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sitivity switch and the back panel level control. The er-
ror amp (U104-C) amplifies the difference between the
output signal and the input signal from the gain pot,
and drives the voltage translator stage.
From the error amp, the voltage translator stage chan-
nels the signal to the Last Voltage Amplifiers (LVAs),
depending on the signal polarity. The +LVA (Q105)
and the –LVA (Q110), with their push-pull effect
through the bias servo Q318, drive the fully comple-
mentary output stage.
The bias servo Q318 is thermally coupled to the heat
sink, and sets the quiescent bias current in the output
stage to lower the distortion in the crossover region of
the output signal. D301, D302, D303, and D304 are
used to remove the charge on the unused portion of
the output stage, depending on the polarity of the out-
put signal.
With the voltage swing provided by the LVAs, the sig-
nal then gains current amplification through the
Darlington emitter-follower output stage.
The bridge-balanced circuit (U104-D) receives a sig-
nal from the output of the amplifier, and differences it
with the signal at the Vcc supply. The bridge-balanced
circuit then develops a voltage to drive the bridge-bal-
anced output stage. This results in the Vcc supply hav-
ing exactly one half of the output voltage added to their
quiescent voltage. D309, D310, D311 and a trimmer
resistor set the quiescent current point for the bridge-
balanced output stage.
The protection mechanisms that affect the signal path
are implemented to protect the amplifier under real
world conditions. These conditions are high instanta-
neous current, excessive temperature, and output de-
vice operation outside safe conditions.
Q107 and Q108 act as a conventional current limiter,
sensing current in the output stage. When current at
any one instant exceeds the design criteria, the limiters
remove the drive from the LVAs, thus limiting current in
the output stage to a safe level.
To further protect the output stages, the patented
ODEP circuitry is used. It produces an analog output
proportional to the always changing safe operating
area of the output transistor. This output controls the
translator stage previously mentioned, removing any
further drive that may exceed the safe operating area
of the output stage.
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Com-Tech 200/400/800/1600 Power Amplifiers
Thermal sensors S100 and S200 give the ODEP cir-
cuits vital information on the operating temperature of
the heat sink on which the output devices are
mounted.
Should the amplifier fail in a way that would cause DC
across the output lead, the DC protection circuit
senses this on the negative feedback loop and shuts
down the power supply until the DC is removed.
5.2.2 Bridge-Mono Operation
By setting the dual/mono switch on the back panel to
Bridge-Mono, the user can convert the Com-Tech into
a bridge-mono amplifier. With a signal applied to the
channel 1 input jack, and the load between the posi-
tive (+) output terminals on the back panel, twice the
voltage can be output.
The channel 1 output feeds the channel 2 error amp
U204-A. Because there is a net inversion, the channel
2 output is out of polarity with channel 1. This produces
twice as much voltage across the load. Each of the
channel's protection mechanisms work independently
if a fault occurs.
5.2.3 Parallel-Mono Operation
With the dual/mono switch set to Parallel-Mono, the
output of channel 2 is paralleled with the output of
channel 1. A suitable high current-handling jumper
must be connected across the positive (+) output ter-
minals to gain the benefits of this operating mode.
The signal path for channel 1 is the same as previously
discussed, except that channel 1 also drives the out-
put stage of channel 2. The channel 2 balanced input,
error amp, translators and LVAs are disconnected and
no longer control the channel 2 output stage. Discon-
necting the front-end stages from the output causes
the channel 2 IOC circuit to note that the input wave-
form (which is absent) does not match the output
waveform (which is driven by the channel 1 input). This
is why the channel 2 IOC light is activated any time the
amplifier is switched into Parallel-Mono mode. The
channel 2 output stage and protection mechanisms
are also coupled through S1 and function as one.
In Parallel-Mono mode, twice the current of a single
channel is available. Because the channel 2 ODEP cir-
cuit is coupled through S1, you have added protection
if a fault occurs in the channel 2 output stage. The
channel 2 ODEP circuit will limit the output of both out-
put stages by removing the drive from the channel 1
translator stages.
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