Does this work? This circuit uses an op-amp and a transistor to detect and cut off the loud noises.

Thread Starter


Joined Apr 4, 2023
Microphone: The microphone is connected to the non-inverting input of the op-amp, which is pin 3 in this case.
Potentiometer: The potentiometer is used to set the threshold level for the loud noises. It is connected to the inverting input of the op-amp, which is pin 2 in this case. The center pin of the potentiometer is connected to pin 2, while the other two pins are connected to the positive and negative terminals of the power supply, respectively.
Op-amp: The op-amp used in this circuit is LM358, a dual op-amp. It is powered by a +9V power supply. The output of the op-amp is connected to the base of the transistor, which is pin 1 in this case.
Transistor: The transistor used in this circuit is a PNP transistor, which acts as a switch to turn off the microphone when the threshold is exceeded. The emitter of the transistor is connected to the negative terminal of the power supply, while the collector is connected to the output of the circuit. The base of the transistor is connected to the output of the op-amp through a 1kΩ resistor.
Capacitor: The capacitor is used to filter any noise present in the microphone signal. It is connected between the non-inverting input of the op-amp and the ground.
Resistors: The resistors are used to bias the op-amp and set the gain of the circuit. R1 and R2 are equal value resistors (for example, 10kΩ), and they are connected in a voltage divider configuration between the power supply and ground. The junction between R1 and R2 is connected to the inverting input of the op-amp through a 1kΩ resistor. R3 is a feedback resistor (for example, 100kΩ), and it is connected between the output and the inverting input of the op-amp.

When the microphone signal exceeds the threshold set by the potentiometer, the output of the op-amp goes high, which turns on the transistor and blocks the microphone signal from reaching the output. The specific values of the components used in this circuit depend on the specific requirements of the application and can be adjusted accordingly.


Audioguru again

Joined Oct 21, 2019
Your opamp will not work:
1) Its + input is at 0V then its output will also be at 0V. The + input must be at half the supply voltage with R2 and R5 then have a coupling capacitor from the microphone. You do not say if the microphone is dynamic (coil and magnet) or electret.
2) R4 must have a series capacitor to ground so that it does not amplify the DC on the + input of the opamp.
3) C1 will kill all high audio frequencies.
4) I do not know the function of the pot.
5) The transistor symbol is for a PNP type, use an NPN type instead.

The circuit does not detect a level and it does not cut off a loud level. An "automatic gain control circuit" is needed to do that.
Here is the corrected opamp circuit:



Joined Mar 14, 2008
Do you want to just clip the peaks of the load noise, or momentarily totally kill the sound?

If you want to kill the peaks, then you could use a couple op amp configured as a precision full-wave rectifier, so that both the positive and negative peaks could be detected and clipped.
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