Crown CE 2000, Service Manual

Page: 27 / 581

CE Series Power Amplifier Service Manual
Rev. D
Circuit Theory 3-5
The VI circuitry first senses the output current from the
current sense resistors R152 and R301 on the positive
side, and R159 and R300 on the negative side. This
voltage is then fed to the limiting transistors Q108 and
Q109. Before the output current becomes dangerously
high, the limiting transistor is activated, which in turn
limits the drive voltage at the base of the pre-driver.
When the pre-driver current and the limiter current are
equal to the current available from the constant current
source, a limit point is reached, and the protection cir-
cuitry remains in this state until the overload is removed.
The frequency dependence of the circuitry comes from
the capacitors C113 and C114. The resistors R140
through R143 are referenced to ground and only affect
current limit when the output voltage is higher than
ground. The resistors also serve to drain the charge
from C113 and C114, thus increasing the current limit
as the output voltage is increased. The diodes D113
and D114 serve to block voltage during opposing cycles
so that the positive current limit circuitry is not affected
by negative output signals and vice-versa. The VI lim-
iter is pre-biased by R317 and R318. This is done so
that less current in the output stage is required to acti-
vate it.
3.8.2 Temperature Protection
There are three different temperatures that are moni-
tored on the CE amplifiers; the transformer tempera-
ture and both channel 1 and 2 heat-sink temperature.
The transformer temperature is monitored by an inter-
nal thermal switch which is closed (shorted) during
normal operation. When the transformer reaches its
thermal limit, the switch opens to protect the trans-
former, the fan speed is turned to full, and the relay
K100 is turned on, thereby isolating the load from the
output of the amplifier. When the relay K100 is turned
on, the mute circuitry is simultaneously activated. The
mute circuitry will effectively steal the drive from the
error amp. Essentially, the amplifier is safely shut down
until the transformer is cool enough for the thermal
switch to close.
The heatsink temperature is reported via U106. This
device delivers a current proportional to the absolute
temperature that is set to 1 µA per °K of heatsink tem-
perature by R191. (U106 is attached to the heatsink
with electrically insulating and thermally conducting
epoxy.) This current is delivered through R190 to de-
velop a voltage at the anode of D119. This voltage is
then used to adjust fan speed control, and will also
activate relay K100 and the mute circuitry if the heatsink
reaches dangerous levels.
3.8.3 Low-Frequency and Short-Circuit Protection
The output of the error amp is monitored by the win-
dow comparator, U102-B. The window comparator is
designed to detect a LF (Low Frequency) condition.
When this condition is detected, the comparator U5-A
changes state and the amplifier goes into fault mode.
If the output signal remains in a DC condition for enough
time to charge C123 through R185 to a value above
the threshold of the bilateral switch Q132, then Q132
will conduct and turn on TRIAC Q131. Q131 will remain
on until the voltage at Q131-2 reaches within a few
volts of ground and C123 discharges enough to turn
Q132 off. The detection circuit of C123 and R185 is
designed to only allow Q131 to turn on during a fault
condition, but it is possible to trip the circuit with a rail-
to-rail square wave of 5-Hz. If an output device faults,
typically the rail will short through the output device to
the output. Q131 will then turn on and short the rail
through the shorted output to ground. This will prevent
the DC voltage from destroying the load. R184 is used
as a path to ground for any leakage current from Q132
that may build up charge on Q131-G.
3.8.4 Input Compressor
The output of the error amp is monitored by the win-
dow comparator, U102-D. Since the gain from the error
amplifier output is fixed at 20, the maximum amplitude
of the error amplifier is known, and any clipping will
cause the error amplifier to exceed this maximum value.
The window is designed to detect this maximum value
plus a predetermined tolerance. When this value is
exceeded, the output of the comparator goes low.
U101-D, Q100 and U100 are the major components
comprising a fast-attack, slow-decay circuit used to
compress the signal coming out of the VGS. This com-
pression action only occurs when the output of the com-
parator, U102-D, detects clipping on the output of the
error amp. It is important to note that in the event of a
signal being clipped at the error amp, not only is the
compressor activated, but, the red clip LED is also
turned on. In this way the amplifier will not produce a
distorted output, but will visually inform the user that
the input signal is too large and is being compressed.