The preamplifier is designed for use with dynamic microphones (moving coil – MC) with impedances up to 200 Ω and balanced connectors. This is a fairly simple design that can also be thought of as a single-stage instrument amplifier based on the NE5534 style op amp. To achieve maximum common-mode rejection (CMR) with a balanced signal, the split ratios of the dividers (R1-R4 and R2-R5) at the op amp input, respectively, must be identical.
As this can be difficult to achieve in practice, a preset potentiometer, P1, is connected in series with R5. The preset is used to optimally set the common mode rejection. Capacitor C1 suppresses any DC voltage at the input, while resistor R7 ensures stability of the amplifier with capacitive loads. Resistor R3 prevents the amplifier from oscillating when the input is open circuited. If the microphone cable is of reasonable length, R3 is not necessary because the parasitic capacitance of the cable ensures the stability of the amplifier.
Note, however, that R3 improves the CMR from >70 dB to >80 dB. The performance of the preamplifier is very good. THD+N (total harmonic distortion plus noise) is less than 0.1% with 1 mV input signal and 50 Ω source impedance. Under the same conditions, the signal-to-noise ratio is –62.5 dBA. With component values specified, the gain of the amplifier is 50 dB ('316). After carefully tuning P1 at 1 kHz, the CMR, without R3, is 120 dB. The supply voltage is ±15 V. The amplifier produces a current at this voltage of about 5.5 mA. Note the separation of feed lines with L1, L2, C2 – C5.