The NE5561 provides pulse-proportional drive and speed control based on de tachometer feedback. This simple switching circuit consists of transistor 2N4920 pnp with a commutation diode used to deliver programmed pulse energy to the motor. A frequency of approximately 20 kHz is used to eliminate audio noise. The de tach delivers 2.7 V/1000 RPM. Negative feedback occurs when this voltage is applied to the error amplifier of the NE5561.
The duty cycle varies directly with load torque demand. The no-load current is = 0.3 A and full load is 0.6 A.