The audio information stored in a record's groove can be in an area as small as a micron (one-thousandth of a millimetre), so the scale of the task to retrieve it and playback through your speakers is immense - one that your standard line-level stereo amplifier isn't able to do on its own.
The physical limitations of vinyl mean that the original audio signal has to be altered before it can be recorded onto its tiny grooves – low frequencies are reduced in level and the highs are boosted. The curve that governs this equalisation standard was set by the RIAA (Record Industry Association of America) in 1954.
This is where the phono stage (or phono preamp) comes in. It has two jobs. Firstly, it has the reverse response built into it – one that boosts bass and flattens treble to exactly the right degree, which should result in a tonally even presentation for the audio signal.
Secondly, it acts as an amplifier. The cartridge signals from tracking the groove can be as low as a thousandth of a volt (CD’s output is specified at 2V, for instance) so the signal has to be amplified massively before the line-level stage of a stereo amplifier can take over.
Learn more about how a vinyl record makes a sound