A new analog processing unit (APU) with an extended frequency band, which is based on an electromagnetic principle, has been designed and constructed at the Istituto Nazionale di Ricerca Metrologica (INRIM). The unit can be employed in power measurements in the audio-frequency band. The sum and the difference of two waveforms from 20 Hz to 100 kHz are simultaneously performed. The unit has been characterized to evaluate its behavior from power frequencies up to 20 kHz. The output signals of the APU and the phase relation between the sections of the unit have been characterized, and the results of the measurement and the model adopted are reported here. The additional devices that constitute the input structure of the thermal wattmeter acting as a scaling factor (shunts and alternating-current (ac) resistive dividers), as well as the automatic ac/direct-current transfer procedure, have been characterized.