High Voltage Supply; Constant Current Source; Protection Circuitry; Time Dependent Vi Limit - Crown CE-1000A Service Manual

current source would drop enough voltage to require a higher
rail voltage. This would increase the dissipation of the out-
puts since they would never reach this voltage. By using a
bootstrap supply, the bias string never limits the available
voltage swing of the amplifier.

3.7.3 High Voltage Supply

The high voltage supply is bipolar and produces the rail
voltages +Vcc and -Vcc. It is full-wave rectified and capaci-
tively filtered. The transformer scales the line voltage to the
voltages required by the amplifier. It also provides isolation
between the line voltage appearing at the primary winding
of the transformer and the rest of the amplifier.

3.7.4 Constant Current Source

Q105 and R135 form a constant current source utilizing the
bootstrap supply HI-V and the rail voltage +Vcc. This source
is derived from the difference between HI-V and +Vcc (which
is +10V) being developed across R135 and Q105 base-
emitter junction. Since this difference is presented across
the base-emitter junction, the ripple of HI-V relative to +Vcc
must be zero for a constant current to be produced. An-
other very important component is C138. C138 overpowers
the base-collector capacitance of Q105. This ensures that
the slew limit of the LVA is set by a more constant capaci-
tance rather than one that is much more variable with the
applied voltage. This lowers distortion by making the rate of
change of the waveform less dependent on the output volt-
age.
Fan Speed
Control
Circuit Theory 3-4
CE Series Power Amplifier Service Manual
CONDITIONS
Turn-On
Delay
Heatsink
Temperature
Transformer
Temperature
LF and
Short Circuit
Figure 3.2 Block Diagram of Protection Circuitry Logic
The constant current source is required for proper operation
of the bias servo circuit. It also helps to provide isolation
between the front end input stage supply and the rail volt-
age.

3.8 Protection Circuitry

The CE series of amplifiers are equipped with a great deal
of protection circuitry to protect the amplifier under a wide
and varied array of fault conditions. Each of the fault condi-
tions and fault circuitry will be reviewed in this section. Also,
the CE amplifiers provide an output fault connector to allow
remote monitoring of the amplifier's condition. This remote
fault connector will also be covered. A block diagram of the
Protection Circuitry Logic is shown in Figure 3.2.

3.8.1 Time Dependent VI Limit

There is a special type of VI limiting in the CE amplifiers. It is
called Time Dependent VI limiting. While most current limit-
ing circuitry is independent of frequency, that is, it limits VI
regardless of what the frequency is, Time Dependent VI
limiting will actually adjust the VI limiting of the amplifier
based on the frequency of the signal.
The time/frequency dependence of the limiter also allows
for higher, non-repetitive peak-currents than is allowed for
continuous wave output signals. The result is an amplifier
more suited to reproducing music.
Fault
Condition
Rev. D
FAULT
PROTECTION
Output
Relay Mute
Input
Mute
Fault
LED
Auxiliary
Output Jack