Op-Amp Mic Preamp
Circuit :   Andy Collinson
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Description:
A high quality microphone preamplifier using a single power supply, suitable for dynamic or electret microphones. The op-amp used can be any low noise, high performance type, e.g. NE5534,TL071, OPA 371 etc



Notes
Nothing too special here. The schematic is shown using a dynamic microphone, for use with an electret a suitable biasing resistor is required to power the electret microphone, see this page for more help. The pinout for the TL071 is shown below:


TL071 Pinout


The design is a standard non-inverting design, the input is applied to the non-inverting input of the op-amp which is pin 3 in most cases. The input impedance is 23.5k, the overall voltage gain is determined by R2 and R1according to the following formula:

Vo = ( R2 / R1) + 1


With the values of R2 and R1 on the diagram the voltage gain (for mid band, 1KHz) is approximately 23x or 27.2dB The gain bandwidth (bode) plot is shown below. This plot is simulated using the TL071 op-amplifier.

Operational amplifiers feature high gain bandwidth products, have a fast slewing rate and have extremely low noise. It is difficult to achieve the same performance using discrete components. Finally the overall signal to noise ratio has been calculated, the source was a 1k impedance microphone generating a 1mV pk-pk sine wave.


An impressive result of 110dB S/n at 1KHz is achieved. In practice this figure will be lower and determined by the quality of the microphone used.


Variable Gain Modification
For anyone wanting to build a variable gain amplifier, then R1 can be change to a 2.2k preset or potentiometer. Line out levels depends on the microphone used and proximity and loudness of the person speaking.

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