The article discusses how a balanced microphone preamplifier circuit functions and also provides a simple circuit which may be easily built at home and used for personal entertainment in conjunction with your existing power amplifier unit.

A balanced microphone preamplifier system may be defined as an amplifier circuit or a device consisting of two discrete inputs where only the difference in the applied signals is amplified. Therefore this amplifier is also termed as differential amplifier.

Now for understanding the working of such differential amplifiers, let’s consider the diagram provided below.

The diagram shows an Opamp configuration, consisting of two resistors R1 (@ inverting

i/p) and R4 (@non-inv i/p) connected to its two inputs, one resistor R2 (9 times R1 and R4) is connected across the non-inverting input and the output of the opamp, and another resistor R3(same as R2) connected across the inverting input and ground.For keeping things simpler, let’s assume the gain of the opamp to be 9, such that R1=R4= 1 and R2=R3=9. The precise units are irrelevant here as ae are only concerned with the ratio values.

Let’s also assume the end of

R1 to be at 0 potential and that of R4 to be at 100mV If we consider the opamp to be an ideal one, we may expect two things from the set configuration.No current will be taken inside the inputs and the inputs will adjust such that the output shows a zero potential. The above terms means that there must be a 100mV intact across R4 and subsequently a potential of 900mV across the feedback resistor R2.

This will lead to an output of -900mV. Now if we reverse the input voltage conditions such that end of R1 is held at +100mV and the end of R4 at 0, then the voltage at the inverting input can be calculated as:

V at end of R1 * R3/R1+R4 = 90mV

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