If you connect a microphone directly to computer speakers, bypassing the computer, you get a small megaphone powered from the mains. To match the microphone with the speakers, a transistor cascade is required.
Instructions
Step 1
Make sure the speakers you want to use with the microphone are active. A sign of this is the presence of a separate power supply from the mains - through the built-in or external unit.
Step 2
Use an electret microphone. One of its pins is connected to the body - it is this pin that is negative. Connect it to the common wire of the amplifier. Connect the positive terminal through a resistor with a resistance of several kilo-ohms to the power bus.
Step 3
Take a low-power low-frequency bipolar transistor of the p-n-p structure, having a current transfer coefficient of at least 50 (this is a dimensionless quantity). Connect its emitter to the common wire of the amplifier, and connect the collector to the power bus through a second resistor with a resistance of about a kilo-ohm. Connect the base through a capacitor with a capacity of about 0.5 μF to the connection point of the microphone with the first resistor.
Step 4
Connect the point of connection of the second resistor with the collector of the transistor to the connected inputs of the left and right channels of the speakers. Connect the common terminal of the speakers to the common wire of the amplifier. Get an unnecessary but serviceable cell phone charger. Using a voltmeter, determine the polarity of the voltage at its output. Connect to its output, observing the polarity, an electrolytic capacitor with a capacity of 1000 μF, designed for a voltage of at least 16 V. Connect the negative wire to the common wire of the amplifier, and the positive wire to the power bus.
Step 5
Assemble a chain of a 1 MΩ variable resistor and a 2.7 kΩ constant resistor. Connect it between the base and the collector of the transistor. Set the variable resistor to the position corresponding to the maximum resistance.
Step 6
Set the speaker volume control to the middle position. Turn on the power to the microphone amplifier and speakers. While speaking into the microphone, slowly decrease the variable resistor resistance. First the amplifier will start to amplify, then the distortion will decrease. As soon as further rotation will not lead to a decrease in distortion, slightly increase the resistance.
Step 7
Disconnect power to the microphone amplifier and speakers. Solder the control circuit, measure its resistance, and then replace it with a constant resistor of a similar value. Then reapply power to both units. To avoid acoustic feedback, keep the microphone as far away from the speakers as possible and do not turn the volume too loud.
