As you can see, this scheme is very simple. The use of a dual operational amplifier type NE5532 is a large part in simplicity, although it was quite possible to simplify even more by using a single transistor.

Polarization of the microphone
The electret microphone is fed Due to the resistor R1, 4.7 K. The value of this resistor should be adjusted (increased) if the power is chosen higher than the one proposed here (9V). The capacitor C1 prevents this voltage reaches the microphone in the first op amp, the reference voltage can be shifted and a problem of distortion of the high levels (asymmetrical clipping).

Small electret microphone gain
The op amp provides a first amplification of 10 (20 dB), this value is equal to the ratio of resistors R3 / R2. The second op amp is mounted in exactly the same way as the first, and also provides a gain of 10, and the ratio R5 / R4. The overall gain is equal to 100 (40 dB). If you want to be able to vary the gain, simply replace the resistor R5 of 100K with a 220K potentiometer in series with a resistance of 10K. The overall gain in this way can vary from 10 to 200.

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Parts list :

R1 : 470
R3 : 82K
R4 : 1K
R5 : 1K5
C1 : 47uF
C2 : 22uF
C3 : 100uF
C4 : 56pF
Q1, Q2 : BC109

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Sources : This setting is described by J. England Smith, the magazine ETI in January 1979. It is also found in an old French electronic magazine, the same scheme but with different component values​​.

Parts list :

R1 : 470/220K
R2 : 27K/22K
C1 : 33uF/4U7
C2 : 470nF/2u2
Q1 : 2N2907

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Mic Preamplifier described here is designed for dynamic type microphones, the impedance between 200 ohms and 1 ohm. Its uniqueness lies in the low power consumption, no more than 30 uA in the “worst” case, and make it very portable for this, powered by 1.5 V battery which will allow for longer operating time. This scheme is based on records from the Texas Instruments application.

TLC251 is a Programmable Low-Power Operational Amplifiers. This is indeed a control input, called BIAS (pin 8), which determines the mode of operation. When the pin is worn on the positive potential of power supply, the consumption of the circuit is reduced to a minimum, which is 10 uA (ten micro-amps). When the pin is taken to the potential 0V (ground), the consumption of the circuit back to 30 uA, three times greater than in low power mode. Why you should choose the mode in which consumption is more important? Because of bandwidth. In low power mode, the latter reduced to about 5 KHz trickle to the upper limit, while it increased to more than 20 KHz in another mode. Consider the possibility of intermediate mode, when the terminal 8 (BIAS) increased to 0.75 V, half the supply voltage.

The two resistors R2 and R3 are used effectively to create a point of reference for the AF signals from the microphone, a reference point called virtual ground, which is to “place” AF signals on a median between the two poles of power , so they can move as much towards the positive to negative, without any awkward clipping. The coupling capacitor C3 is added to stop the continuous component created by the virtual ground and is reflected in the output.

Gain assembly
The gain is determined by the ratio between the value of the potentiometer RV1 (mounted variable resistor) and the value of the resistor R4. The report here is 100 maximum, which corresponds to a gain of 40 dB.

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This Simple preamplifier circuit provides sufficient gain to weak audio signals from the microphone. This circuit can also be supplied from the voltage baterry 3 to 9 volts if necessary. Take a note that the microphone used in this pre-amp circuit is electret type.

If you are using high fidelity sound system, the this mic pre-amp will not become a good choice, you should be find other circuits.

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