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. The strings are provided with electrical current which interacts with magnetic flux generated by the driver for driving the string in a second orthogonal plane of vibration. Electromagnetic interference between separate sustain devices is reduced by placing the components of each pickup means equidistant to each of the drivers. Automatic level control means is provided to each of a plurality of sustain devices to minimize the effects of magnetic damping on the strings. A hand-held sustaining device is provided with an encapsulating means for separating the driver from other elements of the device.