Mini-Electret Microphone

Circuit Description
Super Mike designed by L . Gendron ( Aug , 2001 )
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The Super Mike Amlifier was redesigned to use an mini-condenser microphone element . The microphone element is very small about 1/4" dia . by 1/4 " long capsule with two coloured coded wires . This microphone is also known as an " Electret " as it contains an FET transistor in its
capsule .
The microphone can be obtained from Radio Shack " Mini-electret microphone element " cat # 270-085 . With application instructions.
The microphone requires a positive voltage input of 4 to 10 volts and a coupling capacitor to the amplifier .

The microphone is now connected while observing its connection polarity between the base of Q1 and ground . As shown in the redesigned circuit , Q1circuit has been modified to accept the mini-mike input by adding R1a which supplies 4.5 volts operating voltage to the mike and C2 value now 1uF is used as the coupling capacitor to the base of Q1, the emitter is now connected to ground and R1 has been changed to 10K to allow more current for amplification .

Not mentionned before is the function of C3 , in both circuits , C3 is used to filter out any stray radio signal from being detected and amplified . You can check that my removing C3 and you should now have a local radio receiver . If C3 value of .01uF does not entirely prevent radio detection you can increase the value of C3 up to .047uF but any value greater than .047 will seriously filter out the microhpone higher frequencies .
Q1 is biased in the same manner as the previous circuit except that due the higher impedance input the feed back voltage from Q2 emitter needs to be greatly reduced and those changes are reflected by the new values of R2 , RV1 , and R4 .

By having to use C2 as the coupling capacitor to the base of Q1 , we are prevented to use the AGC circuit as in the previous amplifier as this would require to add a transistor stage for this function . RV1 now has the function of setting the gain of Q1 at will for sensitivity as needed and allows Q1 to amplify all frequencies equally .

A further change which is optional was made to the RV2 filter where C8 value was changed to .01uF for slightly higher frequency response. Other than the changes mentionned the circuit is the same . Constructions and phones connections as detailed on the previous page apply .

Mini-Mike with Shotgun

Using the mini-mike with the shotgun can easily be adapted by using a small plastic funnel to fit the diameter of the tube cap . The mike is inserted into the small end of the funnel and the larger end cemented to the tube cap after having previously cut out the hole in its face as described previously . The wire connection can be made in the same manner as the speaker . I used a short lenght of shrink tubing around the mike and part of the wiring to seal the capsule before inserting it into the small end of the funnel for a tight fit .

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