It should be fbx, but some amplifiers have fbk. fbkswitch means feedback suppression switch, also called anti-howling, which is a special function for microphones. When this function is turned on, the microphone enters the anti-howling mode, and the singing voice can be louder, controlling the howling and harsh sound. But at this time, the sound will be a little trembling. Turning it off will stop the sound from trembling, but it is easy to have howling.