DATA SHEET
No.
135
DATE: January 1942
SUBJECT:
Model 750B Crystal
Military-Type Hand Microphone
MODEL 750B MILITARY-TYPE HAND MICROPHONE
GENERAL :
Model 750B "Military-Type" Hand Micro-
phone is apressure-actuated diaphragm-
type crystal microphone designed es-
pecially for efficient reproduction of speech.
The crystal used is a grafoil (high capacity)
bimorph unit, triple moisture-sealed to with-
stand adverse climatic conditions. The Micro-
phone fits naturally and comfortably in the palm
of the hand. It is light, compact and takes
minimum space in portable equipment. It has a
specially designed "On-Off" switch which locks
in the "On" position when pushed up, operating
"On-Off" in the down position. The sturdy die
cast case is attractive Iridescent Gray with
Satin chrome finish grille. The Microphone is
complete with removable suspension hook, 7 foot
shielded rubber-covered cable and spring cable
protector.
APPLICATIONS:
Model 750B is suitable for effi-
cient transmission of speech, and
it is ideally suited to portable
transmitters, Announcing Systems, and for all
other uses where a hand microphone is required.
The frequency response characteristic is design-
ed for high intelligibility. The pick-up charac-
teristic is of the conventional semi-directional
type. (See "Acoustic Considerations").
INSTALLATION:
Model 750B may be conveniently
installed by using the attached
hook to hang the Microphone in a
convenient position. For permanent placement in
portable or mobile equipment, Mounting Bracket
Assembly No. 99-160, including Mounting Button
No. 31-204, will be supplied on special order.
CONNECTIONS:
The inner conductor or "hot" lead
should be connected to the grid of
the first tube In the amplifier
across a load resistance of 1/2 to 5 megohms.
Input resistance as low as 1/2 megohm may be
used if necessary but higher values are recom-
mended If good low-frequency response isdesired.
The shield lead should be connected to the chas-
sis. See Fig. A.
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, regard-
less of length.
Total Cable Length Output Level* at
Cable Terminals
7 Ft.
-52.5 db
25 Ft. (Standard) -55.0 db
50 Ft. -58.0 db
75 Ft. -60.0 db
100 Ft. -61.5 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 de-
crease 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 should be of high quality and low
capacity. 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.
On special order, the Microphone can be
furnished with switches to control relay or other
circuits.
OPERATION:
No polarizing voltage is required
for crystal microphones.
Crystal microphones may be seriously dam-
aged if accidently connected to loud speaker or
power supply outlets 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 consider-
able length of time - or left in closed automo-
biles parked in the sun during hot weather, as
the temperature inside the automobile may easily
be built up to over 125° and permanently damage
the crystal.
When used near a radio transmitter, use the
minimum length of cable consistent with require-
ments. Careful grounding of the cable shield is
advisable.
ACOUSTIC Model 750B is a semi-directional
CONSIDERATION:
microphone. It has a frequency
response adapted especially to
the transmission of speech, with a rise at
approximately 3000 c.p.s. The low frequency
response may be easily attenuated through the
choice of sufficiently low terminating resis-
tance (See Fig. B). This permits more efficient
Copyright, Shure Brothers, 1942
(over)
Printed In U.S.A.
