Program Mix Amplifier; Feedback Filters; Program Output; Monitor Mixer Applications - Shure SR101 Series 2 Operation And Service Manual

POWER PA
AMPLIFIER SPEAKERS
LEVEL
-
S R l l O STAGE
MONITOR
POWER MONITOR
M I X E R
AMPLIFIER
SPEAKER
I
S R l l O
MONITOR
MIXER
STEREO
RECORDER
MONITOR
FIGURE 5. SR110 MONITOR MIXER APPLICATIONS
PROGRAM MIX AMPLIFIER
The program mix amplifier is an active mixing amplifier
in which gain remains constant independent of the num-
ber of individual channel PROGRAM Switches (13) that are
activated. The output of the program mix amplifier is con-
nected through a 560-ohm mixing resistor to the LlNK
Jacks (28,31).
LlNK JACKS
The LlNK Jacks (28,31) on the rear panel of the Console
enable the user to interconnect more Consoles for addi-
tional inputs or add external equipment, such as equalizers,
compressors, or limiters. When connecting two or more
SR101 Audio Consoles together to provide many channel
inputs, connect the LlNK OUT Jacks (31) of all the units
together. It should be noted that the LlNK OUT Jack is
actually a two-way jack; the impedance at this point is
actually 600 ohms and any number of units may be tied
together at this point. The LlNK IN Jack (28) is an input-only
jack and has switching contacts that disconnect the output
of the program mix amplifier from the MASTER Volume
Control (20).
If an equalizer, limiter or compressor is connected to the
Console, the LlNK OUT Jack (31) is connected to the input
of the external unit and the output of the external unit is
connected to the LlNK IN Jack (28).
The signals at the LlNK JACKS are typically at a level 10
dB below line level. These jacks will accommodate signal
levels in the range between -30 to + I 0 dBm. The LlNK IN
input impedance is greater than 20 kilohms and may be
considered a bridging impedance. The output of the LlNK
Jacks feeds the MASTER Volume Control (20) which is a
two-section control similar to those used in the individual
channels. The POST LINK/PRE LlNK Switch (29) on the
rear panel, in the monitor circuit, allows the operator to
monitor the program before (PRE) or after (POST) the LlNK
Jacks.
FEEDBACK FILTERS
The output signal from the MASTER Volume Control (20)
attenuator section is sent to the feedback filter circuit at
this point. The FEEDBACK FILTERS (15) are four notch
filters whose center frequencies are 130 Hz, 800 Hz, 2 kHz,
and 5 kHz. T h e s e jil'lers a r e designed to minimize the
acoustic feedback (speaker howl or squeal) that may occur
through some combination of room acoustics, microphone
and speaker placement, volume increase, or equalization
control boost. Each filter modifies the frequency response
of the program output, with the lowest (130 Hz) filter af-
fecting the lowest feedback pitch (howl) and the highest (5
kHz) filter affecting the highest feedback pitch (squeal).
The two center frequency filters affect feedback modes in
the middle ranges of the audio frequency spectrum.
The three upper frequency filters have little effect on
voice tonal qualities. The 130 Hz filter, generally used to
eliminate low-frequency room reverberations, causes some
decrease in bass tones. This may be compensated for by
increasing the FREQ EQ-LO Controls (5) slightly for the
individual channels in use.
If feedback is present, locate the one FEEDBACK FILTER
Switch (15) which eliminates it. Then increase the VOLUME
(1) or FREQ EQ Controls (5,7) as desired until another feed-
back pitch occurs. Then locate the filter to eliminate the
new feedback mode. IMPORTANT: Up to two FEEDBACK
FILTERS may be used at one time; more than two filters
will reduce overall gain and significantly affect system tonal
quality.
Note that the PHASE Switch (26), which allows the Con-
sole operator to change the output phase, functions to
reduce or eliminate acoustic feedback, too. It is normally
used to obtain maximum gain before feedback, or to obtain
the highest pitched feedback for elimination by the feed-
back filters.
PROGRAM OUTPUT
The output signal from the feedback filters drives the
program output amplifier. The gain of this amplifier is con-
trolled by one-half of the MASTER Volume Control (20) (the
other half is a preamplifier output attenuator). The output of
the program amplifier is sent to the PHASE Switch (26)
mounted on the rear panel of the Console. This switch
allows the user to change the phase of the program output,
which may be helpful in eliminating or reducing low-fre-
quency acoustic feedback. Generally, this switch is ad-
justed to the position which either gives the most gain
before feedback or, if both positions give the same gain
before feedback, use the position that produces the highest
pitch feedback. The program signal, after leaving the
PHASE Switch, is routed to the output transformer which
provides both LlNE LEVEL and MIC LEVEL outputs. The
LlNE LEVEL output is connected to one professional three-
pin, male, audio output connector (27) and two three-circuit
phone jacks (25). The MIC LEVEL output is a low-im-
pedance output, 50 dB below the LlNE LEVEL output, and is
connected to a professional three-pin male, audio output
connector (24).
Note that all of the program outputs are balanced with
respect to ground. If a phone plug is used to connect the
mixer line level output to an amplifier or tape recorder and
the phone plug is a two-circuit type, the line outputs will
automatically become unbalanced. If the 3-pin unbalanced
output is used and a phone jack output is
also
to be used,
obtain a stereo phone plug and connect the tip and ring of
the plug only. (The sleeve is a shield and would cause a
ground loop.)