A circuit is provided to detect which of a simultaneous plurality of varying input signals has the dominant maximum amplitude. Two simultaneous input signals are compared with each other; or each of more than two simultaneous input signals is compared individually with a common, varying, comparing signal, provided in response to the summing of the input signals, and the subsequent scaling of the summation. The input signals, and a provided comparing signal, are different from each other, only, in amplitude. Normally there is expected, at least in relation to the input signal having the dominant maximum amplitude, to be a wide dynamic extent of amplitudes; whereas each signal supplied to a difference amplifier for such a comparison is arranged to have only a low amplitude, at least the dominant input signal being supplied so as to have only a range of amplitudes, as transmitted by an associated limiting amplifier in response to the receipt of the signal. The rate of change with time of a varying signal amplitude, resulting from each comparison of two signals, substantially is proportional to the difference between the maximum amplitudes of the signals compared. Thus, the detector is arranged so as not to be required to operate upon the whole of said normally expected wide dynamic extent of amplitudes of at least the dominant input signal.