Electro-Voice 7600 - Calculating the Maximum Length of Cable for a Specified Damping Factor

Operating and Servicing Instructions for the Electro-Voice 7600 Power Amplifier
Figure 5 High-voltage Distribution System
greater the ability of the amplifier
to control unwanted speaker cone
movements. When a signal drives a
woofer, current flowing through
the voice coil creates a magnetic
field. This field interacts with the
permanent magnetic field in the
gap and forces the combination
cone and voice coil assembly to
move outward. When the signal is
removed, the assembly moves in¬
ward but its momentum causes it
to overshoot its resting point. This
overshoot will dampen itself out
eventually but the unwanted move¬
ments can add considerable dis¬
tortion products to the sound.
In the process of moving inward
through the magnetic field, the
voice coil assembly generates a cur¬
rent of opposite polarity to the
original signal. This current in¬
duces a voltage or “back EMF”
which travels through the speaker
wire to the amplifier’s output. The
lower the amplifier’s output impe¬
dance, the faster the overshoot of
the voice coil will dampen out. The
output impedance of an amplifier
can be calculated by dividing the
rated output impedance, typically 8
ohms, by the damping factor. The
7600 has a damping factor rating
of 250 which corresponds to an
output impedance of 0.032 ohms.
3.5.1 Calculating the Max¬
imum Length of Cable
for a Specified Damping
Factor Specification 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 factor at the
load should be 30 for general pag¬
ing systems and 50 for high fidelity
music systems. Economics usually
dictate, however, that these num¬
bers are cut-in-half. The resulting
damping factor at the load should
be based on experience and cus¬
tomer satisfaction. Once a mini¬
mum damping factor is determined
for a particular type of installation,
the following equation can cal¬
culate 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
= _DF_
DCR/ft
where
ZL is the load impedance
to connect to the amplifier;
Zo is the amplifier’s
output impedance (0.032
ohms for the 7600);
DF is the minimum
permissible damping factor
2at the load; and
DCR/ft is the DC
resistance of the 2-wire
cable per foot from Table I.
Table I 7600 Power Losses in 2-wire Speaker Cable
Power Cable Cross-
Power
AWG
DCR/fl Loss/n
Sectional DCR/meter
Loss+neier
(GA)
(n/«)
(watts/ft) area (mm 2 )
(fi/m)
(watts/m)
6
0.00081 0.0402 13.30 0.00264
0.1314
8 0.00121
0.0603 8.36
0.00421
0.2082
10
0.00204
0.1013 5.26 0.00669
0.3288
12
0.00324
0.1606
3.31 0.01063
0.5175
14 0.00515
0.2542 2.08 0.01691 0.8108
16 0.00819 0.4011
1.31
0.02685 1.2567
18
0.01302 0.6303
0.82 0.04289
1.9319
20
0.02070 0.9834
0.52
0.06764 2.8752
22 0.03292 1.5185
0.33 0.10658 4.1497
4
Electro-Voice • a Mark IV Company