Most modern amplifiers do not have provision for a phono cartridge, so a separate phono preamplifier is necessary. And the phono preamps included in many older integrated amplifiers are fairly basic. You can achieve a worthwhile improvement in sound quality with a separate phono preamp.
When you consider that the signal that the phono cartridge extracts from the groove has to be amplified many thousands of times before it gets to the speakers any improvement in the signal at the front end will be magnified greatly.
Why do you need a phono preamplifier? (answer courtesy of Graham Slee)
Bass takes space. Vinyl is a mechanical medium. The groove contains the wavelength and velocity information for all the musical frequencies contained in it. The signal's wavelength is inversely proportional to frequency. Using the equation:- wavelength (?) = velocity (V) / frequency (f)
We find a bass frequency of 50Hz has a wavelength ( groove wiggle) of 0.1cm (1mm) at the peak recorded velocity of a vinyl LP (5cm/sec). With 50:50 spacing, you'd only get 40 revolutions, and at 33? RPM that's just 1 minute and 12 seconds!
At 1kHz (middle of the band) the wiggle is 0.05mm - enough for 24 minutes, but at 10kHz, the wiggle is so small it would be swamped by the noise of the vinyl's polymorphic structure.
The cure is to turn the bass down and the treble up while cutting the record master. This is done using a precise formula (RIAA). So when you play a record you need a clever bit of electronics to change it back. That's the job of the Phono Stage.
The more accurate in frequency and phase performance the phono stage is - the better the sound. Not forgetting that the phono stage must not add to the background noise already on the record.
The best phono stages are: Fast (wide bandwidth); Quiet (low noise) and distortion should be inaudible. And that is not an easy task!