Balanced Microphone Preamplifier

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The preamplifier is
intended for use with dynamic (moving coil–MC) microphones with an
impedance up to 200 Ω and balanced terminals. It is a fairly simple
design, which may also be considered as a single stage instrument
amplifier based on a Type NE5534 op amp. To achieve maximum common-mode
rejection (CMR) with a balanced signal, the
division ratios of the dividers (R1-R4 and R2-R5 respectively) at the
inputs of the op amp must be identical. Since this may be difficult to
achieve in practice, a preset potentiometer, P1, is connected in series
with R5. The preset enables the common-mode rejection to be set
optimally. Capacitor C1 prevents any direct voltage at the input, while
resistor R7 ensures stability of the amplifier with capacitive loads.

Balanced Microphone Preamplifier Circuit

Balanced Microphone Preamplifier Circuit Diagram

Power Supply For Balanced Microphone Preamplifier

Power Supply For Balanced Microphone Preamplifier

Resistor R3 prevents the amplifier going into oscillation when the
input is open circuit. If the microphone cable is of reasonable length,
R3 is not necessary, since the parasitic capacitance of the cable
ensures stability of the amplifier. It should be noted, however, that R3
improves the CMR from >70 dB to >80 dB. Performance of the preamplifier is very good. The THD+N
(total harmonic distortion plus noise) is smaller than 0.1% with an
input signal of 1 mV and a source impedance of 50 Ω. Under the same
conditions, the signal-to-noise ratio is –62.5 dBA. With component
values as specified, the gain of the amplifier is 50 dB (´316). After
careful adjustment of P1 at 1 kHz, the CMR,
without R3, is 120 dB. The supply voltage is ±15 V. The amplifier draws a
current at that voltage of about 5.5 mA. Note the decoupling of the
supply lines with L1, L2, C2–C5.
Author: T. Giesberts
Copyright: Elektor Electronics 1998