Art DJ Pre II Phono Preamplifier - Manual (Page 4)

The Facts About Turntable Preamps
Phonograph Records Have Three Basic Problems
1. It is physically impossible to press them with ridges
that allow low frequencies to come out at the same
reproduction level as mid and high frequencies. Bass
compensation is therefore needed during playback.
2. 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.
3. 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,
the Recording Industry Association of America
established compensation standards. 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 that does not have one
built in.