Shure 708A User Guide

DATA SHEET No. 129

DATE: March, 1941

SUBJECT: Model 708A

"Stratoliner"

MODEL 708A "STRATOLINER" CRYSTAL MICROPHONE

Crystal Microphone

CONNECTIONS :

megohms. Input resistances as low as 1 megaohm may be used

if necessary but higher values are recommended because of
the better low-frequency response obtained. The shield
should be connected to the chassis. See Fig. A.

by a decrease in output level as given in the table below.
There is no frequency discrimination introduced by the
cable, regardless of length.

margin of gain to make up for the decrease in output
levels shown in the table. If the amplifier does not have
the necessary gain a preamplifier at the microphone or
near the main amplifier, is suggested. Preamplifiers
with low impedance output are recommended if the main
amplifier system has low impedance transformer or mixed

input.

ty. The inner leads should be soldered and insulated
with a good grade of rubber tape. Metal braid sleeve or a

serve of fine wire should be soldered between the shields
of the cable to complete the shielding. Longer lengths of
cables with standard plug attached may be purchased at

GENERAL:

high quality reproduction of sound. The crystal used is
a grafoil (high capacity) bimorph unit, moisture-sealed
to withstand adverse climatic conditions. The case is at­tractively streamlined for improved acoustical performance
and modern appearance. The microphone is provided with
built-in receptacle and 7-foot shielded rubber- jacketed
cable with microphone plug attached.

APPLICATIONS:

B) is suitable for either voice or music, and is free from
undesirable peaks. When placed horizontally, the 708A ex­hibits the semi-directional properties shown in Fig. C.
Used vertically, (microphone point straight up), it be­comes non-directional in the horizontal plane, and per­formers may be placed all around it without frequency dis­crimination. A swivel is provided to permit tilting of
the microphone through an angle of 90°.

INSTALLATION: Model 708A is equipped with the standard

overhead suspension, an A35A Suspension Adapter may be
used. Convenient cable changing is possible due to the
built-in receptacle. A 7-foot single-conductor shielded
rubber-Jacketed cable is furnished with plug attached.

(See catalog for bulk length and special cables with plug
attached.) External flexible couplings are unnecessary
since the internal elements of the microphone are sub­stantially isolated from the outer case.

Model 708A “Stratoliner” is a pressure-ac-

tuated diaphragm-type semi-directional crys­tal microphone with wide range response for

The Model 708A is designed for public ad­dress, home recording, and all general pur­pose use. The frequency response (see Fig.

5/8”-27 thread and may be mounted on any
Shure desk, banquet, or floor stand. For

(over)

small cost, and may be used interchangeably with shorter

cables. For unusually long cable lengths or for applica­tions where the hum conditions are bad, Shure Type C30A
Super-shielded cable is recommended. (See catalog).

OPERATION:

cidently connected to loud speaker or power supply outlets
carrying high voltage.

places where the temperature exceeds 125°F. They should
not be exposed to the rays of the sun in very hot weather
for any considerable length of time - or left in closed
automobiles parked in the sun during hot weather, as the
temperature inside the automobile may easily build up to
over 125° and permanently damage the crystal.

length of cable consistent with placement requirements.
Careful grounding of the cable shield is advisable.

ACOUSTIC
CONSIDERATION:

is non-directional In the horizontal plane, and is ideal
for the grouping of artists around it without frequency
discrimination. It is also well-suited for “large-group
pickup of orchestras, etc.

The inner conductor of the cable should be
connected to the grid of the first tube in
the amplifier across a load resistance of 5

Added lengths of connecting cable will be accompanied

Total Cable Length

7

Ft.

(Standard)

25

Ft.

50

Ft.

75

Ft.

100

Ft.

150

Ft.

200

Ft.

*(Expressed in db below 1 volt per bar.)

Most modern high-gain amplifiers have a sufficient

Added cable should be of high quality and low capaci-

No polarizing voltage is required for crys­tal microphones.

Crystal microphones may be seriously damaged if ac-

Check your connections carefully.

Crystal microphones should not

When used near a radio transmitter, use the minimum

Model 708A is a semi-directional microphone
with polar characteristics as shown in
Fig. C.

When turned to the vertical position, the microphone

Output Level* at
Cable Terminals

-53.0 db

-55.0 db

-58.0 db

-60.0

db

-61.5 db

-64.0

db

-66.0 db

be

used or kept in

Copyright Shure Brothers, 1941.

Printed in U.S.A.

No. 129

Voltage Sensitivity: 2.2 millivolts r.m.s. per bar at the

equivalent to 53 db below 1 volt per bar at the terminals

of the 7-foot cable.
Internal Output

Recommended Load

Height, Overall

Height, Case
Width
Thickness
Finish

Net Weight

Shipping Weight
Code Word (7’ Cable)
List Price

*

See Fig. D.

Impedance :

Impedance :

(a)*. . . . . . . . .

(h)+. . . . . . . . .

(b)*. . . . . . . . .

(c)*. . . . . . . . .

................

..............

...........

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DATA SHEET

SPECIFICATIONS

end of a 7-foot cable across 1 to 5

megohms at 400 cycles. This i

Equivalent to a 1,500 micro-micro-

farad condenser.

1 to 5 megohms.

708A

4"
2-l/2"
2-l/2"
4-7/8"

Irridesc.Gray Chrome

1 Lb.

2-1/4 Lb.

.........

RUDUM
$17.50

S

Guarantee :

from date of shipment from the factory, provided all In­structions are complied with fully. In case of damage,
return the microphone to the factory for repairs. Our

guarantee is voided if the microphone case is opened.
License Notice:

Each microphone is guaranteed to be

free from electrical and mechanical

defects for a period of one year

Shure patents pending. Licensed un­der patents of the Brush Development
Company.

FREQUENCY CHARACTERISTIC OF MODEL 708A

FIG. B. FREQUENCY-CYCLES PER SECOND