Early CDs were recorded with pre-emphasis (50/15 µs) to reduce high frequency noise. This is part of the Red Book standard IEC 60908.
Audio CD players automatically apply a de-emphasis filter when the corresponding flag in the subcode data is present.
Some ripping software (e.g. iTunes) can cope with it and apply the necessary de-emphasis, however most of them (such as dBpoweramp) will not do it so the corresponding wav file needs manual post-processing to apply de-emphasis.
WaveEmph by Th. Ahlersmeyer is a dedicated program to apply de-emphasis:
Jos Van Dyck