An amplifier is a device that electronically increases the voltage or current of signals connected to its input. The signals at its output are strong enough to drive loudspeakers, headphones or other devices according to the amplifier’s type. Different kinds of amplifiers have varying amounts of gain, the factor by which the amplifier increases signals. Most audio equipment, including CD players, FM receivers and televisions, put out a standard signal strength called line level, which amounts to 0.316 volts RMS. Most audio inputs, including those of a headphone amplifier, are designed to take line level signals.
A headphone amplifier has one or more line level inputs and enough output power to drive a pair of headphones. Some have several headphone jacks, with independent volume controls for each jack, so more than one person can listen at a comfortable level. Musicians use them for live performance and studio recording, as the amplifier can mix signals from several sources at the same time.
The turntable’s magnetic cartridge puts out a signal weaker than the line level. If you plug the turntable into a headphone amp, the music in the headphones will sound faint, even with the volume turned up. In addition to the strength of the signal, the frequency response of music on vinyl isn't flat across the human hearing range, unlike CDs. Instead, it has a curve developed by the Recording Industry Association of America (RIAA), designed to compensate for dips and peaks in the response of vinyl. A headphone amp doesn't have the circuitry to compensate for the RIAA frequency curve.
A turntable preamplifier, or preamp, is a device designed to increase a turntable’s signal to line level. Connect the turntable to the preamp and the preamp’s output to the headphone amp’s input. The preamp has a filter which compensates for the RIAA frequency curve, restoring music to its original fidelity.