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Contractor Series Power Amplifiers
Reference Manual
operation, but allows the devices to provide high bursts
of peak power when needed. This amplifier output to-
pology offers a good combination of low quiescent am-
plifier heating, great distortion performance at high
powers, and relative simplicity, with impressive reliabil-
ity and value.
All output power is delivered through a channel-inde-
pendent, dead front barrier block on the back panel.
When the Channel Operation switch is set to 4/8 Ohm,
only the negative and 4/8 ohm connections on the bar-
rier block have audio present. This provides power to
low impedance speakers. When the Channel Opera-
tion switch is set to 70V/100V, only the negative and 70V
and 100V connections on the barrier block have audio
present. This provides power to distributed speakers in
a high-impedance "constant-voltage" application.
The output relay, in conjunction with input signal mute
circuit, assures a quiet turn-on and turn-off. In the event
of an amplifier output failure, a triac will activate to turn
off the offending channel to protect the speakers.
The turn-on delay circuit functions to keep the output
relay open until all the voltages are up and stable, both
in the amplifier, and in all the components in the system
ahead of the amplifier.
Heatsink temperature is monitored by a thermal probe
attached to the heatsink. As the temperature rises, the
probe sends a proportional current to the proportional
speed fan circuit which starts the fan. Should the power
transformer reach its maximum safe temperature, an
internal thermal switch opens and the fan circuit turns
on full speed to quickly cool down the amplifier. It also
disconnects the load via the output relay, removing any
output current and further speeding a cool-down cycle.
This point is set both to protect speakers and to guard
against nuisance tripping.
Whenever the heatsinks or the transformer reach a
maximum temperature, or during the normal turn on
delay window, the front panel Fault Indicators will blink.
An RJ11 modular jack is mounted on the back panel.
Pins 2 and 5 are connected to an opto-isolator which is
always in a low-resistance state whenever the unit is on
and operational. 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 re-
motely 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
at the amplifier front end.
The Clip Iindicator is driven from the output of the com-
pressor circuitry and lights to indicate the onset of au-
dible distortion.
The Power Indicator LED is driven from the low-voltage
supply.
A positive and negative regulator form the ±15-volt
power supplies. Add to that the main transformer, a full-
wave bridge rectifier, and high energy electrolytic to
form the main power supply. They are protected by the
front-panel line circuit breaker and controlled by the
front-panel power switch.
Figure 6.2 CL1 & CL2 Circuit Block Diagram
