4
EARPHONE VOLUME CONTROL
A simple volume control circuit can be built to enable the
SM12A earphone receiver to be connected to the speaker
output of a power amplifier with an output of up to 200 watts
into 4 ohms. (The volume control need not be connected di-
rectly to the power amplifier. It can also be connected to an
unused phone jack on a working stage monitor.)
WARNING
Although the SM12A’s receiver circuit is protected
against excessive level, it is capable of an output of over
121 dB SPL sufficient to cause hearing damage with
prolonged exposure. Always take care to turn the vol-
ume control on this circuit all the way down before insert-
ing the earpiece into your ear. Then turn the control up
gradually until a confortable listening level is reached.
VOLUME CONTROL
FIGURE 6
PARTS LIST
2
Two-circuit phone jacks
. . . . . . . . . . . . . . . . . . . . . . . . .
1
Resistor, 2.2k, 1W
. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
Potentiometer, 25k, 1W (or larger)
. . . . . . . . . . . . . . .
1
Plastic or metal box
. . . . . . . . . . . . . . . . . . . . . . . . . . . .
Connecting leads
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ARCHITECT’S SPECIFICATIONS
The microphone with receiver shall be the Shure Model
SM12A or equivalent. The microphone shall be a moving–
coil (dynamic) type with a frequency response of 50 to
15,000 Hz. The unit shall have a cardioid polar characteris-
tic. The cancellation at the rear shall be 15 to 20 dB. The mi-
crophone shall be low impedance with a rated impedance of
150 ohms for connection to microphone inputs rated at 19
to 300 ohms.
The microphone output shall be – 66.0 dB where 0 dB =
1 milliwatt per 10 microbars.
The receiver shall be a dynamic type with a frequency re-
sponse of 70 to 12,000 Hz. The receiver shall be sup–plied
with an integral connector–mounted matching transformer
designed with an impedance of 2,000 ohms.
The microphone with receiver shall be a head-worn type,
and shall be provided with a 1.5m (5 ft), non-detachable,
four-conductor (two-conductor shielded) cable with a pro-
fessional three-pin audio connector designed to mate with
Cannon XL series, 3-Pin Mini Connector (TA3M) or equiva-
lent connectors. An additional 762 mm (2.5 ft) cable length
shall be provided between microphone and receiver (phone
plug) connectors. The microphone shall also be provided
with a headband, two connector belt clips, and a foam wind-
screen.
The overall dimensions of the microphone shall be 203
mm (8 in.) in length and 44.5 mm (1–3/4 in.) in maximum
height (pivot housing). The microphone housing shall be
15.9 mm (5/8 in.) in diameter and 14 mm (9/16 in.) in height.
SHURE Incorporated Web Address: http://www.shure.com
222 Hartrey Avenue, Evanston, IL 60202–3696, U.S.A.
Phone: 847-866–2200 Fax: 847-866-2279
In Europe, Phone: 49-7131-72140 Fax: 49-7131-721414
In Asia, Phone: 852-2893-4290 Fax: 852-2893-4055
Elsewhere, Phone: 847-866–2200 Fax: 847-866-2585
