Abstract: A circuit to control the frequency response of a variable filter by means of a control signal based on the amplitude of the information signal to minimize the effect of noise in a signal transmission system. The variable filter is connected to an output circuit of an information signal amplifier and may feed the frequency-modified information signal to a system output terminal, if the system is to compress the dynamic range of the information signal by amplifying low amplitude signals more than high amplitude signals. Alternatively, the filter may feed the frequency-modified signal back to the input of the amplifier by way of connecting means, such as switch, to expand the dynamic range. Control of the filter for low frequency signals is achieved by rectifying and filtering the signals to the control amplifier. Negative-going transients are inverted and by-pass the rectifier and filter and are applied directly to the control amplifier.