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Schumann resonances are very weak natural electromagnetic signals produced in the earth-ionosphere cavity located in the extremely low frequency (ELF) band (7-60 Hz), and the sensors that measure them produce amplitudes of few microvolts. Strong signals from power lines (50-60 Hz) occur in the same frequency range. Amplification techniques play a key role in acquiring resonance modes with the best signal-to-noise (S/N) ratio. This paper presents a study of the various structures of amplification systems that optimize the S/N ratio for the signal of interest. The aim of this paper is to measure all possible resonance modes with low time acquisition. To this end, we compare four instrumentation amplifiers and design a new indirect method for obtaining the noise floor of the system with sensors manufactured on magnetic cores that are several meters long. We present the measurements of the Schumann resonance achieved using these techniques at the ELF electromagnetic wave observatory at Calar Alto (Spain). The solutions adopted allow measurement of seven resonance modes with an acquisition time of 30 min, where the S/N ratio in the fundamental mode was 39 dB.