The Microphone Power Supply Type 2804 is a portable supply for Brüel &Kjær 1/2″ Microphone Preamplifier Types 2639, 2645, 2669 B, 2669 L and 1/ 4″ Types 2670 and 2633.
It supplies 200 V, 28 V or 0 V polarization voltage for the condenser microphone, and 28 V supply for powering the preamplifier.