Pre-amps boost or amplify the small signal coming out of your cartridge on the tonearm to get the signal up to what is called "line-level". This will fix the very common symptom of having extremely low sound from your turntable. Around 10% of what you might expect. This will occur if you try to hookup an older turntable to a newer amp or receiver that does not have an input on the back that specifically says "phono" near the red and white 'rca' inputs. If you connect a turntable to anything else without a preamp with labels such as AUX , LINE IN, TAPE, CD, VIDEO you will get super low volume until you install the preamp "in series" between the turntable and the amp or receiver.
