Altec Lansing 9444B - Manual (Page 6)

Operating and Service Instructions for the Altec Lansing 9444B Power Amplifier
Damping Factor Spec­
ification at the Load
The damping factor rating
is ^rpically 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 ^sterns and 50 for
high fidelity music ^sterns. 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 partic­
ular type of installation, the fo­
llowing equation can calculate the
maximum length of 2-wire cable
which can be used to achieve the
minimum damping factor specified
at the load;
Max. Length of 2-wire cable in
feet
ZL — Zo
Table I 9444B Power Losses in 2-wire Speaker Cable
= DF
DCR/ft
where
ZL is the load impedance
to connect to the amp­
lifier;
Zo
is the amplifier’s
output impedance (0.04
ohms for the 9444B);
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 leng­
th in meters by substituting the
DCR per meter value from Table
I.
Let’s use the equation. Suppose
ZL equEils 8 ohms, Zo = 0.04
ohms, and the minimum damping
Power Cable Croaa- Power
AWQ DCR/ft Loss/ft SccI ional DCR/nietcr Loss/raotc
(GA) (n/ft)
(walWft) area (mm*) (n/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 0.006CO 0.1669
12 0.00.124 0.0809 3.31 0.01003 0.2650
14 0.00510 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
factor at the load is 25. In add­
ition, 18 GA cable is preferred.
Then, the maximum length of 18
GA cable which can be used to
achieve a damping factor of 25 at
the load is:
8 — (0.04)
^ 5 ___________________ = 21.5 feet
0.01302 n/ft
Sometimes it may be necessary to
locate the speaker 100 feet or
more away from the amplifier. In
this situation, a much larger
gauge cable is required. However,
this may not be practiced or eco­
nomical. 'The size of the 2-wire
cable can be greatly reduced by
stepping up the output voltage of
the amplifier to 70, 100, 140, or
210 volt, using an output trans­
former, then stepping down the
voltage at the load. Such a system
is shown in Figure 5.
The maximum length of 2-wire ca­
ble in this situation can be ap­
proximated from the following
equation:
Max. Length of 2-wire cable in
feet
1 Zo
(Pout)(DCR/ft) DF ZL
where
V is the stepped-up ii.
age of the system;
Pout is the rated output
power of the amplifier:
Zo is the output impetl
ance of the amplifier (O.G-
ohms for the 9444B);
ZL is the load impedaiic.;
DCR/ft is the DC resis­
tance of the 2-wire cabL
per foot from Table I; anti
DF is the minimum pf
missible damping facte r
the load.
Suppose a 210 volt ^stem werv
used at a 600 watt power level '
drive an 8 ohm load with a min:
mum damping factor of 25. Using
the same 18 GA cable as before,
the maximum length can now t)6
198 feet. Power companies
this teclinique to transfer large
amounts of power over great dis
tances.
3.6 Speaker Protection.
Fuse Selection
Sometimes it may be des­
irable to use in-line fuses (fuses i:.
series with the output) to protect
loudspeciker ^sterns (or the amp­
lifier). It is difficult, however,
determine the proper fuse valttv
with the correct time lag anrl
overload characteristics to match
the limitations of a speaker sy-s-
tem. The values shown in Table II
should serve only as a guide. To
ALTEC LANSING* CORPORATION • a Mark IV Company