The Microphone Power Supply Type 2804 is a portable supply for Brüel &Kjær 1/2″ Microphone Preamplifier Types 2639, 2645, 2669 B, 2669 L (with adaptor ZG 0350) and 1/4″ Types 2670 (with Adaptor ZG 0350) 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.
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 Microdot sockets are available at the back, each supplying 28 V DC. Maximum current is 2 × 2 mA or 1 × 4 mA.
For use with the wide range of condenser microphones available from Brüel & Kjær, 0 V, 28 V or 200 V microphone polarization may be selected by a simple internal modification to the printed circuit board located under the bottom panel of the 2804.
The polarization voltage setting can be checked from a socket on the rear panel of the instrument. Half-inch Condenser Microphone Type 4130 is designed for use with a nominal polarization voltage of 28 V, while 0 V is used for Prepolarized Condenser Microphone Types 4155, 4188 and 4189
To prevent battery drain the heating elements in the Types 2639 and 2645 are 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.
INPUTS:
Two 7-pin sockets for direct connection of Microphone
Preamplifiers: 1/2″ Types 2639, 2645, 2669 B, 2669 L (with Adaptor ZG 0350) and 1/4″ Types 2670 (with Adaptor ZG 0350) and 2633
SIGNAL OUTPUTS:
Two standard 14 mm Brüel & Kjær sockets.
Input to output connections interchangeable by front panel switch
28 V DC OUTLETS:
Two 10 – 32 NF Microdot sockets. Max. current 2 × 2 mA or 1 × 4 mA with no preamplifier at the input. Short circuit protected
POLARIZATION VOLTAGE:
200 V, 28 V, selectable with jumper on the circuit board. 0 V selection requires a simple circuit adjustment.
Stability ±2% for all conditions mentioned above and battery voltage from 3 to 5 V
NOISE AND RIPPLE:
Gives no additional noise on output of preamplifiers
ATTENUATION:
0 dB, both channels
CROSS-TALK ATTENUATION:
>100 dB for frequencies below 20 kHz
>80 dB up to 200 kHz
Power Supply
BATTERIES:
3 × 1.5 V Non-rechargeable alkaline batteries, type LR 20 (“D” size).
40 hours battery life for continuous operation with two preamplifiers.
Battery condition continuously displayed on meter during operation
PREAMPLIFIER SUPPLY VOLTAGE:
+28 V (2 × 2 mA or 1 × 4 mA) on input sockets or on output Microdot sockets at the rear
SOCKETS FOR HEATING ELEMENT:
Sockets are provided for an external DC source (Max. 12 V) for heating element in Types 2639 and 2645
Bruel & Kjaer - 2804 Microphone Power Supply 
