of the frequency signals f 39 may be varied this means that the recording and reproducing frequency characteristics of a magnetic tape sound recorder can be maintained 34 flat 34 or constant at all frequencies of interest if the signal levels er and ep viz the potentials at the output terminal of the recording line amplifier 16 and the input terminal of the playback line amplifier 38 in the arrangement shown in fig1 are substantially equal to each other if the magnetic tape is replaced with another tape having a recording sensitivity higher by for example 2 decibels than that of the former and the new tape is used to record and reproduce the relatively low frequency signals f from the first frequency generator 10 then the signals f will be recorded at a sensitivity higher by about 2 decibels than the recording sensitivity achieved with the former magnetic tape the recording sensitivity of the sound recorder can thus be corrected properly if the recording signal level is adjusted to make the signal levels er and ep equal to each other if the recording signal level is further adjusted to make the signal levels er and ep equal to each other when the relatively high frequency signals f 39 from the second frequency generator 12 are recorded and played back with use of the new magnetic tape the recording and reproducing frequency characteristics of the sound recorder can be made 34 flat 34 or constant for the new magnetic tape the recording signal level control.