Dynamic Microphone Preamp

This circuit is capable of amplifying the output from a 150-ohm dynamic microphone to line level in a single stage, courtesy of a little thing called an instrumentation amplifier. They're like op-amps but with settable gain and much better common-mode rejection. They can have very high gain, high precision, low noise, and decent cost per chip; not particularly fast, but there's no such thing as a free lunch, and it's good enough for audio anyway.

Analog Devices makes some nice ones, in particular the AD620AN I use in this circuit. The standard workhorse is the AMP02F, but it works better off of higher voltages than you get from a single 9v.

They're designed for circuits with a positive, negative, and ground supply, so a virtual ground is needed, a constant 4.5 volts when operating off a convenient 9v battery. The sole purpose of the 741 op-amp is to buffer this virtual ground, since the things feeding into it would easily overwhelm a pair of 1K resistors.

A side-effect of this virtual ground is that this circuit MUST operate from a iosolated power supply, like a battery - anything else would short the 4.5v virtual ground directly to REAL ground. If you have an actual +/- power supply handy, use it and forget the virtual ground entirely to avoid the shorting problem. It'll have less noise with a real +/- supply, too.
Copyright(C) Tyler Montbriand, 2004. Full permission is given to use these schematics and plans for personal use. For commercial use or publishing, contact the author. Send comments, etc to Tyler Montbriand, tsm@accesscomm.ca.