A microphone preamp is used to increase Gain of a microphone.
This simple preamp has a gain from 100 to 1000,
depending on the supply voltage, and the used FETs.
It is a Shunt regulated push-pull stage, with low distortion
and relatively low noise, high input impedance, low output impedance and high gain.
FET Low Noise, High Gain Microphone preamplifier
If the Circuit is used on batterys, You should place a 100uF cap over the supply rails.
Originally the design was a tube
Amplifier for turntables.
The FETs were one stage of a 12AX7.
Remarkably is the fact, that this circuit has no feedback, but has very good linearity.
It is possible to raise R2 and R3
to obtain higher gain.
They are required when using low impedance microphones, usually condenser mics, because these mics don’t have enough voltage to connect directly in to a mixing consoles input.
There are some mixers, line level mixers for example, that don’t have mic inputs at all and a preamp is required to get the mics output signal to line level.
Mic Preamps can be found in mixers, recorders, and other devices that use a microphone.
There are also mic preamps that operate as stand alone devices while others are used in combination with Analog-To-Digital converters to create an Audio Interface or sound card
as they were originally called.