Brüel&Kjaer
2804

The Microphone Power Supply type 2804 powers the Bruel & Kjaer transistorized preamplifiers, especially the 2619 half-inch preamplifier for condenser microphones. It supplies 200 V or 28 V polarization voltage for the condenser microphone, and 28 V for the preamplifier.
Two channels are provided. The inputs and outputs can be interchanged by a single switch to facilitate measurement at two positions without reconnecting the equipment.
Two additional power output microsockets are available at the back, each supplying 28 V DC. Maximum current is 2 x 2 mA or 1 x 4 mA.
The polarization voltage can be selected to be 200 V or 28 V by a contact on the printed circuit. The voltage can be checked from a socket at the back of the instrument. The 28 V polarization is used with type 2619 and the ½" condenser microphone type 4148.
To prevent battery drain the heating element in the 2619 is not powered from type 2804. However provision is made on the rear of the 2804 for feeding the heating element from an external 6 V DC supply.

Please find a corresponding data sheet here.