An electret condenser microphone element is a cheap, high-quality microphone that requires a bias resistor and battery to operate

An electret condenser microphone element is a two-terminal capsule that contains a permanently charged capacitor diaphragm and a built-in JFET buffer transistor. Unlike dynamic microphones, it requires a small bias voltage (1.5-9V) through a resistor (typically 2.2kΩ) to the positive terminal. One terminal is positive (signal/power), the other is ground/shield. These elements are manufactured in huge quantities for consumer electronics (answering machines, phones, toys) and are inexpensive but have surprisingly flat frequency response from 20 Hz to 20 kHz. Unlike piezo contact mics, electrets pick up airborne sound and produce a low-impedance output compatible with standard mixer mic inputs. They require no phantom power—just a battery and resistor. Collins’ design uses a 9V battery, 2.2kΩ bias resistor, and a series capacitor to block DC from the amplifier input.

Examples

Wire a 2.2kΩ resistor from the +9V battery lead to the positive terminal of the electret element; connect element ground to battery negative and shield; connect element positive through a 10μF capacitor to amp input. The result is a condenser mic that costs under $1.

Assessment

Why does an electret element need a bias resistor and battery even though it has a ‘built-in’ transistor? What would happen if you connected an electret element directly to an amplifier input without the bias circuit?