Microphone Applications
Microphones from Bedford Technical are based on the Primo EM-158 and EM-172/173 electret condenser microphone (ECM) elements.

Important Precautions !

Because of their unique construction, these high-sensitivity, low-noise microphones must have a PIP (plug-in-power) voltage equal or close to that specified in the data sheet.  Operating at too low a voltage will result in degraded performance.

Also, because of their unique construction, they may be more susceptible to damage from high-temperature soldering than other ECMs. If at all possible use the wires already attached.

When using these ECMs (electret condenser microphone) with some portable equipment it is possible that the PIP voltage will fall below exceptable levels because the equipment cannot provide adequate power under load. 

Example:  When a stereo pair of BT-EM158s were plugged into a Sony Hi-MD MZ-NH900 the PIP voltage dropped to 0.47 volts.  Even though they still operated at this low voltage a low freq hum/noise was experienced. A 3Volt battery box was then used and the expected high performance was again experienced.  An outdoor application where the mic(s) are located a great distance from the audio equipment input will result in even more noise. The solution again is the use of a battery box near the microphone(s).