This professional Brüel & Kjær condenser microphone pair consists of two ½" preamplifiers type 2671 and two prepolarized (electret) microphone capsules 4188. The preamps are "phantom" powered over the BNC connectors with 28 V and 4 mA, typically. Since the capsules are prepolarized no high voltage bias voltage is necessary. A simple battery powered constant current supply is sufficient, as shown in the link. The capsules are pressure type, i.e. have an omnidirectional directivity pattern suitable for AB-stereophonic recording or for general use in loudspeaker measurement.
https://www.bksv.com/-/media/literature/Product-Data/bp1446.ashx
https://www.bksv.com/-/media/literature/Product-Data/bp2209.ashx
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