Microphone Preamplifier and Noise Limiter

Engineeering Projects

The circuit posted here is of a microphone preamplifier and noise limiter. The main role of a microphone preamplifier circuit is to raise the signal level of the microphone output signal. A microphone is a transducer that converts sound signals to electrical signals. The output signal of the microphone is very small in a range of a few millivolts. This signal needs some level of amplification before the power amplification. Generally, the gain of the preamplifier is set to between 30 to 100. Besides pre-amplification, the circuit also limits noise. The noise can be reduced by limiting the bandwidth of the signal from 200-300Hz to around 3.3kHz – 5kHz

Circuit description Microphone Preamplifier and Noise Limiter

The circuit is divided into three main sections:

  1. preamplifier
  2. High pass filter
  3. Low pass filter

Preamplifier

The preamplifier is designed around two-transistor T1 and T2. The output of the microphone is given to the base of transistor T1 through an RC coupled network. The output from the collector of the transistor T1 is given to the base of transistor T2.

Microphone Preamplifier and Noise Limiter

Gain of preamplifier

The overall gain of the preamplifier section is determined by two resistors R4 and R5. The gain is approx equal to -R5/R4.

Let the value of resistor R4 = 100-ohm and resistor R5 = 3.3K then gain = 33.

The overall current of the preamplifier is determined by resistor R7.

High pass Filter

A high pass filter circuit is designed to limit the bandwidth of the signal to around 200Hz – 300Hz. It consists of three RC networks as shown in the circuit diagram

The formula of cut off frequency is F_c = \dfrac{1}{6.28 \times R \times C}. By using this formula cut-off frequency of high pass and low pass can be calculated.

F1 = \dfrac{1}{6.28 \times R12 \times C5} = \dfrac{1}{6.28 \ times 0.33 \mu f \times 2 k\Omega} = \dfrac{1}{6.28 \ times 0.33 \times 10^{-6} \times 2 \times 1000} = 241 Hz

F2 = \dfrac{1}{6.28 \times R13 \times C6} = \dfrac{1}{6.28 \ times 33 nf \times 20 k\Omega} = \dfrac{1}{6.28 \ times 33 \times 10^{-9} \times 20 \times 1000} \approx 241 Hz

F3 = \dfrac{1}{6.28 \times R14 \times C7} = \dfrac{1}{6.28 \ times 3.3 nf \times 200 k\Omega} = \dfrac{1}{6.28 \ times 3.3 \times 10^{-9} \times 200 \times 1000} \approx 241 Hz

From the above calculation, we can say that high pass cut-off frequency F_C = F_1 = F_2 = F_3 \approx 241

The value of resistor and capacitor can be selected according to the desired cut-off frequency of the high pass filter.

Low pass filter

A low pass filter circuit is designed to limit the bandwidth of the signal to around 3.3kHz – 5kHz. It also consists of three RC networks.

F4 = \dfrac{1}{6.28 \times R18 \times C9} = \dfrac{1}{6.28 \ times 22 nf \times 2 k\Omega} = \dfrac{1}{6.28 \ times 22 \times 10^{-9} \times 2 \times 1000} \approx 3619 Hz

F5 = \dfrac{1}{6.28 \times R19 \times C10} = \dfrac{1}{6.28 \ times 2.2 nf \times 20 k\Omega} = \dfrac{1}{6.28 \ times 2.2 \times 10^{-9} \times 20 \times 1000} \approx 3619 Hz

F6 = \dfrac{1}{6.28 \times R20 \times C11} = \dfrac{1}{6.28 \ times 0.22 nf \times 200 k\Omega} = \dfrac{1}{6.28 \ times 0.22 \times 10^{-9} \times 200 \times 1000} \approx 3619 Hz .

From the above calculation, we can say that low pass cut-off frequency F_C = F_4 = F_5 = F_6 \approx 3619.

Power supply: Preferred power supply will be above 15V but can work on as low as a 9V supply.

Component Required for Microphone Preamplifier and Noise Limiter

Resistor (all ¼-watt, ± 5% Carbon)
R1 = 4.7 KΩ

R2 = 220Ω

R3, R8, R17, R22 = 1 KΩ

R4, R11 = 100 Ω

R5, R10 = 3.3 KΩ

R6, R9, R23 = 330 Ω

R7 = 330 KΩ

R12, R18 = 2 KΩ

R13, R19 = 20 KΩ

R14, R20 = 200 KΩ

R15, R16, R21, R25 = 3.3 MΩ

R24 = 10 KΩ
Capacitors
C1 = 220 µF/ 25V

C2 = 4.7 nF

C3, C5, C8, C12, C14 = 0.33 µF

C4, C11 = 470 µF/25V

C6 = 33 nF

C7 = 3.3 nF

C9 = 22 nF

C10 = 2.2 nF

C11 = 0.22 nF

C13 = 100 µF/25V
Semiconductors
Q1, Q2 = BC550

Q3, Q4 = BC618

ZD1 = 5.1V Zener diode

LED1 = 5mm RED Color LED
Miscellaneous
SW1 = ON/OFF Switch

MIC1 = Microphone

 

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