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
Dynamic Microphone Preamp - schematic

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, [email protected] ca.

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