The requirements for microphones used in radio equipment are not very strict in terms of sound quality. A frequency response ranging from approximately 50 Hz to 5 kHz is entirely sufficient for speech. In fixed CB setups or for radio amateurs, sensitivity is a more critical factor. It ensures clear communication without the need to always hold the microphone very close to the mouth. While high-quality microphones with built-in amplifiers are available for purchase, an existing microphone can work just as effectively with the addition of a small preamplifier.
Simple Low-Power Microphone Preamplifier
This project utilizes minimal discrete components and operates with low power demands. It functions effectively with a supply voltage ranging from 1.5 V to 2 V, drawing a mere 0.8 mA current. For those avoiding batteries, the provided adaptor circuit can generate the required voltage using a 10 kΩ resistor, three series-connected diodes, and two 10 μF electrolytic smoothing capacitors from a standard 13.8 V supply.
Microphone Connection and Amplification
The preamplifier accommodates both ordinary dynamic microphones and more affordable electret capsule types. In the latter case, a 1 kΩ resistor must be connected between the 1.5 V supply voltage and the positive input connection. The impedance of the microphone and subsequent stages in the radio apparatus is not critical due to the substantial gain of 32 dB (a factor of 40). Adjusting P1 to its maximum position is rarely necessary.
Frequency Range, Distortion, and Practical Use
The preamplifier covers a frequency range from 70 Hz to approximately 7 kHz, delivering low distortion and compact size. Its design allows easy retrofitting into radio equipment enclosures or microphone stands. Concerns about distortion should be contextualized: for speech radio, slight distortion can enhance intelligibility. In this context, minor harmonic distortions are not flaws but rather intentional features that improve the overall performance.