Magnetic fono cartridges, which about 99.9999% are, generate very little output. A 'high-output' cartridge generates about 5 thousandths of a Volt (5mV) while playing a standard test tone (which is 3.54 whachamacallits per channel at 1KHz). 'Medium-output' carts generate somewhere around 1- to 2mV, while low-output (LO) carts, almost always moving-coil types, generate no more than around 0.5mV.
The 5mV is 40 deciBels below a nominal 'line' Voltage of 0.5 Volts. This 0.5 Volts would be enough to drive some poweramps (PAs) to full output and about-all PAs to quite-high levels. A line-level preamp will control and amplify that line-level Voltage and have switching circuitry.
So the first function of a fono preamp is to amplify the cart's signal about 40dB, and some will have enough additional gain, usually 20- to 30dB more and usually switchable, to amplify the outputs of LO carts. ('Prepreamps' are amplifiers that perform only this latter amplification.)
The other function of a fono preamp is to apply the RIAA equalization curve to the cart's output. The Recording Industry Association of America invented this equalization curve to help make records more listenable and less noisy. The playback half is a downslope beginning in the bass, with the treble info (and surface noise) down greatly compared with bass. A picture of it is here
http://www.euronet.nl/~mgw/background/riaa/uk_riaa_background_1.html.
So you need a fono preamp if you want to play records with a magnetic cartridge. It's fed by the cable coming out of the recordplayer's arm, and it feeds (usually) the linestage preamp.
I hope this helps.
