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The next stage is an input amplifier which has a gain
of 14 to 34 dB depending on the setting of the rear panel
GAlN Control (12). The output of the amplifier is fed to
two cascaded differential amplifier stages which com-
prise the equalizer boost and cut circuitry. The individual
active gyrator resonators for 31 Hz, 125 Hz, 500 Hz, 2 kHz
and 8 kHz are connected to the first of the differential
amplifiers and provide control of these frequency bands.
The second amplifier is connected to the 63 Hz, 250 Hz,
1 kHz, 4 kHz, and 16 kHz active resonators for control of
the remaining five frequency bands. Each of the ten
octave-spaced filters is equipped with a front panel Filter
Control (1) which may be adjusted for up to 15 dB boost
or cut relative to its zero or flat position (see Figure 5).
This configuration of minimum-phase networks assures
correct filter characteristic combinations, minimum am-
plitude ripple, and minimum phase shift.
The output of the second Equalizer differential ampli-
fier is fed to the EQUALIZER IN/BYPASS Switch (2)
which selects the input to the line output stage from
either the equalizer amplifier in the IN position, or from
the output of the input amplifier stage in the BYPASS
position.
The Equalizer LEVEL Control (3) provides adjustment
of the Equalizer output level over the range of 2 1 5 dB
relative to its zero setting. Reduction in output is accom-
plished by attenuating the signal input to the input ampli-
fier. An increase in output is produced by changing the
gain in the output amplifier. The LEVEL Control operates
only when the EQUALIZER INIBYPASS switch is set to
the IN position.
I
- 1 0
z
U
- 5
g
0
P
$
- 5
-10
u
2 0
100
K O 0
IOPOO 20,000
FREWENCY IN HERTZ
FIGURE 5A- E m FILTER AT
7.5
dB
BOOST OR CUT
t 2 0
+ I 5
t 1 0
1 5
U
H
O
p
-5
u
:
-I0
g
- I 5
- 2 0
2
4
6
8
1
2
4
6
8
1
2
4
6
8
1
2
2 0
100
1000
IO.WO
20.000
FREWENCV IN HERTZ
FIGURE50 -EACH FILTER AT MAXIMUM BOOST OR CUT
FIGURE 5.
TYPICAL FILTER CHARACTERISTICS
The output amplifier has a gain of 5 to 20 dB depend-
ing on the setting of the LEVEL Control, and is capable
of delivering
+
18 dBm through the output transformer to
a 600-ohm load at either of the balanced LlNE OUTPUT
Connectors (9, 11). The three-pin male OUTPUT Con-
nector may be switched from balanced line level to low-
impedance microphone level output (50 dB below line
level) by means of the MIC/LINE Slide Switch (10) which
selects one of two secondary windings on the output
transformer. The AUX OUTPUT Phone Jack (8) is sup-
plied with an output signal that is 20 dB below that of the
LlNE OUTPUT. This is obtained from an attenuator net-
work at the output of the output amplifier. The AUX OUT-
PUT can be used to feed the link input of an audio
console or mixer so equipped.
An LED driver stage connected to the output amplifier
illuminates the front panel OVERLOAD LED (4) when the
audio signal approaches the output clipping level. The
OVERLOAD Indicator operates in both the EQUALIZER
IN and BYPASS modes.
Input and Output Connections
Two LlNE INPUT Connectors are located on the rear
panel of the SR107. A professional, three-pin female
audio connector (13) provides a balanced connection
from the audio console or mixer-preamplifier. A standard
1/4
in. three-conductor (stereo) phone jack (14) provides
either a balanced or unbalanced input connection de-
pending upon the phone plug (three-conductor or two-
conductor) used.
The gain of the SR107 can be adjusted by means of
the back panel GAlN Control from UNITY to +20 dB in
either the EQUALIZER IN or BYPASS modes. The 20 dB
gain capability may be used in hi-fi or Link applications
to maintain desired signal-to-noise performance through
the SR107. It also allows the Equalizer to accommodate a
variety of normal input and output levels while maintain-
ing an adequate operating signal-to-noise ratio.
Three output connectors are provided on the rear
panel. A professional three-pin male audio OUTPUT con-
nector provides a balanced low-impedance output at
either microphone or line level depending upon the
setting of the associated MIC/LINE Slide Switch. The
LlNE OUTPUT Connector is a standard
1/4
in. three-
conductor (stereo) phone jack which provides a low-
impedance line level output that can be either balanced
or unbalanced. The AUX OUTPUT Connector is
a
stand-
ard
1/4
in. two-conductor phone jack providing a low-
impedance unbalanced output suitable for connection to
a hi-fi power amplifier or the link input of a mixer or
audio console. The LINE, microphone level and AUX out-
puts are isolated and may be used simultaneously.
APPLICATIONS
The primary applications for the SR107 Audio Equal-
izer are as follows:
1. Feedback Control -Adjustment
of sound system
frequency response to provide maximum gain be-
fore feedback and improved sound quality in a
variety of live performance situations. The SR107
is equally adaptable to feedback control functions
in either a house sound system or stage monitor
(foldback) system.
2. Playback Equalization -Adjustment
of audio play-
back system frequency response to compensate for
any variations in electrical or acoustical response
that may alter the natural sound of the recorded
material.
3. Live Performance Equalization
-
Adjustment of
sound system frequency response to provide a
tonal balance appropriate to the performance and
to reduce the tendency for feedback.
The following sections present procedures and recom-
mendations for the use of the SR107 in each of these
applications. Also included is a section discussing acous-
tic measurement instrumentation and its use with the
SR107.
Feedback Control
In many live performance sound reinforcement applica-
tions, feedback is often the limiting factor in achieving
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