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LW1DSE > TECH 04.01.18 01:05l 120 Lines 5638 Bytes #999 (0) @ WW
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Subj: Electret microphones
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From: LW1DSE@LU7DQP.#LAN.BA.ARG.SOAM
To : TECH@WW
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An electret microphone is a type of condenser microphone, which
eliminates the need for a polarizing power supply by using a permanently
charged material.
An electret is a stable dielectric material with a permanently
embedded static electric charge (which, due to the high resistance and
chemical stability of the material, will not decay for hundreds of years).
The name comes from electrostatic and magnet; drawing analogy to the
formation of a magnet by alignment of magnetic domains in a piece of iron.
Electrets are commonly made by first melting a suitable dielectric material
such as a plastic or wax that contains polar molecules, and then allowing it
to re-solidify in a powerful electrostatic field. The polar molecules of the
dielectric align themselves to the direction of the electrostatic field,
producing a permanent electrostatic "bias". Modern electret microphones use
PTFE plastic, either in film or solute form, to form the electret.
History
Electret materials have been known since the 1920s and were proposed
as condenser microphone elements several times, but they were considered
impractical until the foil electret type was invented at Bell Laboratories in
1962 by Gerhard Sessler and Jim West, using a thin metallized Teflon foil.
This became the most common type, used in many applications from high-quality
recording and lavalier use to built-in microphones in small sound recording
devices and telephones.
Types
There are three major types of electret microphone, differing in the
way the electret material is used:
Foil-type or diaphragm-type
A film of electret material is used as the diaphragm itself. This is
the most common type, but also the lowest quality, since the electret material
doesn't make a particularly good diaphragm.
Back electret
An electret film is applied to the back plate of the microphone
capsule and the diaphragm is made of an uncharged material, which may be
mechanically more suitable for the transducer design being realized.
Front electret
In this newer type, the back plate is eliminated from the design,
and the condenser is formed by the diaphragm and the inside surface of the
capsule. The electret film is adhered to the inside front cover and the
metalized diaphragm is connected to the input (Gate )of the JFET. It is
equivalent to the back electret in that any conductive film may be used for
the diaphragm.
Unlike other condenser microphones, electret types require no
polarizing voltage, but they normally contain an integrated preamplifier,
which does require a small amount of power (often incorrectly called
polarizing power or bias). This preamp is frequently phantom powered in sound
reinforcement and studio applications. Other types simply include a 1.5 V
battery in the microphone housing, which is often left permanently connected
as the current drain is usually very small.
Ϊ---------------Ώ
| ΪΔΔΔo Drain
| ³ |
| ΗΔΩ |
| ΪΔΔΔΔΔ>Ί | Fig. 1: Internal schematic of a 2
| ³ ΗΔΏ | terminal elecret capsule.
|³(M) ³ |
| ³ ³ |
| ΐΔΔΔΔΔΔΔΔΑΔΔΔo Ground
ΐ---------------Ω
+VDD
o
³ JF1 Fig. 2: A schematic of an microphone
ΐΔ¶ preamplifier designed of
ΊΔΏ my own. JF1 and JF2 are
ΪΔ¶ ³ both 2SK117 or 2SK246.
³ ³ ³³C
ΓΔΔΔΔ(ΔΔ΄ΓΔΔo Audio out R χ 10Kκ (Depending on JFET's and
³ ³ ³³ mike chararcteristics.
± ³
± R ³ C = 1ζF or more depending
± ³ on load impedance.
³ ³
ΓΔΔΔΔΩ JF2 and the internal mike JFET
³ makes a `solid state' cascode.
ΐΔ¶ JF2 is like an SRPP and provides
ΊΔΏ JF2 lower output impedance togeter to
ΪΔ¶ ³ high gain and low audio distortion.
³ ³
D o ³
³ ³ VDD is about 7.2V (A NiCd or NiMH
³(M) ³ version of the 9V rechargeable
³ ³ standard battery).
S o ³
ΓΔΔΔΔΩ
³
ΔΑΔ
///
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Ί Compilled from Wikipedia.com . Translatted to ASCII by LW1DSE Osvaldo Ί
Ί F. Zappacosta. Lomas de Zamora, Buenos Aires, Argentina. Ί
Ί Made with MSDOS 7.10's Text Editor (edit.com) in my AMD's 80486. Ί
Ί May 06, 2013 Ί
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