Turntables have low-level signals. However, most audio units can only process line-level signals. Between a turntable and another audio unit (a mixer, an amplifier or a recording unit), the PP400 acts as intermediary, as follows:
1. The PP400 equalizes a turntable signal according to RIAA specifications,
2. The PP400 amplifies the RIAA equalized signal
3. The PP400 outputs a line-level signal

In terms of sound you dont have such problem like with some preamps which produce HUMMING noise. It does not add anything to the sound just amplifying it.