Bruel & Kjaer - 2609 Documentation, here :

http://www.technicalaudio.com/pdf/Bruel&Kjaer/Bruel&Kjaer_Measuring_Amplifier_Audio_Frequency_2609.pdf

Forum :

http://www.diyaudio.com/forums/equipment-and-tools/289411-bruel-kjaer-2609-measuring-amplifier.html

Bruel & Kjaer - 2609 - Measuring Amplifier

Features :

• Frequency range 20 Hz to 20 kHz
• Voltage measuring rang e 100 µV to 30 V RMS FSD
• "Fast" and "Slow" RMS indication to IEC 651 Type 1
• A-weighting network to IEC 651
• Direct Input and Mic . Preamp . Input with O V, 28 V or 200 V microphone polarization  
• AC and DC output signals 
• Interchangeable meter scales

Uses

• General purpose audio frequency amplifier (90 dB gain)
• Microphone amplifier for use with Tape or Level Recorders
• Sound level meter conforming with IEC 651 Type 1 (with suitable microphone arrangement)
• Measuring amplifier in multiple microphone arrays
• Audio frequency amplifier with selectable gain steps  

The Measuring Amplifier Type 2609 is intended for use in the audio frequency range where its principal application will be as a microphone amplifier.

The meter measures RMS voltage accord ing to iE C 651 and includes an A-weighting network. The Type 2609 features connect.ions for a condenser microphone preamplifier via a 7-pin B & K socket , so that with a suitably positioned condenser microphone, measurements can be made that conform witht he IEC 651 standard (Type 1).

The Measuring Amplifier also includes provision for the direct connection of accelerometer preamplifiers. It can be used as an input amplifier when recording sound or vibration signals with tape recorders, where gain adjustable in 10 dB steps is a useful capability. 

The Type 2609 is the ideal amplifier and meter in multichannel noise monitoring systems, and as an audio frequency compressor amplifier in feedback controlled systems.