Why do mic preamps sound different? Electrical parameters of microphones
In typical use, the output voltage from a microphone might be as small as 1 mV - one millivolt or one thousandth of a volt. This is around 60 decibels lower than the level required by a professional mixing console or recorder. So a gain of 60 dB is necessary to bring the signal from the mic up to par.
The output impedance of a microphone gives a measure of its ability to supply current (it's the width of the pipe in the water tank analogy). Most modern microphones have an output impedance of around 200 ohms. If the impedance were higher, then the mic's ability to drive long cables would suffer. If it were lower, then the output voltage would also be lower, potentially degrading the signal-to-noise ratio of the system.
Since this is a small and delicate signal, it would make sense to transfer as much of it as possible to the microphone preamplifier. This would require that the mic preamp had an input impedance of 200 ohms too, which is a measure of how much current the preamp needs to draw from the mic. However, the consequence of this is that any slight variations in the output impedance of the microphone at different frequencies will lead to differences in voltage level, which is bad. So most microphone preamps have a higher input impedance of around 2000 ohms, which is a compromise that has come to be accepted over the years.