During the practical sessions it may be interesting to illustrate the operation of a wireless transmission or an electronic device with a real audio signal. After having described some time ago on this site an adapter assembly for an MP3 player, I propose in this article a description of a small pre-amplifier module for small electret microphone and runs on 2 AAA batteries.
The proposed scheme is shown in figure below in which use a dual operational amplifier working under a low supply voltage (3V). Under these conditions has a bias voltage to 1.5V (bridge divider R6-R7) on the terminals 2 + with an effective decoupling capacity C10. Include a polarization vector for the electret microphone followed by a first amplifier whose gain is variable by means of the potentiometer P. The second operational amplifier has a fixed gain of 20dB and a filter whose bandwidth can be secured by J1 & J2 jumpers:
The capacitor C7 is used to delete the DC component at the output of the assembly and this assembly can easily connect the Mod Ext Input inputs of different signal generators (AFG3022B, SM300, etc ...). Although it is not shown on the diagram, we add a small slide switch and a power indicator LED for complete realization.
The proposed scheme is shown in figure below in which use a dual operational amplifier working under a low supply voltage (3V). Under these conditions has a bias voltage to 1.5V (bridge divider R6-R7) on the terminals 2 + with an effective decoupling capacity C10. Include a polarization vector for the electret microphone followed by a first amplifier whose gain is variable by means of the potentiometer P. The second operational amplifier has a fixed gain of 20dB and a filter whose bandwidth can be secured by J1 & J2 jumpers:
- For the low cutoff frequency: 30Hz ~ fcb (J1 ON) fcb ~ 300Hz (J1 OFF)
- For high cut-off frequency: 10kHz ~ fch (J2 OFF) FCH ~ 3,3kHz (J2 ON)
The capacitor C7 is used to delete the DC component at the output of the assembly and this assembly can easily connect the Mod Ext Input inputs of different signal generators (AFG3022B, SM300, etc ...). Although it is not shown on the diagram, we add a small slide switch and a power indicator LED for complete realization.
The entire installation can take place in a small plastic box as shown in the photo below.Components & batteries are voluntarily available to facilitate measures, the replacement & settings (Jumper & potentiometers).