Shure 708S User Guide
DATA SHEET
DATE: January, 1942
No.
139
SUBJECT:
Model 708S
"Stratoliner" Crystal Microphone
MODEL 708S "STRATOLINER" CRYSTAL MICROPHONE
Added lengths of connecting cable will be accompanied
by a decrease in output level as given in the table below.
There is no frequency discrimination introduced by the
cable, regardless of length.
Total Cable Length Output Level* at
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
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.
Cable Terminals
-53.0 db
-55.0 db
-58.0 db
-60.0 db
-61.5 db
-64.0 db
-66.0 db
city. The inner leads should be soldered and insulated
with a good grade of rubber tape. Metal braid sleeve or
GENERAL:
for high quality reproduction of speech. Thecrystal used
is a grafoil (high capacity) bimorph unit, moisture-sealed
to withstand adverse climatic conditions. The Microphone
has a built-in filter circuit for protection against R.F.
burnouts. The case is attractively streamlined for improved acoustical performance and modern appearance. The
microphone is provided with built-in receptacle, a 7-foot
shielded rubber-jacketed cable with microphone plug attached, and an S36A Desk Stand. The same Microphone without a stand is available under the Model No. 708SH.
APPLICATION: The Model 708S is designed for public ad-
production of speech is of primary importance. The 708S
exhibits the semi-directional properties shown in Fig. C.
A swivel is provided to permit tilting of the microphone
through an angle of 90°.
INSTALLATION :
Shure desk, banquet, or floor stand. For 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.)
CONNECTIONS:
of ½ to 5 megohms. Input resistance of approximately l/2
megohm should be used to decrease the low frequency response of the Microphone, for improvement in speech transmission efficiency; but higher values are recommended if
good low frequency response is desired. The shield should
be connected to the chassis. See Fig. A.
Model 708S "Stratoliner" is a pressure-ac-
tuated diaphragm-type semi-directional
crystal microphone with wide range response
dress, amateur broadcast, and all general
purpose use, where clear, intelligible re-
Model 708S is supplied with a heavy die-
cast desk mount, and is equipped with the
standard 5/8"-27 thread fitting any other
The inner conductor of the cable should be
connected to the grid of the first tube
in the amplifier across a load resistance
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 small cost, and may be used interchangeably
with shorter cables.
OPERATION:
lets carrying high voltage. Check your connections
carefully.
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: phone with polar characteristics as shown
mum intelligibility, and is smooth and free from undes-
irable peaks. The low frequency response is flat, but
may be attenuated if necessary through the use of low
values of termination resistance. (See Fig. B). When
turned to the vertical position, the microphone is nondirectional in the horizontal plane, and is ideal for
the grouping of artists around it without frequency dis-
crimination.
Added cable should be of high quality and low capa-
No polarizing voltage is required for
crystal microphones.
Crystal microphones may be seriously damaged if ac-
cidently connected to loud speaker or power supply out-
Crystal microphones should not be used or kept in
When used near a radio transmitter, use the minimum
Model 708S is a semi-directional micro-
in Fig. C.
The high frequency response is accentuated for maxi-
Copyright Shure Brothers, 1942
(Over)
Printed in U.S.A.
No.
139
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
Model................
Height, Overall
Height, Case
Width
Thickness
Finish. . . . . . . . . . . . . . . .
Net Weight. . . . . . . . . . . . . .
Shipping Weight . . . . . . . . . . .
Code Word (7' Cable). . . . . . . . .
* See Fig. D.
Impedance:
Impedance:
DATA SHEET
SPECIFICATIONS
end of a 7-foot cable across 1 to 5
megohms at 400 cycles. This is
Equivalent to a 1,500 micro-micro-
farad condenser.
1 to 5 megohms.
708S
(a)*. . . . . . . . .
(h)*. . . . . . . . .
(b)*. . . . . . . . .
(c)*. . . . . . . . .
Irridesc. Gray Chrome
4"
2-1/2"
2-1/2"
4-7/8"
1 Lb.
2-1/4 Lb.
RUPOC
GUARANTEE:
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: Shure patents pending. Licensed under
Each microphone is guaranteed to be free
from electrical and mechanical defects
for a period of one year from date of
patents of the Brush Development Company.
FIG. B--FREQUENCY CHARACTERISTIC OF MODEL 708S
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