Meyer Sound 750-LFC, Operating Instructions Manual

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750-LFC OPERATING INSTRUCTIONS
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Pins 2 and 3 carry the input as a differential signal. Pin 1 is
connected to earth through a 1 kΩ, 1000 pF, 15 V clamped
network. This circuitry provides virtual ground lift for audio
frequencies while allowing unwanted signals to bleed to
ground. Make sure to use balanced XLR audio cables with
pins 1–3 connected on both ends. Telescopic grounding is
not recommended and shorting an input connector pin to
the case may cause a ground loop, resulting in hum.
TIP: If unwanted noise or hiss is produced by the
loudspeaker, disconnect its input cable. If the
noise stops, there is most likely nothing wrong
with the loudspeaker. To locate the source of the noise,
check the audio cable, source audio, and AC power.
Audio Loop Output (XLR 3-Pin or 5-Pin Male)
The XLR 3-pin or 5-pin male Loop output connector allows
multiple loudspeakers to be looped from a single audio
source. The Loop output connector uses the same wiring
scheme as the Input connector (see “Audio Input (XLR 3-Pin
or 5-Pin Female)” above). For applications that require
multiple 750
-
LFC loudspeakers, connect the Loop output of
the first loudspeaker to the Input of the second loudspeaker
and so forth.
NOTE: The Loop output connector is wired in
parallel to the Input connector and transmits
the unbuffered source signal even when the
loudspeaker is powered off.
Calculating Load Impedance for Looped Audio
Signals
To avoid distortion when looping multiple loudspeakers,
make sure the source device can drive the total load
impedance of the looped loudspeakers. In addition, the
source device must be capable of delivering approximately
20 dBV (10 V rms into 600 Ω) to yield the maximum SPL
over the operating bandwidth of the loudspeakers.
To calculate the load impedance for the looped
loudspeakers, divide 10 kΩ (the input impedance for a
single loudspeaker) by the number of looped loudspeakers.
For example, the load impedance for ten 750
-
LFC
loudspeakers is 1000 Ω (10 kΩ /10). To drive this number of
looped loudspeakers, the source device should have an
output impedance of 100 Ω or less. This same rule applies
when looping 750
-
LFC units with other Meyer Sound
self
-
powered loudspeakers.
NOTE: Most source devices are capable of
driving loads no smaller than 10 times their
output impedance.
TIP: Audio outputs from Meyer Sound’s Galileo
GALAXY Network Platform are rated at 50 Ω.
This rating allows each output to drive up to 20
Meyer Sound (10 kΩ) loudspeakers without distortion.
CAUTION: Make sure that all cables for looped
loudspeakers are wired correctly (Pin 1 to
Pin 1, Pin 2 to Pin 2, and so forth) to prevent
the polarity from being reversed. If one or more
loudspeakers in a system have reversed polarity,
frequency response and coverage will be significantly
degraded.
CABLE RINGS
Two cable rings (Figure 14) are provided on the rear of the
750
-
LFC cabinet. Power and audio cables should be tied off
to the rings to reduce strain on the cables and prevent
damage to them during installation.
CAUTION: 750
-
LFC cable rings should only be
used to reduce strain on cables. The cable
rings should not be used for any other purpose.
TRUPOWER LIMITING
The 750
-
LFC loudspeaker employs Meyer Sound’s
advanced TruPower ® limiting. Conventional limiters assume
a constant loudspeaker impedance and set the limiting
threshold by measuring voltage alone. This method is
inaccurate, because loudspeaker impedances change as
frequency content in the source material changes, and as
thermal values for the loudspeaker’s voice coil and magnet
vary. Consequently, conventional limiters often begin
limiting prematurely, which reduces system headroom and
dynamic range.
Figure 14: Cables Tied Off to Cable Ring
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