Shure 750B User Guide

DATA SHEET

DATE: January 1942

No.

135

SUBJECT:

MODEL 750B MILITARY-TYPE HAND 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

25 Ft. (Standard)
50 Ft.
75 Ft.

100 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

GENERAL :

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:

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:

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:

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 recom­mended If good low-frequency response isdesired.
The shield lead should be connected to the chas­sis. See Fig. A.

Model 750B "Military-Type" Hand Micro-

phone is apressure-actuated diaphragm-

type crystal microphone designed es-

Model 750B is suitable for effi­cient transmission of speech, and
it is ideally suited to portable

Model 750B may be conveniently

installed by using the attached

hook to hang the Microphone in a

The inner conductor or "hot" lead
should be connected to the grid of
the first tube In the amplifier

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:

Crystal microphones may be seriously dam­aged 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 consider­able length of time - or left in closed automo­biles 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 require­ments. Careful grounding of the cable shield is
advisable.

ACOUSTIC
CONSIDERATION:

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 resis­tance (See Fig. B). This permits more efficient

Model 750B Crystal
Military-Type Hand Microphone

Output Level* at
Cable Terminals

7 Ft.

No polarizing voltage is required

for crystal microphones.

Model 750B is a semi-directional
microphone. It has a frequency
response adapted especially to

-52.5 db

-55.0 db

-58.0 db

-60.0 db

-61.5 db

Copyright, Shure Brothers, 1942

(over)

Printed In U.S.A.

No. 135

transmission of high frequency sounds, contribu­ting greatly toward improved intelligibility
without overloading the power stage of the amp-

lifier.

DATA SHEET

SPECIFICATIONS

Voltage sensitivity:

megohms at 400 cycles. This is equivalent to

32.5 db below 1 volt for a 10 bar signal at the
end of a 7 foot cable.

Internal Output

Recommended Load

Model . . . . . . . . . 750B

Height, Overall (a)*. .
Height, Case (h)*. .
Width (b)*. .
Thickness

Finish . . . . . . . . .

Net Weight. . . . . . .

Shipping Weight.. . . .

Code Word . . . . . . .

* See Fig. C.

Impedance:

Impedance:

(c)*. .

2.36 millivolts r.m.s. per
bar at the end of a 25­foot cable across 1 to 5

Equivalent to a 1,500 mic­ro-microfarad condenser.

1/2 to 5 megohms.

4-7/16"
3-13/16"
2-3/4"
1-7/8"

Iridescent Gray

14 ounces

1 lb. 3 ounces

RUSEL

Fig. C. Overall Dimensions 750B microphone, and
Mounting Bracket (Hanger A Included as Standard
Equipment, Stud B, on special order).

GUARANTEE:

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 from

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

Fig. B: Frequency Response 750B