Electro-Voice 7300A - Calculating the Maximum Length of Cable for a Specified Damping Factor

Operating and Service Instructions for the Electro-Voice 7300A Power Amplifier
impedance of 0.04 ohms.
3.5.1 Calculating the Maxi¬
mum Length of Cable
for a Specified Damp¬
ing Factor Specifica¬
tion at the Load
The damping factor rating
is typically never realized at the
load because of the resistance of
the cable (and other factors such
as the contact resistance of an
output relay or the resistance of
an output fuse). The damping fac¬
tor at the load should be 30 for
general paging systems and 50 for
high fidelity music systems. Econ¬
omics usually dictate, however,
that these numbers are cut-in¬
half. The resulting damping factor
at the load should be based on
experience and customer satis¬
faction. Once a minimum damping
factor is determined for a parti¬
cular type of installation, the
following equation can calculate
the maximum length of 2-wire
cable which can be used to ach¬
ieve the minimum damping factor
specified at the load:
Max. Length of 2-wire cable in
feet
ZL — Zo
= _M _
DCR/ft
where
ZL is the load impedance
to connect to the
amplifier;
Zo is the amplifier’s
output impedance (0.04
ohms for the 7300A);
DF is the minimum
permissible damping
factor at the load; and
DCR/ft is the DC resis¬
tance of the 2-wire cable
per foot from Table I.
The same equation can be used to
calculate the maximum cable
length in meters by substituting
Table I 7300A Power Losses in 2-wire Speaker Cable
Power Cable Cross-
Power
AWG
DCR/ft
Loss/ft Sectional DCR/meter
LossAneter
(GA)
(a/ft)
(watts/ft)
area (mm^) (a/m)
(watts/m)
6 0.00081
0.0201
13.30 0.00264
0.0661
8
0.00121 0.0302
8.36
0.00421
0.1051
10 0.00204
0.0509 5.26 0.00669
0.1669
12 0.00324 0.0809 3.31 0.01063
0.2650
14
0.00515
0.1286 2.08 0.01691
0.4210
16 0.00819
0.2043
1.31 0.02685
0.6667
18 0.01302
0.3244
0.82 0.04289
1.0609
20
0.02070 0.5148
0.52 0.06764
1.6627
22
0.03292 0.8163 0.33 0.10658
2.5950
the DCR per meter value
from
/
1 Zo \
Table 1.
(Pout)(DCR/ft) \
DF ZL 1
Let’s use the equation. Suppose
ZL equals 8 ohms, Zo = 0.04
ohms, and the minimum damping
factor at the load is 25. In
addition, 18 GA cable is preferred.
Then, the maximum length of 18
GA cable which can be used to
achieve a damp-ing factor of 25 at
the load is:
8 — (0.04)
_25_ = 21.5 feet
0.01302 O/ft
Sometimes it may be necessary
to locate the speaker 100 feet or
more away from the amp-lifier.
In this situation, a much larger
gauge cable is required. How¬
ever, this may not be practical or
economical. The size of the 2-
wire cable can be greatly reduced
by stepping up the output volt¬
age of the amplifier to 70, 100,
140, or 210 volt, using an output
transformer, then stepping down
the voltage at the load. Such a
system is shown in Figure 5.
The maximum length of 2-wire
cable in this situation can be ap¬
proximated from the following
equation:
Max. Length of 2-wire cable
in feet
where
V is the stepped-up
voltage of the system;
Pout is the rated output
power of the amplifier;
Zo is the output
impedance of the amplifier
(0.04 ohms for the
7300A);
ZL is the load impedance;
DCR/ft is the DC
resistance of the 2-wire
cable per foot from Table
I; and
DF is the minimum
permissible damping
factor at the load.
Suppose a 210 volt system were
used at a 600 watt power level to
drive an 8 ohm load with a mini¬
mum damping factor of 25. Using
the same 18 GA cable as before,
the maximum length can now be
198 feet. Power companies use
this technique to trsinsfer large
amounts of power over great dis¬
tances.
3.6 Speaker Protection
Fuse Selection
Sometimes it may be de¬
sirable to use in-line fuses (fuses
in series with the output) to pro-
4
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