DATA SHEET
No.
139
DATE: January, 1942
SUBJECT:
Model 708S
"Stratoliner" Crystal Microphone
MODEL 708S "STRATOLINER" CRYSTAL MICROPHONE
GENERAL: Model 708S "Stratoliner" is a pressure-ac-
tuated diaphragm-type semi-directional
crystal microphone with wide range response
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 im-
proved acoustical performance and modern appearance. The
microphone is provided with built-in receptacle, a 7-foot
shielded rubber-jacketed cable with microphone plug at-
tached, and an S36A Desk Stand. The same Microphone with-
out a stand is available under the Model No. 708SH.
APPLICATION: The Model 708S is designed for public ad-
dress, amateur broadcast, and all general
purpose use, where clear, intelligible re-
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 :
Model 708S is supplied with a heavy die-
cast desk mount, and is equipped with the
standard 5/8"-27 thread fitting any other
Shure desk, banquet, or floor stand. For overhead sus-
pension, an A35A Suspension Adapter may be used. Con-
venient 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 at-
tached.)
CONNECTIONS: The inner conductor of the cable should be
connected to the grid of the first tube
in the amplifier across a load resistance
of ½ to 5 megohms. Input resistance of approximately l/2
megohm should be used to decrease the low frequency re-
sponse of the Microphone, for improvement in speech trans-
mission efficiency; but higher values are recommended if
good low frequency response is desired. The shield should
be connected to the chassis. See Fig. A.
Copyright Shure Brothers, 1942
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
Cable Terminals
7 Ft.
(Standard) -53.0 db
25 Ft.
-55.0 db
50 Ft.
-58.0 db
75 Ft.
-60.0 db
100 Ft.
-61.5 db
150 Ft.
-64.0 db
200 Ft.
-66.0 db
*(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.
Added cable should be of high quality and low capa-
city. 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 small cost, and may be used interchangeably
with shorter cables.
OPERATION:
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-
lets carrying high voltage. Check your connections
carefully.
Crystal microphones should not be used or kept in
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.
When used near a radio transmitter, use the minimum
length of cable consistent with placement requirements.
Careful grounding of the cable shield is advisable.
ACOUSTIC
Model 708S is a semi-directional micro-
CONSIDERATION: phone with polar characteristics as shown
in Fig. C.
The high frequency response is accentuated for maxi-
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 non-
directional in the horizontal plane, and is ideal for
the grouping of artists around it without frequency dis-
crimination.
(Over)
Printed in U.S.A.
