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The JFET is externally-powered by the DC voltage V + through a resistor which sets the gain and output impedance. The output audio signal is received though a DC blocking capacitor. An electret microphone is a microphone whose diaphragm forms a capacitor (historically-termed a condenser) that incorporates an electret.
In this configuration the output impedance of the microphone should be negligible in comparison with the input impedance of the pre-amplifier (in practice a pre-amp impedance at least 10 times greater than the microphone impedance is recommended). By doing so, the signal is attenuated minimally and almost no power is used in the process. [59]
Contact microphones based on piezoelectric materials are passive and high-impedance, and they sound tinny without a matching preamp. Instead of being used as a microphone, they alternatively may be used to produce sound (typical used as the buzzer in computer motherboards ) by sending voltages to them.
The electronics of the IEPE sensor (typically implemented as FET circuit) converts the high impedance signal of the piezoelectric material into a voltage signal with a low impedance of typically 100 Ω. A low impedance signal is advantageous because it can be transmitted across long cable lengths without a loss of signal quality.
Instead, line level circuits use the impedance bridging principle, in which a low impedance output drives a high impedance input. A typical line out connection has an output impedance from 100 to 600 Ω, with lower values being more common in newer equipment. Line inputs present a much higher impedance, typically 10 kΩ or more. [5]
Two pins are used for the mono headphone signal and two pins for the unbalanced microphone signal. The 4-pin XLR connector is also commonly used on amateur radio microphones, but transferring unbalanced audio instead, and using the 4th pin (with the common ground) for a push-to-talk (PTT) circuit activated by a button on the microphone.
The electrical purpose of a direct box is to bridge the impedance of the input and output. Most electronic instruments and microphones cannot be plugged directly into the pre-amplifier inputs of mixing boards or recording devices because of impedance mismatch. Instruments are typically high impedance, whereas microphone inputs are low impedance.
A user presses a key which transfers information to a computer. In computing, an input device is a piece of equipment used to provide data and control signals to an information processing system, such as a computer or information appliance. Examples of input devices include keyboards, computer mice, scanners, cameras, joysticks, and microphones.