Shure 750B User Guide

GENERAL :

DATA SHEET No. 135

DATE: January 1942

S U B J E C T : Model 750B Crystal

Military-Type Hand Microphone

MODEL 750B MILITARY-TYPE HAND MICROPHONE

Model 750B "Military-Type" Hand Microphone is apressure-actuated diaphragmtype crystal microphone designed es-

pecially for efficient reproduction of speech. The crystal used is a grafoil (high capacity) bimorph unit, triple moisture-sealed to withstand adverse climatic conditions. The Microphone 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 designed for high intelligibility. The pick-up characteristic 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, regardless of length.

Total	Cable Length	Output Level* at
Total	Cable Length	Cable	Terminals
		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 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 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 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	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 requirements. 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 resistance (See Fig. B). This permits more efficient

(over)

Printed In U.S.A.

No. 135 DATA SHEET

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

SPECIFICATIONS

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

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
	Impedance: Equivalent				to	a 1,500 mic-
			ro-microfarad			condenser.
Recommended		Load
	Impedance: 1/2 to 5 megohms.

Model . . . . . . . . .					750B
Height,	Overall (a)*. .				4-7/16"
Height,	Case	(h)*. .			3-13/16"
Width		(b)*. .			2-3/4"
Thickness		(c)*. .			1-7/8"
Finish . . . . . . . . .				Iridescent Gray

Net Weight. . . . . . .					14	ounces
Shipping Weight.. . . .				1	lb.	3 ounces
Code Word . . . . . . .					RUSEL
* See Fig. C.

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

GUARANTEE: Each microphone is guaranteed to be free from electrical and mechanical defects for a period of one year 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: Shure patents pending. Licensed under patents of the Brush Development Company.

Fig. B: Frequency Response 750B