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I have a few electret microphones (KEAG45NPF-W). But these are all without a FET. The datasheet tells me I need to add a 'FET impedance converter.' Now, I'm still learning, and I have a very basic understanding of FET (and JFET.) But the bottom line is, I have not a clue how to interpret this.

Any tips/help how to make this work would be great!

Here is the schematic they provided:

enter image description here

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  • \$\begingroup\$ This link is pretty helpful in explaining how a JFET buffer is supposed to work: wildlife-sound.org/equipment/newcomersguide/pip.html Only remember that although that site states the FET is in the capsule, it apparently isn't in your mic. So you have implement simething similar. \$\endgroup\$
    – jippie
    Commented Aug 18, 2013 at 15:06
  • \$\begingroup\$ Googling for electret mic schematic will bring up hundreds of schematics. \$\endgroup\$
    – user3624
    Commented Aug 18, 2013 at 15:20
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    \$\begingroup\$ @DavidKessner, true (did that). But the those i have seen are for with FET. And those schematics don't work. \$\endgroup\$
    – Roger
    Commented Aug 18, 2013 at 15:27

2 Answers 2

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Let's start with the electret microphone basics.

The actual microphone is formed by a (very very) thin sheet of mylar film that is charged. This is connected to the metal body of the electret capsule but insulated from the pick-up plate. The mylar film and the pick-up plate form a capacitor. The charge on the mylar film produces a voltage difference between it and the pick-up plate. (V = Q/C)

As the mylar film is moved by the sound waves it changes the distance between it and the pick-up plate and changes the capacitance value as the distance between the 'plates' increases and decreases.

This is effectively what your electret microphones consist of.

Most electrets contain a FET which converts the very high impedance of the 'capacitor' voltage into a more useful form. It consists of a FET with the gate connected to the pick-up plate, the drain connection as a '+ pin' and the source connected to the ground (metal body).

enter image description here

You could add a FET but given that electrets are so inexpensive, by the time and money you would spend adding a FET to your electrets it would be a lot easier and cheaper and more reliable just to buy a new electret microphone with the FET built into it.

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a look at electret pages on the net will show that some electret fans are after capsules with no fet. electrets are usually noisy. better ones usually have 2sk170 fets. there are other fets with other virtues. with no fet in capsule you can change all sorts of perameters without trying to separate the mic, type fet, number of fets, schematic, voltage of supply. many happy hours of voo-doo contemplation are to be had. love my primo capsules, will be trying to find some transound fetless ones

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    \$\begingroup\$ Welcome to EE. Please consider using proper capitalization and strip out irrelevant things like "many happy hours of voo-doo contemplation are to be had", this would improve your answer. \$\endgroup\$
    – Rev
    Commented Sep 2, 2014 at 6:56

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