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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 systems,
one benefit of such a system is that it eliminates
complicated impedance calculations when set-
ting 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 (actually
70.7V) are most commonly used in commercial
sound distribution systems in the U.S.
Because of the high power capability of the
M•2600 amplifier, it can be used to directly
drive 70V constant-voltage distribution systems
without the use of a step-up transformer.
We recommend that you
do not use the M•2600 in
BRIDGED
mode with a 70V
system, as it can produce
up to 1700 watts into 8
ohms, or 116V. This may
saturate the small 70V transformers. Use the
STEREO
or
MONO
mode instead, and you can
run one or two separate 70V lines.
In
STEREO
, into a nominal 70V system, the
M•2600 output can produce a maximum of 500
watts into 8 ohms (equal to 63V). Since this is
less than 70.7V, you can recalculate the actual
power delivered to each tap by multiplying 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,
and P
2
is the power delivered by 70.7V into 8
ohms (625W).
M•2600: K = 500W/625W = 0.8. Thus, a
2.5W tap becomes 2W, a 5W tap becomes 4W
and a 10 W tap becomes 8W.
CAUTION: A characteristic of many tapped
transformers is that they saturate at very low
frequencies, 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•2600 amplifier in a 70V
distribution system, set the
LOW CUT FILTER
to 100Hz or higher. In addition, install an RC
network at the
SPEAKER OUTPUT
of the am-
plifier, as shown in the figure below.
CH 1
RC NETWORK
RC NETWORK VALUES
C1 = C2 = 680µF @ 250V
R1 = 4Ω @ 100W
Note: You can substitute
a single capacitor for C1/C2.
C3 = 330µF @ 250VDC,
NON-POLARIZED.
C1 C2
+
++
–
+
–
M•2600
POWER AMPLIFIER
IN STEREO MODE
+
–
+
–
70V LINE
CONSTANT
VOLTAGE
TRANSFORME
R
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