A method of operation of a variable frequency metal detector having a driver circuit for establishing an alternating magnetic field in a coil system so as to generate an output signal at a given frequency, said driver circuit comprises a plurality of switches being arranged to cause the coil system to be driven at a frequency determined by the operation of the plurality switches, the method comprising the steps of generating an adjustable balance signal, combining the adjustable balance signal with the output signal of the detector, and varying the adjustable balance signal so as to provide a compensated signal whereby the output signal and/or the adjustable balance signal is filtered to remove one or more harmonics.