An input device for operating a musical instrument having musical behavior of string instruments, which comprises a key plate unit containing a set of input keys to be pressed by a player, in order togenerate a first set of electric input signals, which emulate a combination of pressed keys of a musical instrument or a combination of pressed strings; a stick unit with variable tension of displacement including a stick to be moved by the player between predetermined discrete positions, which emulate a combination of strummed strings of a musical instrument; a set of electric contacts, each corresponding to a different discrete position, for transmitting stick position signals; a processing unit with input buttons for setting musical parameters; a set of interface inputs for receiving the stick position signals; a second set of interface inputs for receiving the first set of electric input signals; a memory for storing processing software; a processor, connected to the memory, for jointly processing the stick position signals and the first set of electric input signals using the processing software, and for outputting electric output signals representing codes of musical notes thatcorresponds to movements of the input keys between pressed and released positions, and movements of the stick between the discrete positions; interface connections, for transmitting electric output signals representing codes of musical notes, from the processing unit to a computerized tone generating unit that converts the electric output signals to sounds.