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Contractor Series Power Amplifiers
Reference Manual
An RJ11 modular jack is mounted on the back panel.
Pins 2 and 5 are connected to an opto-isolator that is
always in a low-resistance state whenever the unit is on
and happy. Should a fault be detected or should the
amplifier lose AC power, the opto-isolator will change to
a high resistance, allowing the user to remotely detect
the status of the amplifier.
The Signal Presence Indicators tap the signal chain just
before the level controls and prior to the power amplifier
chain. They are not amplifier output indicators and
should only be used to indicate the presence of signal
to the amplifier front end.
The Clip indicators are driven from the output of the
compressor circuitry and light to indicate the onset of
audible distortion. The Power indicator LED is driven
from the low-voltage supply.
6.2.2 Power Supply Operation
AC power enters the amplifier through a power cord
equipped with an IEC (unplugable) connector. It then is
passed through the EMI filter. Circuits that use switch-
ing technology will normally send a small amount of
high-frequency noise back down the power cord and
into the power distribution system. This noise must be
removed in order to sell the unit in certain parts of the
world. Since the CH4/CL4 is a worldwide product, the
EMI filter removes this noise so that it does not exit the
box.
The power then enters the Power Factor Correction
(PFC) Boost stage. This stage is what allows the CH4/
CL4 to be plugged into any outlet in the world without
any modifications to the amplifier. The PFC stage uses
switching power supply technology to take whatever
AC line voltage comes in, convert it to DC and boost it
to 400 Volts. The circuit also uses intelligence to draw
the current from the line sinusoidally and in phase with
the line voltage. This reduces the load on the power
companies and also allows the amplifier to pull more
peak power from the power source (the outlet). The
power is drawn in small amounts 62,500 times each
second and is used to provide power to the isolation
stage and to fill the large energy reservoir capacitors.
The power then goes to the “buck” isolation stage. This
stage takes the 400 Volt PFC voltage and, again using
switching power supply technology, converts it down
(“bucks” it down) to the level needed to power the au-
dio output stage. The isolation stage also satisfies a
safety requirement by providing isolation, using a trans-
former, between the AC mains power and the power
that is delivered to the speakers. The isolation stage
moves power 125,000 times each second from the pri-
mary to the secondary to power the audio output stage
and keep its large energy reservoir capacitors full.
In order to keep the power supply controllers, protec-
tion circuits, and the audio signal path components
powered, another switching power supply is used, this
one also running at 125 kHz. This one is also a “buck”
type supply in that it takes voltage from the 400 Volt
PFC bus and converts it down to the low voltages
needed. This circuit also uses a transformer to provide
safety isolation.
Like the audio signal path parts of the amplifier, there
are many ways that the power supply protects itself.
Part of the start-up time delay mentioned above occurs
while the power supply is ramping up all of its voltages
(soft-start) so that large inrush currents are avoided.
Current limiters and over-current detectors are used to
protect the power supply output devices. The power
supply will also detect severe brownouts and shut off
the supply until the brown-out is over if the line voltage
is drastically less than normal.
