This is a low cost microphone pre-amplifier. The circuit has a mute switch (S2 push-button), which speakers can use when they want to clear their throat. The preamplifier has an unbalanced input and supports high and low impedance matching. Switching between high and low impedance is possible with S1 switch.
The circuit is based on NE5532, a low noise dual-operation amplifier, suitable for superb quality audio applications. The first amplification stage is build around IC1-A (1/2 of IC1) and boosts the input signal at about 27db. This stage is a typical non-inverting amplifier which uses C5 to suppress high frequencies in order to ensure stable operation. Any DC at the input is blocked from C1, and only the remaining AC signal is amplified.
The electronic circuit of the microphone preamplifier
After the first amplification stage, there is a FET (Field Effect Transistor) which normally contacts and the output of IC1-A is further amplified in IC1-B. The second amplification stage is a typical inverting amplifier which provides a voltage gain of about 14db. Finally the signal is taken to the output terminal via high pass filter C3-R9. Due to lack of a voltage follower at the output, for optimum performance, the load should be greater than 10KΩ.
Printed circuit board artwork for the "microphone preamplifier with mute" (copper side view)
While the mute switch is pressed, the FET is switched off and the circuit is muted. C4 determines the speed with which muting occurs within certain limits.
You may build the unit on a test board (breadboard) or on the printed circuit provided above. You may easily assemble all the components on the specific printed circuit board, by following the assembly drawing provided here:
Assembly guide for the Microphone preamplifier with mute
