Abstract: Apparatus for controlling a stepping motor drive system during start up and maintenance of a desired oeprating speed. A dynamically variable loop filter i.e. a boot strap loop filter is used in a phase locked loop circuit to ramp the stepping motor to operative speed faster and with less transient instability than heretofore available. Protection from transients tending to cause slower or faster speeds is obtained with the same circuits. Stall sensing and restart capabilities are provided and the circuits use fewer components and less PCB real estate than prior art.