This preamp is very simple, and will work under tough conditions. The input impedance is pretty high, so it won`t load down electric guitars. It will also amplify microphones. When I have a moment, I`ll make some additions that will optimize it for microphones, and also show how to power a condenser microphone. The power source is typically a 9-volt battery, but the circuit will tolerate 6 to 30 volts. Battery drain at idle is about 1 milliamp. The two red LEDs are important in the circuit - they act as a voltage reference to set the operating point of the opamp.