The circuit was designed to provide more efficient output with a dynamic microphone by using a low noise preamplifier.
http://www.zen22142.zen.co.uk/Circuits/Audio/dyn_mic/dyn_mic.htm
A microphone amplifier that may be used with either Electret Condenser Microphone (ECM) inserts or dynamic inserts, made with discrete components.
http://english.cxem.net/preamplifier/preamplifier6.php
This is a design for a low noise microphone preamplifier, which is ideally suited to low impedance microphones. One limitation is that it is not balanced, which is not a problem in a home recording environment, but will allow the mic lead to pick up noise with long cable runs or in a hostile environment.
http://sound.westhost.com/project13.htm99
This circuit is mainly intended to provide common home stereo amplifiers with a microphone input. The battery supply is a good compromise- in this manner the input circuit is free from mains low frequency hum pick-up and connection to the amplifier is more simple, due to the absence of mains cable and power supply.
http://www.redcircuits.com/Page49.htm
This is a simple microphone preamplifier circuit which you can use between your microphone and stereo amplifier. This circuit amplifier microphone suitable for use with normal home stereo amplifier line/CD/aux/tape inputs.
http://www.tkk.fi/Misc/Electronics/circuits/micamp.html9
JP-11P-1 Input transformer improvement for electronically balanced input.
http://www.jensen-transformers.com/as/as069.pdf
JT-16-A Improves Performance of SSM2017 Mic Preamp IC.
http://www.jensen-transformers.com/as/as064.pdf
JT-MB-C Isolates both Mic and Line Inputs of Inexpensive Mixer.
http://www.jensen-transformers.com/as/as015.pdf
JT-MB-C Mic Input Isolation for Mackie Mixers.
http://www.jensen-transformers.com/as/as057.pdf
Provides 5 to 6dB better S/N, better common mode hum and buzz rejection and inceased RFI immunity.
http://www.jensen-transformers.com/as/as063.pdf