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, firstname.lastname@example.org.