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CHAPTER 3: AMPLIFICATION AND AUDIO
Calculating Load Impedance for Looped Audio
Signals
To avoid distortion when looping multiple loudspeakers,
make sure the source device can drive the total load imped-
ance of the looped loudspeakers. In addition, the source
device must be capable of delivering approximately 20 dBV
(10 V rms into 600 ohms) to yield the maximum SPL over the
operating bandwidth of the loudspeakers.
To calculate the load impedance for the looped loudspeak-
ers, divide 10 kOhms (the input impedance for a single loud-
speaker) by the number of looped loudspeakers. For
example, the load impedance for 10 750-LFCs is 1000 ohms
(10 kOhms / 10). To drive this number of looped loudspeak-
ers, the source device should have an output impedance of
100 ohms or less. This same rule applies when looping
750-LFCs with other Meyer Sound self-powered loudspeak-
ers.
NOTE:
Most source devices are capable of
driving loads no smaller than 10 times their
output impedance.
TIP:
Audio outputs from Meyer Sound's GALAXY,
Galileo, and Callisto loudspeaker processors are
rated at 50 ohms. This allows each output to drive up
to 20 Meyer Sound (10 kohm) loudspeakers without
distortion.
CAUTION:
Make sure that all cabling for
looped loudspeakers is 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 loud-
speakers in a system have reversed polarity, fre-
quency response and coverage will be significantly
degraded.
16
CABLE RINGS
Two cable rings 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.
Figure 13: Cables Tied Off to Cable Ring
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 employs Meyer Sound's advanced TruPower
limiting. Conventional limiters assume a constant loud-
speaker impedance and set the limiting threshold by mea-
suring 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.
In contrast, TruPower limiting calculates varying loud-
speaker impedances by measuring both current and voltage
to compute the actual power dissipation in the voice coil.
This improves performance, both before and during limiting,
by allowing the driver to produce the maximum SPL across
its entire frequency range, while also retaining signal peaks.
TruPower limiting also eliminates power compression at high
levels over lengthy periods, which helps regulate voice coil
temperatures, thereby extending the life of the driver.
®
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