MACKIE M1400/M1400i - Manual (Page 25)

25
70V CONSTANT VOLTAGE DISTRIBUTION SYSTEM
CH 1
RC NETWORK
CH 2
RC NETWORK VALUES
C1 = C2 = 680
µF @ 250VDC
R1 = 4
Ω @ 100W
Note: You can substitute
a single capacitor for C1/C2.
C3 = 330
µF @ 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 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-
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.
Because of the high power capability of the
M•1400/M•1400i amplifier, it can be used to
directly drive 70V constant-voltage distribution
systems without the use of a step-up trans-
former. When the M•1400/M•1400i amplifier is
operated in
BRIDGE mode, it can produce
850 watts into 8 ohms, or 82.5V. 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•1400i: K = 850W/625W = 1.36. Thus, a
2.5W tap becomes 3.4W, a 5W tap becomes
6.8W and a 10 W tap becomes 13.6W.
CAUTION: A characteristic of many tapped
transformers is that they saturate at very low fre-
quencies, which causes their impedance to
decrease, approaching the DC resistance of the
copper wire. This can result in overloading the
amplifier if the signal contains lots of low frequen-
cies. When using an M•1400/M•1400i amplifier in
a 70V distribution system, set the
LOW CUT
FILTER to 100Hz or higher. In addition, in-
stall an RC network at the SPEAKER OUTPUT
of the amplifier, as shown in the figure below.