Measuring Amplifier with Power Supply MN 921
The MN 921 is a one-channel measuring amplifier with a power supply unit for measurement microphones. Between the input and output there is a precision measuring amplifier with adjustable gain. This enables optimisation of the dynamic range of the measured signal for the connected evaluation system. Additionally it is a driver for capacitive loading, e. g. long BNC-cables at signal outputs.
The measuring amplifier can be powered by batteries or mains. It provides all necessary voltages for condenser measuring microphones (preamplifier-, polarization- and heater voltage). These voltages are indirectly-coupled (no mains hum on the output signal) and are available on the LEMO-socket (MIC). The output for amplifier operating voltage (120 V) is short-circuit protected and the polarization voltage is maintained at ± 0.1 V. For pre-polarized measuring microphones, the polarization voltage can be switched off. The MN 921 can supply measurement microphones with IEPE-connection in use with the adaptor A 92, LEMO to BNC.
There is a level indicator for visual control of the output signal at the BNC connector (OUT), which ensures that it is within the operating range of the amplifier. To supress infrasonic disturbances on the signal you can switch a high pass second order before the output.
The MN921 switches automatically from battery to mains adaptor operation on connection to mains, i.e the battery can be left in place with no current drain. While the device runs in accumulator or battery mode the heater cannot be activated, preserving battery life. Likewise, the level indicator switches from line-LED-display to single-LED-display. To ensure a correct operation of the measuring amplifier with accumulators or batteries, at all times, the MN 921 shuts down as soon the minimum battery charge level is reached. The LED (B-Low) lights up just before this takes place.