Electro-Voice 7600 - Speaker Protection Fuse Selection; Compression Driver Protection Capacitors

Operating and Servicing Instructions for the Electro-Voice 7600 Power Amplifier
The same equation can be used to
calculate the maximum cable len¬
gth in meters by substituting the
DCR per meter value from Table I.
Let’s use the equation. Suppose ZL
equals 8 ohms, Zo = 0.032 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
damping factor of 25 at the load is:
8 — (0.032)
25 _ = 22.1 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 practical or economical.
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 transformer, 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
V 2 /_1_Zo_\
(Pout)(DCR/ft)\ DF ZL/
where
V is the stepped-up
voltage of the system;
Pout is the rated output
power of the amplifier;
Zo is the output impeda¬
nce of the amplifier (0.032
ohms for the 7600);
ZL is the load impedance;
DCR/ft is the DC resis¬
tance of the 2-wire cable
per foot from Table I; and
DF is the minimum per¬
missible damping factor at
the load.
Suppose a 210 volt system were
used at a 1200 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
102 feet. Power companies use
this technique to transfer 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¬
tect loudspeaker systems (or the
amplifier). It is difficult, however,
to determine the proper fuse value
with the correct time lag and
overload characteristics to match
the limitations of a speaker sys¬
tem. The values shown in Table II
should serve only as a guide. To
use, determine the power rating
and load value. Then, select a
standard value fuse of the next
smaller value to the one listed in
the table.
The values are calculated for fast-
blow fuses which carry 135% of
their current rating for an hour
but will blow within 1 second at
200%. Other fuse values may be
calculated for different power
levels from the following equation:
Fuse value =
(Pout x ZL) H amps
ZL x 1.35
Table II Calculated Output Fuse
Values
Power 4 n 8 fl
160
(watts)
Load lead
Load
100 3.70
2.62 1.85
150 4.54
3.21
2.27
200
5.24 3.70
2.62
300 6.42
4.54
3.21
400
7.41
5.24 3.70
600
9.07
6.42
4.54
where
Pout is the output power
rating of the amplifier;
and
ZL is the load impedance.
Use 32 volt fuses if possible; they
typically have the lowest internal
resistance which will help
minimize deterioration of the
damping factor at the load. Refer
to the example in Figure 4.
3.7 Compression Driver
Protection Capacitors
Compression drivers, used
for mid to high frequency sound
reproduction, are much more sus¬
ceptible to damage from low
frequencies than large cone
loudspeakers. Even though an
electronic crossover may be
employed, problems may arise in
the cables between the crossover
and the power amplifier, or from
misadjustment of the crossover.
Either of these situations could
apply low frequency signals or
hum to the driver and cause dam¬
age. To prevent a potential mis¬
hap, Electro-Voice recommends .
using a capacitor between the
amplifier and the compression
driver to suppress low frequencies
and possible DC. Refer to the
example in Figure 4.
Table III Compression Driver Pro¬
tection Capacitors
Crossover 80
160
Frequency
Driver
Driver
500 Hz
80 (iF
40 (iF
800 Hz
50 (iF 25 nF
1000 Hz
40 |iF
20 |iF
1250 Hz
33 jiF
16 |iF
2000 Hz
20 |iF
10 |*F
3150 Hz
12 |iF
6 |iF
6300 Hz
6 (iF 3 !iF
In choosing a value, one must be
careful not to interfere with the
crossover frequency. As a general
rule, select a capacitor whose
Electro-Voice • a Mark IV Company
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