure and forms no part of the claimed design.