Specifications

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(4- 3/8 IN.)
FIGURE E.
OVERALL DIMENSIONS
CIRCUIT DESCRIPTION
In conventional, full-range, single-amplifier opera-
tion, input signals from the power amplifier are con-
nected to the
SR108 through the uppermost set of
parallel-wired jacks to a series-type, passive LC
crossover network composed of two capacitors and
two inductors (see Figure F). The crossover provides
a 6
dB1octave rolloff for the low-frequency speakers
and an 18
dB/octave rolloff for the high-frequency
drivers. The inductance of the low-frequency speaker
voice coils provides an additional 6
dB1octave rolloff,
resulting in a total of 12 dB1octave low-frequency
rolloff.
Low-frequency signals are routed from the cross-
over directly to the six low-frequency speakers through
the rear-panel LOUDSPEAKER OPERATION Switch.
The low-frequency speakers are wired in two parallel
sets of three series-connected speakers, providing
a total nominal impedance of 16 ohms. High-frequency
signals are routed from the crossover through the
LOUDSPEAKER OPERATION Switch attenuator net-
work which provides high-frequency level adjustments
of -4, -2, 0, and
+2 dB in the first four positions
of the switch. From the attenuator network, high-
frequency signals enter a protection circuit designed
to prevent damage to the high-frequency drivers by
extremely high signal levels. Signal levels greater than
28 volts rms (approximately 50 watts) for longer than
15 milliseconds are sensed through a diode bridge,
causing a relay to engage high-wattage resistors
which decrease the high-level signals to the high-
frequency drivers by approximately
7 dB. The cir-
cuitry automatically resets itself when the
high-fre-
quency signals drops below a safe threshold level.
The output of the protection circuitry feeds the four
CONENTICUM
FULL-RANGE
PASSIVE
HIGH-FI1EWENCY
lpUr n mgmER
ATTENUATION
I FULL RANGE O\
1 1
FULL-RANGE 1
BUW
LOI-FREOUENCI
w
WUT 0-
8mCn SHOWN N FULL RAW€ WKN
FIGURE F.
SR108 BLOCK DIAGRAM
high-frequency drivers which are wired in series-
parallel and provide a total nominal impedance of
16 ohms.
Operation in the biamplification mode is accom-
plished by feeding separate high- and low-frequency
signals from an external 2600 Hz electronic crossover,
such as the Shure Model
SR106 Electronic Crossover,
through separate high- and low-frequency power am-
plifiers, such as the Shure Model
SR105 Power Am-
plifier, to the
HlGH and LOW FREQUENCY input jacks
of the
SR108 (see Figure G). With the rear-panel
LOUDSPEAKER OPERATION Switch in the
BlAMP
position, the internal passive crossover of the SR108
is bypassed, and the low-frequency signals are routed
directly to the low-frequency speakers. High-frequency
signals also bypass the attenuator network, going di-
rectly from the LOUDSPEAKER OPERATION Switch
through the high-frequency protection circuit and a
12
dB1octave filter to the high-frequency drivers. The
filter serves to protect the drivers from low-frequency
transient signals produced by connecting input cables
to the power amplifier, turn-on thumps, or operating
errors.
SRlOB
SPEAKER SYSTEM
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HIGH POWER HIGH-FREQUENCY I '
AMPLIFIER I DRIVERS !
AUDIO CWSOLE ELECTRONIC
CROSSOVER
(2600HZ1
FIGURE G.
BIAMPLlFlED OPERATION
INSTALLATION
General
In planning a sound system using the SR108 Ex-
tended Range Speaker System, care must be taken
to observe the horizontal (140") and vertical (90")
sound distribution. Maximum coverage for sound re-
inforcement installations is generally obtained with
SR108's on each side of the sound source and as
far forward as possible. Assuming a single
SR108
to either side of the sound source, the SR108's should
be positioned so that an imaginary line from the
center of each
SR108 runs to the back row of the
audience area. For "clustered"
SR108 installations
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