I just got some little microphones from ECSC.  (www.shopeio.com)  They are Electret Condensers microphone capsules.  They have the benefits of a condenser microphone without the high voltage required to electrostatically charge the plates.  These microphones have permanently charged plates.  They do however require a small amount of power to operate the FET Impedance Converter.

All Electret Microphones I've seen require a voltage of a couple volts minimum to operate the FET impedance converter.  Below is a simple schematic of a microphone bias circuit that I have used to power the microphone and get a signal out.  You can also connect them directly to the microphone input of tape, MD or DAT recorders.  Most, if not all consumer recording devices with microphone inputs have the microphone bias circuit for powering Electrets.

R1 and R2 provide the power that the Impedance Converter requires to operate.  C1 is used to filter out power supply noise from getting into the audio.  C2 is used to AC couple the output of the microphone to your circuit.  None of values are very important.  If you have power supply noise in your audio try increasing the value of C1 and/or R1 & R2.  Increasing C2 will push the high frequency rolloff point highier.

Above is a schematic of a simple Stereo Microphone.  I connected two microphones to my MiniDisc recorder like this and it worked beautifully and sounded great.  The left channel is the tip of the jack, the right channel is the middle connection and the base of course is ground.  Below is a picture of the actual stereo microphone setup.  I took a 1/8" stereo and cut one end off and soldered the microphone right to the ends.  This was just to test it out.  I have since unsoldered the yellow and black wires from the back of the microphone and solder my wires on directly and put 5 minute epoxy on to strain relief the joints.

Explaination of different types of microphones

Carbon Microphones
Early microphones used two plates with carbon particles sandwiched in between.  As the plate vibrated with sound pressure they created a varying resistance.  By connecting a fixed resistor in series with a constant voltage source a varying voltage is seen across the microphone.  This microphones had two problems, poor frequency response due to the heavy metalic diaphram and they would actually wear out.  Using the old acoustic cup modems accelerated their degradation with the constant high volumes.  This type of microphone is all but gone.

Dynamic Microphones
These microphone use a light weight diaphram with a coil of wire attached to the back that is suspended in a permanent magnet.  Sound waves cause the diaphram to move and thus the coil of wire to move as well.  From physics you will remember that a moving wire in a magnetic field creates an EMF.  This type of microphone suffers from limited frequency response due to the mass of the diaphram plus the coil of wire.  These microphone are still very common today.

Condenser Microphones
Unlike dynamic microphones that use electro-magnetic effects, condensers use electrostatic effects.  Here there are two rigid plates on either side of a very light weight metalized mylar diaphram.  The plates are sometimes charged to more than 100 volts.  The sound wave cause the diaphram to oscillate between the two plates causing the voltage to vary, but, it's extremely high impedance.  Thus a FET or vacuum tube impedance converter is placed as close as possible to the diaphram to amplify the signal and provide a low impedance output.

Electret Microphones
Electrets are basically exactly the same as condenser microphones except the plates are permenately charged and FETs are typically used.
 

Created by Scott Johnson on August 6th, 2002


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