Western Electric 618A User manual

Western Electric
1VIICROPHQNE
No. 618A, Moving Coil Type
and Associated Apparatus
INSTRUCTIONS
FOR USE
insttucu: n BuQat:n No. !?,'
ri+4m

Moving Coil Type Microphone and
Associated Apparatus
No. 6 i 8A
Instructions for Use
GENERAL
In radio broadcasting, sound recording and public address systems a
microphone is needed which has the utmost fidelity of pick-up, inherent quiet-
ness, stability and mechanical ruggedness. These requirements are met by
the Western Electric No. 618A Moving Coil Type Microphone. This micro-
phone has a low electrical impedance and may be connected to its amplifier
by means of a suitable transformer; it may be used at a considerable distance
from an associated amplifier.
The microphone may be mounted in a suspension, table, or floor type of
mounting; it is shown in Figure 1.
DESCRIPTION
No. 618A Microphone
The microphone has a thin duralumin diaphragm of low mechanical stiff-
ness, which is clamped around its outer edge. To this diaphragm is attached
a coil of aluminum ribbon. Motion of the diaphragm causes the coil to move in
the magnetic field in which it is located. The magnetic field is maintained by
a permanent magnet made from a high grade cobalt steel alloy. The use of
the permanent magnet eliminates the necessity for supplying a polarizing:
voltage to the microphone.
Motion of the coil in the magnetic field, caused by sound pressures acting
on the diaphragm, induces a voltage in the coil which is proportional to the
velocity with which it moves. A number of air chambers and slot openings
connecting them have been associated with the diaphragm in order to obtain
substantially uniform response over a frequency range of from 35 to 9500
cycles per second. One of these acoustic elements, in addition to exerting a
control on the motion of the diaphragm, allows air to be transferred from the
front to the back of the diaphragm. This eliminates effects due to changes in
barometric pressure.
[1]

Figure 1 --No. 618A-13 Moving Coil Type Microphone
The design and principles of operation of the No. 618A Microphone are
such that it is not affected appreciably by any changes in temperature likely
to be encountered in its use.
The electrical impedance of the No. 618A Microphone at low frequencies
is approximately 28 ohms with a phase angle of nearly zero degrees. The
impedance rises gradually to about 38 ohms with a phase angle of about 20
degrees at 10,000 cycles per second.
The efficiency of this microphone is higher than that of the conven-
tional form of condenser microphone. For the condition of a person speaking
with normal conversational intensity at a distance of 3 feet from the micro-
phone, an output level of -84 db relative to a zero level of 6 milliwatts is
representative.
A speaker need not be directly in front of a No. 618A Microphone. If he
talks toward it, he may even be at right angles to the front of the microphone
without great loss of efficiency. The same is true of orchestral instruments
and other program sources.
Nos. 17A, 18A-13 and 19A-13 Microphone Mountings
The No. 17A Type Mounting illustrated in Figure 2 is intended for suspen-
sion from a boom. In this mounting the face of the microphone may be ad-
justed in a horizontal plane.
The No. 18A-13 Mounting illustrated in Figure 3 is a table type mounting
and has an oxidized bronze finish. It has a fixed overall height of 13 inches
[2]

and has a clamp for holding the microphone. This clamp is removable and
may be used interchangeably on the No. 19A-13 Mounting.
The No. 19A-13 Mounting, illustrated in Figure 4 is a floor type mounting
and has an oxidized bronze finish. It may be adjusted in height from 3 feet
8 inches to a maximum height of 5 feet 10 inches. The clamp for holding the
microphone is removable and may be used interchangeably on the No. 18A-13
Mounting.
Plugs and Jacks
The contacts of the plugs and jacks furnished for use with the No. 618A
Microphone and associated apparatus are of the compression type. They are
connected and held in firm contact by pressure applied by means of a cam
lever located on the side of the plug. Figure 5 outlines the various units and
their uses.
Nos. 405A, 405B and 406A Jacks
The No. 405A Jack is of the fixed mounted type and is designed to mount
as a part of the No. 618A Microphone.
The No. 405B Jack consists of a No. 405A Jack mounted on an escutcheon
plate, and is suitable for mounting on a single gang outlet box.
The No. 406A Jack is of the portable type and is designed for use on the
end of a cord.
Figure 2-No. 618A-13 Moving Coil
Type Microphone mounted in
No. 17A-13 Mounting
[3]
Figure 3-No. 618A-13 Moving Coil
Type Microphone mounted in
No. 18A-13 Mounting

Nos. 284A, 285A, 286A and 286B Plugs
The No. 284A Plug is of the fixed type and is arranged for mounting on
apparatus. The prongs of this plug project at right angles to the mounting
surface.
The No. 285A Plug is of the portable type and is designed for use on the
end of a cord. The No. 286B Plug is of the fixed
type and is furnished with a plate for
mounting on a single gang outlet box.
This plug also may be had without the
mounting plate and is then known as
the No. 286A. The prongs of this plug
are parallel to the mounting surface.
Nos. M3J, M3K and M3T Cords
The No. M3J Cord is a black rubber
covered cord with a standard length
of 100 feet and is intended for use
with the No. 17A-3 Mounting. It
has a black crystalline lacquer finished
No. 285A-3 Plug for engaging the jack
on the microphone on one end and a
No. 406A-3 Jack on the other end.
This cord is shown in Figure 6.
The No. M3K Cord is a brown
cotton covered cord and is intended
for use with the Nos. 18A and 19A
Mountings. Its standard lengths are
12 feet or 5 feet and it has an oxidized
bronze finished No. 285A-13 Plug for
engaging the jack on the microphone
on one end. It is equipped with a
No. 406A-13 Jack on the other end.
Figure 4 -No. 618A-13 Moving Coil It is furnished 12 feet long unless other-
Type Microphone mounted in wise specified.
No. 19A-13 Mounting The No. M3T Cord is a brown
rubber covered cord with standard
lengths of 5, 12 or 20 feet, otherwise the same as the No. M3K. It is also
furnished in the 12 foot length unless otherwise specified.
NOTE: On special order, the No. 618A Microphone, the No. 17A Mounting,
and the associated plugs and jacks may be finished in black crystalline
lacquer instead of oxidized bronze. The black finish is designated by
the figure "-3" after the code number of the apparatus, and the bronze
finish by the figure "-13".
[4]

CABLE
OUTLET
CABLE
OUTLET
OMPRESSION
CAM
NO. 285A PLUG
¡THICK
N0.286A PLUG "I
I
I
2a- .1
181THICK
NO 2868 PLUG
Tr- Fi=
e
NO. 2136A PLUG e
THE NO. 285 TYPE PLUG MAY BE USED
WITH ANY ONE OF THE JACKS SHOWN
THE NO.405 TYPE JACK MAY BE USED
WITH ANY ONE OF THE PLUGS SHOWN
NO.405A JACK
tBRASS ESCUTCHEON PLATE
SCREWED 10 STEEL PLATE
ARRANGED FOR STANDARD
OUTLET BOX
THICK
CABLE
OUTLET
NO.4058 JACK
0
NO.405A JACK.'
BRASS ESCUTCHEON PLATE
SCREWED TO STEEL PLATE
ARRANGED FOR STANDARD
OUTLET BOX
NO. 406A JACK
5"8
rc THICK
CABLE
OUTLET
CODE NO. FINISH TYPE
284A PLUG ALUMINUM PANEL MOUNTED-VERTICALLY
285A-I3 PLUG OXIDIZED BRONZE PORTABLE
286A-I3 PLUG PANEL MOUNTED -HORIZONTALLY
2868 -13 PLUG " OUTLET BOX MOUNTED
406A-13 JACK " PORTABLE
405A-13 JACK " u PANEL MOUNTED- HORIZONTALLY
4058-13 JACK .I .. OUTLET BOX MOUNTED
Figure 5-Plugs and Jacks for Use with No. 618A Moving Coil
Type Microphone
OPERATION
The microphone may be placed in any of the mountings by loosening the
knurled clamping ring nut sufficiently to allow the microphone to be readily
inserted. The nut should then be tightened until the instrument is firmly held.
The plug of the cord being used should be inserted in the jack forming part
of the microphone and the cam lever on the plug pressed down flush with the
plug body. This not only compresses the contacts to insure good contact, but
locks the plug and jack together mechanically.
If the table mounting (No. 18A) or floor mounting (No. 19A) is being used
the cord should be led through the hook in the base. This will not only present
a better appearance than if the cord is left loose, but will also prevent the
mounting being overturned if the cord is accidentally pulled. The other end
of the cord equipped with the jack should be plugged into a plug connected to
the amplifying equipment. As the impedance of the microphone is low, it
may be used at a considerable distance (several hundred feet) from this equip-
ment.
[5]

A preliminary amplifier is required with this microphone. Such a unit is
available in the No. 80A Amplifier. This is a single-stage rack-mounted unit
having an input impedance suited to that of the microphone, an output im-
pedance of 200 or 50 ohms, and a uniform gain of approximately 31 db over a
frequency range of from 35 to 10,000 cycles per second. Full details of this
amplifier may be obtained from any distributor listed on the last page of this
bulletin.
Because of the low energy level generated by the microphone, the entire
circuit between it and the associated amplifier equipment should be shielded.
The cords described previously have two insulated conductors surrounded by
a braided copper shield, this shield being connected to the middle contact of
the plug and jack and the two conductors to the outside contacts. The case
of the microphone is also connected to the middle contact of the jack.
The heads of the Nos. 18 and 19 Type Mountings are interchangeable so
that a microphone may be transferred from one to the other without delay.
This should be done, however, only when the instrument is not in circuit, as
otherwise the noise of moving it will be picked up.
The actual field calibration of the No. 61M Microphone, as in the case of
all microphones, is dependent not only upon the construction of the instrument
but also upon the conditions under which it is used.
The acoustic conditions encountered in the practical use of a microphone
vary so widely that no single response curve can be considered as representative
under all conditions of use. These variables include such factors as the nature
of the sound source or sources, the configuration and acoustic properties of
the reflecting surfaces which surround the microphone, etc.
11111111
i
Figure 6-No. MJJ Cord for Use with No. 618A Moving Coil
Type Microphone
[ 6 ]

MAINTENANCE
The No. 618A Moving Coil Microphone is a precision instrument and
should be handled as such. The instrument is tool tightened during manu-
facture and should not be opened in the field. Should it become defective for
any reason it should be returned for repair to the nearest distributor listed on
the last page of this bulletin.
If a cord breaks, it may be readily changed. The two screws on the back of
the plug and the jack may be removed, exposing the connections. These
should be unsoldered and the new cord soldered in place, care being taken that
solder or loose strands of wire do not touch the body or the cover of the plug
or jack. The red conductor should be soldered to the contact nearest the cam
lever of the plug or the strike plate of the jack, the shield conductor to the
middle contact and the green conductor to the third contact. After the con-
ductors have been soldered in, the cover plate should be replaced and screwed
home. This plate not only protects the connections but acts as a strain relief,
preventing strains on the cord from being transmitted to the soldered connec-
tions.
REPLACEMENT PARTS
Black rubber covered cord-Western Electric No. M3J Cord, standard length
100 feet, order as needed
Brown cotton covered cord-Western Electric No. M3K Cord, standard length
12 feet or 5 feet, order as needed
Brown rubber covered cord-Western Electric No. M3T Cord, standard length
5 feet, 12 feet or 20 feet, order as needed
The apparatus described in this Bulletin
was developed for the
Western Electric Company
by
BELL TELEPHONE LABORATORIES
[7I

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