THE SHURE

TECHNICAL BULLETIN - --- - - - - pp - -- - -

D E V O T E D T O T H E A D V A N C E M E N T O F M I C R O P H O N E T E C H N I Q U E

V01. 2 No. 3 AUGUST L934

TWO-BUTTON MICROPHONE CIRCUITS HUM REDUCING INPUT CIRCUIT tional advantage of the cir-

cuit is that no connections to Hum ~roblems immediately arise the high-impedance grid

when an input transformer is added to a circuit are required, thus high gain amplifier which has integral avoiding hum A.C. power supply. Usually, despite all which might arise in running attempts to orient the input transformer grid-circuit connectors to

for minimum induction, stray fields switches. To switch off the from the power transformers and filter low impedance source, sim- chokes introduce high hum ply short circuit the terminating resistor The button current for carbon micro- voltages in the input transformer sec- as shown. phone can be obtained easily from the ondary. For any given conditions of amplifier power supply system. Several stray the hum in the amp1ifier If volume control of the low-impe- precautions, however, should beobserved output increases in direct proportion dance source is required, the arrange- to prevent momentary application of to the amp1ifier gain and the qrnber merit of Fig. 1.C is suitable. The excessive voltage to the microphone. of turns On the input transformer 2C)Odhm resistor is replaced by a 200. Fig. 2-A S ~ O W S the basic circuit for ondary. The difficulties become very ohm potentiometer connected as shown. obtaining the button current from the ~ronounced, for instance, in ort table

~ ~ t h of these circuits will be found bleeder system of the amplifier. Usually sound-on-film amplifiers where the over- all gain is frequently more than 100 db.

especially suitable for adding low-im- about 10 to 20 ( 5 to 10 Per pedance input to existing highgain A.C. will be required, which aP*

Fig. 1-A shows the usual arrange- operated amplifiers, especially in port- proaches the total current flowing in the bleeder, and hence the microphone

ment of the input transformer in the grid able and theatre sound picture equip- circuit must be tapped across a resist-

circuit of the first stage; the hum level merit. ance which is high compared with the

is, oi course, relatively high. Nowt in effective resistance of the microphone high gain sound-on-film amplifiers, a rather low voltage is sufficient for full

A.C. SUPPLY load so that practically all of the bleeder CARBON MICROPHONES current will flow through the micro-

amplifier output. Fig. 1-B is a circuit It is especially desirable to eliminate phone. Thus the portion of the bleeder

which takes advantage of this fact. The batteries in portable sound equipment system shunted by the microphone cir-

transformer may be a 200-to-200 ohm to reduce the weight of the equipment. cuit is practically short-circuited when

unit, a mixing transformer for instance, Sound systems which are operated by the microphone is connected, and this with the secondary connected into the

non-technical personnel are logically will upset the voltages supplied to the cathode circuit and terminated with a

made fully-A.C. operated to minimize various tubes unless the bleeder is re- 200-ohm resistance. Since there are rela-

service fa i lures due to run-down designed. A disadvantage of the element- tively few turns on such transformers

batteries. ary circuit of Fig. 2-A is the momentary (compared with the usual input trans- former secondary) the induced hum voltage will be correspondingly low. Impedances are, of course, perfectly matched. Since the D.C. resistance of the transformer secondary will be about 20 ohms or less, the bias voltage will be practically unaffected. One addi- F16. I. LOW-impedencc Cethode Input Circuits.

2 THE SHURE TECHNICAL BULLETIN

high voltage which is applied to the SURGE FILTER FOR TWO-BUTTON

microphone if it is plugged into the MICROPHONES

bleeder system with the amplifier turned Carbon microphones tend to "pack"

on; this is likely to cause cohering and if the circuit is opened while normal

burning of the diaphragm.

Fig. 2-B shows a potentiometer for

current control inserted in series with

the existing bleeder system, a method

which overcomes many of the above

difficulties. The arm of the potenti-

ometer should always be turned back

to minimum current before the power

supply is shut off, which can be

automatically accomplished using the

familiar potentiometer switch combina-

tion employed in radio receivers. The

potentiometer resistance may be approx-

imately 1000 ohms with a current rating

somewhat greater than the normal cur-

rent in the lower part of the bleeder

system. The switch is, of course, wired - in series with the power transformer

primary. The microphone should be

connected to the amplifier with the po-

tentiometer in the "OFF" position, grad-

ually advancing the control until the

proper current is obtained. Remember

that the maximum current for the micro-

phone circuit will be slightly less than

lhe available bleeder current.

button current is flowing. This is caused

by the high transient voltage induced in

the microphone transformer primary

winding when the circuit is o?ened. As

a preventative measure, the current

should always be reduced to a minimum

by turning down the rheostat or poten-

tiometer before disconnecting the.micro-

phone or switching off the battery.

A simple filter section, which is essen-

tially a low pass network, will act as a

shock-absorber for transients, and the

arrangement of Fig. 2-A has been found

to afford rather complete protection

against "surge packing." The chokes

have an inductance of approximately

1.5 millihenries each, and may consist

of universal air-core windings, or ap-

proximately 100 turns of about No. 28

magnet wire, random wound on a stack

of transformer "I" laminations. "I"

laminations taken from a small loud-

speaker output transformer are quite

suitable. The condensers have a capacity

of .02 Mfd. each. Low voltage paper

condensers are satisfactory.

The surge filter accomplishes another

The resistanke of the potentiometer useful result when used with radio-

will be practically short-circuited when telephone transmitters where the antenna

the microphone is in use. If the ampli- and speech-input equipment are close

fier is intended for use without the together, for if the system is thoroughly

microphone, it may be advisable to cut shielded throughout, the surge filter

the potentiometer out of circuit to pre- will effectively prevent R.F. feedback

vent voltage changes. A single-pole through the speech-input equipment.

switch, as shown dotted in Fig. 2-C, will Completely shielded cables should be

accomplish this result very simply. used for microphone connections.

F16.2. Button Current from the Amplifier Poser Supply.

Through oversight, n o cred i t l ine was given o n t he il lustration o f t he announcer speaking in to t he Shure M o d e l 40C Condenser Micro- phone, appearing as Fig. 2 on page 2 o f t h e June-July Bulletin. This artwork was kindly loaned b y POPULAR M E C H A N I C S MAGAZINE and was used originally i n t he M a y issue of t h a t magazine in en ,article entitlfd, "Condemer Microphone Operates o n A.C. b y t he Editor of t he BULLETIN.

SHURE BROTHERS COMPANY 215 W. Huron St., Chicago, U. S. A.

w

I C b

"A utometic

Friction-Lock"

FLOOR STANDS

You simply raise lower or turn the microphone if you have onc' of these ultra-modern "Auto- matic Friction Lock" Hoor Stands . . . no thumb screws no wing nuts no clamps . . . no rattlin or sudden dtopping!' Nothing to tighten or loc& . . . the microphone stays r~ght where you leave it.

The "Automatic Friction Lock" holds the plunger In position through fr~ct ion exerted by spring. supported leather washers. The action is silent smooth and pori t ive-a remarkable refinemen{ providing the ultimate in convenience plus safety even in the hands of careless performers.. The mcchavism is simple dependable and rel~able . . guaranteed to l;st indefinitely.

The Chromium Collar at the top of the barrel identifies Shure "Automat~c Friction Lock" Floor Stands. The proper degree of friction for any type of microphone is initially adjusted by simply otating the collar slightly one wa or the other.

{he construction in no way inteXerer with the convenience, of runnlng the cable through the tubings. Finlrh is rich rubber-blbck w~ th chromium plated collar and ring. Suppl~ed in three base styles with or without ring. List prices, less ring from $16.50 to $21.50, depending on thq style 01. bare. Adapters are ava~lable for attaching pther makes of microphanes. Complete dctarls WIII be found in our Stand and Accessory Catalog Sheet which will be sent on request.