a protective circuit for a transistor amplifier is provided which measures the load impedance and actuates a protective means across an input terminal of the transistor amplifier when a measured value of said load impedance becomes lower than a predetermined value , to protect the transistor amplifier from overload . the protective circuit is applicable for a class b push - pull amplifier and , particularly effective to protect the overload even when the load of the amplifier has a reactance component . various modifications such as relay type protective circuits are also disclosed .