Dynamic microphones are popular in audio recording and sound systems because they are tough, long-lasting and can handle loud sounds really well.
However, they usually produce a weak output signal, which means you often need a preamplifier to make the signal strong enough for further use or amplification.
The circuit shown here is a preamplifier made just for dynamic microphones using the µA739 operational amplifier (op-amp) to give a lot of gain while keeping distortion low.
This preamplifier runs on a positive 18V DC power supply and can boost weak signals from microphones for audio systems.
The design focuses on providing a good signal to noise ratio and stable performance making it great for different audio uses.
Circuit Working:
Parts List:
| Component Type | Value | Quantity | Remarks |
|---|---|---|---|
| Resistors | 1k | 1 | 1/4 watt |
| 47k | 1 | 1/4 watt | |
| 10Ω | 1 | 1/4 watt | |
| 1M | 1 | 1/4 watt | |
| 10k | 1 | 1/4 watt | |
| 100k | 2 | 1/4 watt | |
| Capacitors | Ceramic 0.1µF | 1 | |
| Ceramic 0.0082µF | 1 | ||
| Ceramic 0.001µF | 1 | ||
| Electrolytic 100µF 35V | 2 | ||
| Semiconductors | IC µA739 | 1 | |
| Other Components | Dynamic Microphone | 1 |
This preamplifier circuit is powered by a +18V DC supply.
A dynamic microphone creates a small AC signal when it picks up sound waves.
The input coupling capacitor C1 sends this AC signal to the non inverting input pin 9 of the IC 739 op amp.
The op amp boosts the signal using a feedback network made up of R6, R7 and C5.
Because the op amp has a high input impedance, it does not put much strain on the microphone.
The circuits gain is based on the ratio of R6 to R7 and the value of C5.
The stronger signal can be found at the output pin 13.
The output coupling capacitor C5 blocks any DC part allowing a clear AC signal to go to the next stage.
Decoupling capacitors C4 and C6 help keep power supply noise from interfering with how the op amp works.
Formulas with Calculations:
Below mentioned are the formulas with calculations for Dynamic Microphone Preamplifier Circuit
Gain:
The gain of the circuit is approximately determined by the formula:
Gain = R6 / R7
Substituting the resistor values:
Gain = 1M / 10k = 100
Thus, the circuit amplifies the input signal by a factor of 100.
Low frequency cutoff:
The low frequency cutoff is determined by the input coupling capacitor C1 and the input impedance of the circuit.
The formula is: f_c = 1 / (2 * π * R_in * C1)
Assuming an input impedance of approximately 47k set by R2:
f_c = 1 / (2 * 3.1416 * 47,000 * 0.1 * 10^-6) f_c = 1 / (2 * 3.1416 * 47,000 * 0.0000001) f_c = 34 Hz
This ensures good bass response while rejecting very low frequency noise.
How to Build:
To build the Dynamic Microphone Preamplifier Circuit using IC uA739 follow the below mentioned steps for connections:
- Assemble all the components as mentioned in the above circuit diagram.
- Connect pin 7 of IC uA739 to GND.
- Connect pin 8 of IC uA739 between resistor R6 and R7 which are the feedback network and controls the gain and frequency response of the circuit
- Connect the AC signal from the dynamic microphone from pin 9 of IC uA739 the input coupling capacitor C1.
- Connect pin 10 of IC uA739 to pin 11 through resistor R5 and capacitor C4 to ensure frequency stability and to avoid oscillations to ensure frequency stability and to avoid oscillations.
- Connect pin 13 of IC uA739 in series with resistor R6 and R7 resistor and a coupling capacitor C5 with the output to block DC components before passing the signal to the next stage.
- Connect this pin 14 of IC uA739 to the +18V DC power supply, ensure proper decoupling by connecting capacitor C6 and ground to filter noise.
- Connect R2 and R3 resistors as a voltage divider between pin 14.
- Connect R1 resistor to set the bias current for the microphone.
Conclusion:
The Dynamic Microphone Preamplifier Circuit using IC uA739 is an easy and efficient way to increase weak microphone signals.
The µA739 op amp provides strong amplification with minimal distortion, which is perfect for audio use.
If you stick to the design and assembly instructions you can build a dependable preamp that will improve your audio systems.
This circuit is great for both hobbyists and professionals who use dynamic microphones for recording or live performances.