There is disclosed a constant amplitude controller with feedback control for a vibratory feeder apparatus wherein a controlled rectifier is used to rectify the A. C. line voltage applied to the feeder solenoid and to control the voltage to the feeder solenoid. Triggering of the controlled rectifier is accomplished by a bias control signal which is composed of an A. C. phase shift voltage superpositioned by a variable D. C. bias voltage. Mechanical coupling is provided between the solenoid and a transducer which provides feedback to control logic circuitry. In the control logic circuitry, the signal received from the transducer is compared with a signal received from an operator adjusted potentiometer. In this manner, the desired amplitude of the vibrator apparatus is compared with the actual amplitude of the vibrator apparatus and, if there is a discrepancy, the control logic circuitry modifies the amount of control voltage being transferred by the controlled rectifier.