The preamplifier is designed for use with dynamic (moving coil TM) microphone with an impedance of 200 Ω balanced and terminals. It's a pretty simple design, which can also be seen as an amplification stage based on a single instrument type NE5534 op amps. To get the most common mode rejection (CMR) in a balanced signal, the reasons for the division of the dividers (R1-R4-R5 and R2, respectively) to the inputs of operational amplifier must be identical. Since this can be difficult to achieve in practice, a preset potentiometer P1 is connected in series with R5. The preset allows the common mode rejection optimal.The capacitor C1 prevents input voltage while the resistor R7 ensures the stability of the amplifier with capacitive loads.
Resistor R3 prevents the amplifier goes into oscillation when the input has been interrupted. If the microphone cable is reasonably long, R3 is not necessary, because the parasitic capacitance of the cable to ensure the stability of the amplifier. It should be noted, however, that because the R3 improve the> 70 dB CMR> 80 dB. Performance of the preamplifier is very good. THD + N (total harmonic distortion plus noise) is less than 0.1%, where the input signal is 1 mV and the impedance of 50 Ω current. Under the same conditions, the signal to noise ratio is -62.5 dB. When the values of the components have been defined, the gain is 50 dB ('316). After careful adjustment P1 at 1 kHz, CMR, without R3, is 120 dB. The supply voltage is ± 15 V. The power amplifier pulls the voltage of about 5.5 mA. Note: the removal of power lines with L1, L2, C2-C5.
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