The amplifier can simply understand as a device that amplifies the electrical signal, that is, you put into it an input signal, it will amplify that signal and bring out the transmitter (speakers, headphones).
Amply has many types, depending on the function of each type that people named after it for easy to remember. Currently there are 5 popular forms: Pre-amply, Power Amply, Integrated Amply, Dual mono Amply, Monoblock Amply
The amplifier will also provide power to the speakers in the system, and is also responsible for processing audio signals to suit the needs, such as enhanced bass, mid or treble; add echo effects (echo), parody (repeat) …
In-ear Monitor (IEM, universal IEM, In-ear headphones) is a small size earphone, worn by inserting deeply into the ear canal of the listener, helping enhance the sound insulation level with the outside environment, and carrying to give users high sound quality. IEM is a specialized product for sound engineers, musicians, singers, or audiophiles. IEM headphones help singers can clearly hear the sound of the instruments, the vocals I need to hear more while reducing the sound on stage.
IEM headphones also have the function of filtering environmental sounds due to the design that fits the ear canal. Therefore, in some music stages, there is also a system of multiple microphones so that singers can hear certain audience responses through IEM headphones. Nowadays, IEM has become more popular and there are a lot of products for ordinary users.
IEM for EDM, DJ, Dance music: Sennheiser MM30g and IE80
IEM for Pop music: SoundMAGIC E10s, Sony MH1C
IEM for Classic music: Grado GR8i