Crown CE 2000, Service Manual

Page: 25 / 581

CE Series Power Amplifier Service Manual
Rev. D
Circuit Theory 3-3
when extremely high input resistance and very high h
fe
(cur-
rent gain) are required. Figure 3.1 includes the block dia-
gram of the output stage. The output stages can be further
broken down by Pre-driver, Driver, Output Devices, Flyback
Diodes and Output Terminating Network.
Be sure when replacing the heatsink that the nuts are torqued
properly. The heatsink receives its power through these nuts,
and without them being torqued properly, the amplifier will
not function correctly.
3.6.1 Pre-Driver
There is both a positive side pre-driver Q110 and a negative
side pre-driver Q120. The level-shifted signal from Q107 is
applied directly to the base of Q110 where it is current am-
plified. The level shifted signal applied to the base should
be symmetrical relative to 0 VDC plus the DC offset re-
quired to bias on Q110 and the driver. The positive pre-
driver is connected as an emitter-follower stage with no sign
inversion; the negative side must provide sign inversion
and level shifting so that the driver can be referenced to the
negative rail. Thus, the output is taken off of the collector.
3.6.2 Driver
The positive side driver, Q112, is driven by the pre-driver,
Q110. Likewise, the negative side driver, Q121, is driven
by the negative pre-driver, Q120. The Class AB+B nature of
the output stage means that the drivers are on as Class AB
devices, and the amount of bias can be measured across
R150 or R165. The resistors R150 and R165 are called the
bias resistors because they are connected directly across
the base-emitter junction of the output devices.
3.6.3 Output Devices
The output devices, Q114-Q119 on the positive side and
Q123-Q128 on the negative side are driven directly from the
emitters of the respective drivers. The most important char-
acteristic between output devices is their ability to share
current handling. In ideal current sharing all of the output
devices produce the same amount of current; that is, no
one output device works more than or less than any other
output device. However, not all transistors have identical
current gain. This is why, for optimum performance, it is
absolutely critical that the output devices be matched. When
the output devices are matched, they will have current gains
that are very similar. To ensure optimal performance, num-
bered devices should only be ordered from the Crown Ser-
vice Department.
3.6.4 Flyback Diodes
D114 and D115 are called flyback diodes. In the event that
a back EMF (flyback) pulse exceeds the power supply volt-
age, the flyback diode will shunt this voltage to the supply
in order to protect the output devices.
3.6.5 Output Terminating Network
R158, L102 and C118 form the output terminating network.
This network serves several functions. It allows the amplifier
to better drive very reactive loads and improves amplifier
stability.
3.7 Power Supply
There is one current source and three different power sup-
plies in the CE series amplifiers: the low-voltage supply,
bootstrap supply and the high-voltage supply. Each of these
circuits will be discussed in this section.
3.7.1 Low Voltage Supply
The low-voltage supply is a bipolar supply pro15V
and -15V via two three-terminal regulators. The source of AC
voltage comes from special taps on the main transformer.
This type of low-voltage supply produces an extremely stable
and dependable voltage source for all of the low-voltage
circuitry.
3.7.2 Bootstrap Supply
The bootstrap supply is a voltage doubler network that con-
sists of C1, C3, D6 and D7. The AC leg of the secondary is
applied to R1, which limits the current. Since the voltage at
+Vcc is tied through D6, the voltage on C1+ can be no
lower than +Vcc - 0.7V so the voltage of C1+ will be +Vcc -
0.7VDC when there is no input on WP6. As the voltage on
WP6 goes positive, C1- rises, minus the voltage drop on
R1, and therefore, C1+ rises relative to ground by the same
amount. D7 will conduct, charging C3+. As WP6 completes
its cycle and goes negative, D7 prevents the charge built
up on C3 from travelling back to C1. So, every positive
cycle of WP6 adds charge up to the point that C3+ reaches
twice WP6 peak minus the drop on R1. The voltage devel-
oped at C3+ has a significant amount of ripple, and that
ripple is not equal in amplitude to that on +Vcc. R2 provides
a current path and isolation between the voltage required
(HI-V) and that on C3+. D8 (10V Zener) is placed be-
tween HI-V and +Vcc to limit HI-V to +Vcc +10VDC.
The purpose of this supply is prevent the bias string
from limiting the rail voltage. If the top of the bias string
was connected to the positive rail voltage, the current
required for bias flowing through a resistor to create a