Abstract: A power line filter comprises a filter circuit in the form of an impedor assembly (30) that filters a network supply voltage to be applied to a load. A voltage sensor means (34) monitors a characteristic of the supply that is representative of the interference content of the supply voltage. A microprocessor (36) compares the monitored characteristic with data that is stored in a memory (38) and is representative of a desired supply characteristic. By way of control means (32), the microprocessor (36) cause variation of a filtration characteristic of the impedor assembly (30) in a sense to minimize any divergence between the two above-mentioned characteristics, thereby to better suppress the interference.