Simple Microphone Preamplifier

The amplifier circuit is a common emitter amplifier with collector to base biasing. Most general-purpose transistors such as the BC547, BC548, 2N2222 and 2N3904 are suitable transistors for use in this circuit.

Simple implementation of a microphone preamplifier for an electret microphone.

The resistor R3 provides the necessary supply voltage for the electret microphone itself. If this amplifier is supposed to be used for dynamic microphones or other low-level audio sources (e.g. a guitar), R3 must be omitted and the audio source is directly connected to capacitor C2. C1 and C2 are coupling capacitors that only let the audio signal pass but block any DC voltage. The coupling capacitors have to be sufficiently large to not hinder low-frequency signals from passing. The purpose of C3 is to provide some degree of voltage stabilization. The value of C3 is not critical and values as low as 10 µF should do the job. R1 and R2 are responsible for biasing the operating point of the circuit.

Functional microphone Preamplifier circuit on a small piece of perfboard.

The circuit can easily be soldered onto a small piece of copper clad Perfboard. One thing to note is that the amplifer has an output impedance of ≈10 kΩ. Therefore, it is not possible to connect a low-impedance load like a speaker directly to the output of the amplifer. A common-collector voltage follower stage will have to be added in order to drive speakers or other low-impedance loads directly.

Microphone preamplifer with a common collector voltage follower stage to drive a speaker or other low impedance loads directly.

If more output power is desired, a transistor with higher power-handling capabilities, such as the BD139 audio amplifer BJT, must be used for Q2.

This article goes back to one of my YouTube videos that has been posted all the way back in 2015. The video has since gathered over 200K views and frequent engagement from viewers shows that there is still a strong demand for this circuit.