Microphone are electronics devices that are used to acquire audio signal and convert them to electrical signals. Microphone that uses magnets are also called Magnetic Microphone or Magnetic Mic. The audio signal which is converted into electrical signal have low magnitude and hence a pre-amplifier circuit is needed to increase the signal amplitude. There are many ways we can increase the signal amplitude such as using transistors and operational amplifiers. Here a magnetic mic pre-amplifier design with uA741 operational amplifier using single supply is shown.
uA741 is a general purpose op-amp IC(operational amplifier integrated circuit) that can be used in varieties of analog applications. It has high gain and high operating voltages and can be for building integrators, summing amplifiers, oscillators etc. uA741 is a stated as dual power op-amp(operational amplifier) but it can also be biased to use only single supply.
The op amp ic 741 pin out is shown below.
To make the Magnetic Mic Pre-amplifier with uA741 op-amp we can either use single or dual supply voltage. See Audio Amplifier design with uA741 op-amp tutorial for single and dual supply design. Here we will use single supply voltage of +5V which is suitable for battery powered applications.
The circuit schematic of magnetic micr pre-amplifier with UA741 is shown below.
The uA741 operational amplifier is used in non-inverting amplifier configuration. The input audio signal microphone is ac coupled into the pre-amplifier circuit using the coupling capacitor C1. The resistors R1, R2 and R3 are used to form voltage divider to provide a DC bias of about one half(2.5V) of the power supply(5V) at the non-inverting input. The output is therefore stands about 2.5V dc bias. The gain from the amplifier is set by the feedback components R4 and R5(see Testing Non-inverting Amplifier). The capacitor C5 is used here to control the low frequency roll-off characteristics. The capacitor C5 is used here to reduce the gain at high frequencies. The output can be coupled to next stage using another coupling capacitor.
In this way you can make your own Magnetic Mic Pre-amplifier using uA741 op-amp. For testing the amplifier circuit at home you can use your PC as oscilloscope and spectrum analyzer. To know how you can test this circuit at home example tutorials LM741 non-inverting amplifier with dual supply from single supply.
Here we have shown how to build pre-amplifier circuit for magnetic microphone. Another common microphone for audio application is the electret microphone which uses FET transistors. Here we have used biasing circuit to create a single power supply op amp uA741 but there are also single power supply op amp like LM358. The tutorial How to make electret microphone amplifier using LM358 illustrate how to design pre-amplifier circuit for electret microphone.
