Abstract: A sustaining device for prolonging the vibration of a string of a stringed musical instrument having a magnetic pickup means responsive to vibration of a string, and an electromagnetic string driver means to provide a magnetic drive force to the string. A cancellation circuit is provided for reducing electromagnetic feedback between the pickup and driver by adjusting the relative phase and amplitude between a first and second pickup signal and combining the signals so that the responses to electromagnetic interference cancel. One embodiment of the cancellation circuit includes a second electromagnetic driver that generates an amplitude-adjusted and phase-adjusted electromagnetic field to cancel electromagnetic interference. The driver may be a section of toroidal solenoid that is shaped so that its endpoles are in close proximity to the string for concentrating magnetic flux along the string.