Phonograph records have three basic problems.
- It is physically impossible to press them with ridges that end up allowing low frequencies to come out at the same reproduction level as mid and high frequencies. Bass compensation is therefore needed during playback.
- Records produce a certain amount of hiss which is covered up in post-production by boosting the gain of the high frequencies before pressing. Counter-EQing during playback compensates for this.
- Magnetic cartridges produce a weak signal which must be boosted to match the rest of the amplification and this too is done during playback.
In the mid-1950s, compensation standards were established by the Recording Industry Association of America. The resulting RIAA preamp has been built into every hi-fi and stereo amplifier with phono or turntable inputs since then. A separate RIAA preamp is necessary when you are connecting a turntable to a mixer which does not have one built in.