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A mathematical model for the calculation of optical signal-to-noise ratio (SNR) on cascades of erbium doped fiber amplifiers (EDFA's) with interposed equalizing filters in the fiber spans is presented. The model enables to simulate different types of cascade, whether the filters are placed after each amplifier or after any group of amplifiers. Criteria for the design of the optimal filter are presented for a typical configuration. The relation between preemphasis and penalty on SNR is studied, and for the first time to our knowledge it is shown that some asymmetry may arise when using in line optical filters. A study of the sensitivity of penalty at receiver toward preemphasis at transmitter based on the model is presented.