Electro-Voice 7600 - Octal Accessory Sockets; Protection Systems; Load Protection Circuitry; Amplifier Protection Circuitry

Operating and Servicing Instructions for the Electro-Voice 7600 Power Amplifier
break frequency, with respect to
the load, is 3 dB down at approx¬
imately '/2 of the high pass corner
frequency.
Mylar capacitors with at least a
100 volt ac rating are recommend¬
ed. Table III shows the recomm¬
ended capacitor values for use
with 8 and 16 ohm drivers at
popular crossover frequencies.
4 OCTAL ACCESSORY
SOCKETS
Two octal sockets permit a
variety of plug-in accessories to be
used with the amplifier. Normally,
one “U” jumper is inserted bet¬
ween octal socket pins 8 and 1,
and another between pins 7 and 6.
These jumpers must remain in
place for the amplifier to operate
when not using any accessory mod¬
ules. To use with an accessory
module, remove (and save) the
jumpers and install the module
making sure the key on its center
post aligns with the groove in the
female socket. For operation, refer
to the instructions provided with
the module. Schematically, the
module will be inserted between
the input connector and the bal¬
anced input stage.
Electronic modules are powered
from a bipolar 15 volt supply in
the amplifier. The supply is cap¬
able of supplying up to 25 ma DC
of current. Currents in excess of
25 ma DC may prevent the ampli¬
fier from disengaging from its
built-in protection mechanisms.
5 PROTECTION SYS¬
TEMS
5.1 Load Protection Cir¬
cuitry
Each channel independ¬
ently protects its load from
startup/shutdown transients, DC,
and large subsonic signals.
5.2 Amplifier Protection
Circuitry
A unique current-limiting
circuit was designed specifically
for the amplifier. It features a
variable current limit which is a
function of the output signal volt¬
age. As a result, the amplifier can
deliver the rated currents into rat¬
ed loads but substantially limits
the current into low impedance or
shorted loads (shorted output ter¬
minals). Once the short is re¬
moved, however, the amplifier will
resume normal operation.
A dual speed fan is also incorp¬
orated to provide efficient cooling
under the most demanding condit¬
ions. When the heatsink tempera¬
ture at the fin tips reaches appro¬
ximately 88 °C (190 °F), the fan
automatically switches to high
speed operation. As the tempera¬
ture cools to approximately 78 °C
(172°F), low speed operation is
once again resumed.
The heatsink is also thermally
equalized to prevent the output
devices nearest the fan from oper¬
ating at a cooler temperature than
the devices at the opposite end.
This minimizes the thermal grad¬
ient across the heatsink and for¬
ces the devices to operate at more
nearly the same temperature. This
equalizes the lifetimes and relia¬
bility characteristics of the output
devices so that no one device be¬
comes the weak link in the chain.
Should the heatsink temperature
of a channel remain excessively
high, the affected channel will
shut down automatically. When
the output devices cool to a safe
operating temperature, the chan¬
nel will automatically resume nor¬
mal operation.
5.3 P rotect Indicator
The “PROTECT” LED
does not turn-on abruptly as ot¬
hers may do; its intensity is all¬
owed to vary. As a result, its de¬
gree of brightness serves as a rela¬
tive indicator of the severity of
the current operating conditions
e.g., the brighter the LED, the
greater the stress on the chann-
el(s). This provides a visual notif¬
ication well in advance of any im¬
pending shutdown.
Although the channel may still
operate with the LED apparently
at full brightness, a total shut¬
down will occur within a few sec¬
onds unless the operating condit¬
ions improve. If a shutdown does
occur, the channel will resume
normal operation as soon as its
devices have cooled to an accept¬
able temperature.
6 OPERATION
6.1 Dual Mode of Opera¬
tion
In the dual mode of oper¬
ation, the channels may be opera¬
ted independently. After install¬
ation and hookup, verify that the
mode switch, located on the rear
panel, is in the “DUAL” position
and rotate the level controls fully
counterclockwise (full attenuat¬
ion). Input a 0 dBu (0.775 V rms)
nominal signal level and apply
power. Slowly increase the level
controls until the desired output
power is obtained. If either
“CLIP” LED illuminates, reduce
the output with the channel level
control or reduce the input signal
level at its source.
WARNING: Never attempt to con¬
nect the outputs of the two chan¬
nels in parallel.
6.2 Bridge Mode of Oper¬
ation
After installation and
hookup, verify that the mode swi¬
tch, located on the rear panel, is
in the “BRIDGE” position. Rotate
both levels controls fully counter¬
clockwise (full attenuation). Input
a 0 dBu (0.775 V rms) nominal si¬
gnal level into channel 1 only and
apply power. Slowly increase the
level control of channel 1 until the
6
Electro-Voice • a Mark IV Company