Preamplifiers for Precision Microphone Cartridges
The output from a condenser microphone is a very high-impedance signal and is therefore very sensitive to the capacitive loads of cables. This makes it necessary to introduce a driver with high input impedance and low output impedance. Such a driver is called a preamplifier.
The frequency range of a preamplifier is determined by its electronic circuit and is typically more than 200 kHz at the high end and 1-10 Hz at the lower end. The lower end is determined by the input impedance of the preamplifier and the capacitance of the microphone. High microphone capacitance gives a low cut-off frequency.
The dynamic range of a preamplifier is defined as the range between the highest level that the preamplifier can handle without distortion, and the lowest level it can measure. The highest level is related to the preamplifier’s supply voltage, whereas the lowest level is related to the electrical noise generated by the preamplifier itself.
In the field of acoustics, two primary preamplifier technologies are commonly used. The first is the conventional model designed for externally polarized microphones, often identified by its 7-pin “LEMO” connector, which has become a widely accepted industry standard. This type operates using a voltage-driven approach and is capable of handling high-voltage signals—up to 50V peak—making it suitable for applications demanding robust performance.
The second approach is based on Constant Current Power (CCP) technology, which began gaining prominence in high-precision acoustic measurements around 1996.
Initially, CCP-based preamplifiers did not match the performance levels of their voltage-driven counterparts. However, advancements in design and materials have significantly improved their quality, and today they are widely regarded as reliable alternatives.
CCP preamplifiers operate using a constant current supply, typically within the range of 2 mA to 20 mA, with a standard nominal current of 4 mA. This current maintains a bias voltage of approximately 12 V DC, around which the actual audio signal fluctuates. These fluctuations represent the dynamic response of the microphone to acoustic input.
One of the key benefits of CCP-based systems is their simplified two-wire configuration. In this setup, the signal and power share the same line, making installation and integration more straightforward—particularly useful in compact or complex measurement environments.
Both technologies have their advantages, and the choice between them often depends on the specific requirements of the test setup, including signal range, integration needs, and equipment compatibility. As CCP technology continues to evolve, it offers a flexible and efficient option for modern acoustic measurements.
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