Instruction manual

108

Technical Notes

Preamplifier

Preamplifier Requirement

Since the condenser microphone is a small-capacity transducer, it has high impedance,

especially at low frequencies. Therefore a very high load resistance is required to ensure

uniform response extending to the low frequency range. The relationship between the

microphone capacitance and the low-range cutoff frequency can be expressed as follows.

f0: Low-range cutoff frequency (Hz)

Zin: Preamplifier input impedance (Ω)

Cm: Capacitance of condenser microphone (F)

If the output of the microphone were directly routed through a long shielded cable, the

capacitance between the cable conductors would cause a sharp drop in sensitivity, as is

evident from the following equation.

M0: Output voltage into directly connected shielded cable (V)

Ms: Output voltage in microphone open condition (V)

Cc: Cable capacitance of shielded cable (F)

For the above reasons, a preamplifier is connected directly after the microphone, to pro-

vide a low-impedance output signal.

Preamplifier Specifications

Input impedance: 3 GΩ

Output impedance: Less than 300 Ω

Maximum output current: 2 mA

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