MACKIE M1200/M1400 - Manual (Page 25)

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The thickness of wire is rated in gauges.
Use the chart below to determine the correct
gauge of wire to use according to the distance
between the speakers and the amplifier, and
the impedance of the load the amplifier is driv-
ing. This ensures that the power lost across the
speaker wire is less than 0.5 dB.
tions when setting up a multi-speaker system.
You just add up the total wattages of all the
speakers in the system and make sure that it
doesn’t exceed the total power rating of the
amplifier (allowing at least 10% for insertion
losses). Another benefit is that by using high
voltage and low current, losses in the speaker
distribution wiring are kept to a minimum.
Standard voltage levels include 25V, 70V, and
100V, but 70V systems are most commonly used
in commercial sound distribution systems in
the U.S.
Because of the high power capability of the
M•1200/M•1400 amplifiers, they can be used
to directly drive 70V constant-voltage distribu-
tion systems without the use of a step-up
transformer. When the M•1200 amplifier is
operated in
BRIDGE mode, it can produce
800 watts into 8 ohms, or 80V. This is slightly
higher than the standard 70.7V for which the
system was designed. You can recalculate the
actual power delivered to each tap by multiply-
ing the tap’s rated wattage by a correction
factor (K). The correction factor is P
1
/P
2
,
where P 1 is the power delivered by the amplifier
into 8 ohms ( BRIDGE mode), and P
2 is the
power delivered by 70.7V into 8 ohms (625W).
M•1200: K = 800W/625W = 1.28. Thus, a
2.5W tap becomes 3.2W, a 5W tap becomes
6.4W and a 10W tap becomes 12.8W.
M•1400: K = 850W/625W = 1.32. Thus, a
2.5W tap becomes 3.3W, a 5W tap becomes
6.6W and a 10 W tap becomes 13.2W.
CAUTION: A characteristic of tapped transform-
ers is that they saturate at very low frequencies,
which causes their impedance to decrease, ap-
proaching the DC resistance of the copper wire.
This can result in overloading the amplifier if
the signal contains lots of low frequencies.
When using an FR Series amplifier in a 70V dis-
tribution system, set the
LOW CUT FILTER
to 100Hz or higher. In addition, install an RC
network at the
SPEAKER OUTPUT of the
amplifier, as shown in the figure below.
CH 1
RC NETWORK
CH 2
RC NETWORK VALUES
C1 = C2 = 660
µ F @ 250VDC
R1 = 4
Ω
@ 100W
Note: You can substitute
a single capacitor for C1/C2.
C3 = 330
µ F @ 250VDC,
NON-POLARIZED.
C1 C2
+
+ +
–
+
–
+
–
FR SERIES
POWER AMPLIFIER
IN BRIDGE MODE
+
–
+
–
70V LINE
CONSTANT
VOLTAGE
TRANSFORMER
POWER TAP
SWITCH
2.5W
5W 10W 2.5W
5W 10W 2.5W
5W 10W
R1
ALTERNATE RC NETWORK
C3
R1
70V Constant Voltage
Distribution System
Load Gauge of
Wire Length Impedance Wire
Up to 25 ft. 2
Ω
14 gauge
4
Ω
16 gauge
8
Ω
18 gauge
Up to 40 ft. 2
Ω
12 gauge
4
Ω
14 gauge
8
Ω
18 gauge
Up to 60 ft. 2
Ω
10 gauge
4
Ω
12 gauge
8
Ω
16 gauge
Up to 100 ft. 2
Ω
8 gauge
4
Ω
10 gauge
8
Ω
14 gauge
Up to 150 ft. 2
Ω
6 gauge
4
Ω
8 gauge
8
Ω
12 gauge
Up to 250 ft. 2
Ω
4 gauge
4
Ω
6 gauge
8
Ω
10 gauge
70V DISTRIBUTION SYSTEMS
A distributed sound system uses a constant-
voltage, high-impedance network that feeds a
number of tapped transformers which, in turn,
deliver power to individual speakers. Each tap
is rated in watts, so you can select the amount
of power delivered to the speaker. Developed
for distributed paging and public address sys-
tems, one benefit of such a system is that it
eliminates complicated impedance calcula-