Reliability of Fitting a Plane to Range Data

A protection circuit for a driver amplifier is provided which senses the absence of current to a driven amplifier and decreases the voltage supplied to the driver amplifier to a safe value thereby preventing damage to the same. The driver amplifier may be a semiconductor (transistor) amplifier and the driven amplifier may be a vacuum tube amplifier which has a warm up period much longer than that of the semiconductor amplifier. The protection circuit protects the semiconductor amplifier during warm up of the tube amplifier, or in the event of failure thereof by reducing the operating voltage applied thereto.