Current Limiter; Display Circuitry; Control Circuitry - Crown CE-4000 Service Manual

CE 4000 Service Manual

3.2.2 Current Limiter

To prevent excessive output current, the CE4000
amplifi er incorporates a current limiting protection
circuit. The current sense resistors (R383/R361 and
R384/R362) in series with the Lp and Ln inductors
sense the output current. The voltage dropped
across these two resistors is sent back to U108-A
and U108-B. D109, R163, and R165 set the voltage
reference for U108-B. D109, R164, and R167 set
the voltage reference for U108-A. Due to the circuit
confi guration, only the reference at U108-5 will show
a voltage and it is set to 0.48VDC referenced to node
B1 with no signal input.
The outputs of the comparators are normally open
but since these two outputs are connected together
and to U107-2 and through R161 to B1+15V, the level
sits at one diode drop above B1 or +0.6VDC. Pin 3
of U107 is tied to B1. The output of U107 is inverting
so normally the output is low. U106-A inverts the
output and is used to turn the modulators on and
off. Overall, if the outputs of U108 go low, the output
of U106-A goes low to turn off the modulatorsU101
and U103.
The second input to U106-A is from the protection
circuitry and is used to turn off the modulators (U101
and U103) for various reasons (see Section 4.2.4).

3.2.3 Display Circuitry

Just after the input signal is converted from balanced
to unbalanced, it is routed to the signal detection
circuit. With a signal approximately 45-dB below that
required for full output, the Signal LED will illuminate.
Note that this LED will indicate whether signal is
present at the amplifi er but not whether it is being
output from the amplifi er.
The compressor circuit is initiated by either the Input-
Output Comparator (IOC) circuit or by the Thermal
Limit Control (TLC) circuit. The IOC circuit, composed
of Q100, Q101, R121-R126, and D105-D106, uses
the error signal from the error amplifi er (U100-C). If
the error amplifi er is unable to correct for non-linear
behavior in the amplifi er, the error amplifi er (U100-C)
will generate a voltage spike of approximately ±7V.
The IOC circuit clamps that voltage to the ±7V limit
(normally it would go to the op-amp's rails) and also
generates a voltage greater than the window of ±0.3V
that is detected by U117-C and U117-B. Depending
on the polarity, U117-C or U117-B will go low during
an IOC event. If this happens, U125-D goes high
Theory of Operation 3-4
and Q103 turns on. Q103 turns on the compressor
opto (located on the BFG PWA) which compresses
the input signal and reduces the peak voltage the
amplifier is required to produce. U125-D going
high will activate the "Clip" LED, but only if the IOC
condition occurs long enough to be audible. C312
and R413 set the conditions at which the "Clip" LED
illuminates. Details of the generation of the TLC signal
are discussed in the next section.
The "Fault" LED indicates any condition when there
is a reason for the modulators to be turned off. When
activated, the Fault LED will fl ash at approximately
3Hz.
The "Enable" LED is turned on by the presence of
the +15V supply (see Section 4.3.3).

3.2.4 Control Circuitry

After the low and high voltage rails are powered
up and stable, the C354 timing capacitor, along
with U114-C and associated circuitry holds the
enable line (the net is called "Timing Cap 1") low for
approximately four seconds. If Timing Cap 1 is low,
U115-A stays low which causes Q115 to be high.
This is input to U106-A that is inverted on the output
and holds the modulators off.
Several detectors affect the status of Timing Cap 1.
U114-A, along with scaling resistors R309 and R310
pull the line low if +Vcc rises beyond a safe limit.
In a like fashion, U213-A, R315, and R316 look at
the –Vcc rail. U213-B with scaling resistors R320
and R319 determine if the high voltage rails are
high enough to ensure proper operation. If a large
amount of high frequency energy is being produced
by the output stage (such as during a shorted load
condition), U114-B with scaling resistors R311 and
R312 and time delay cap C176 will pull Timing Cap
1 low. If the airfl ow is blocked or severely clogged
and even the fan on high speed can not keep the
amplifi er cool, U115-D will pull Timing Cap 1 low. If
Timing Cap 1 goes low, it also turns on Q1 on the BFG
PWA, which causes U5's transistor to not conduct.
U5 is connected to pins 2 and 5 of the modular jack
J3 that allows for remote sensing of the amplifi er's
operating condition.
Special Negative Temperature Coefficient (NTC)
resistors are attached to three heatsinks and the
power transformer to tell the amplifi er if action needs
to be taken to reduce the temperature. RT1 and RT2
130485-1 Rev. B
©2002 Crown Audio, Inc.