|
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. |