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Macro-Tech
®
602/1202/2402 Power Amplifiers
The balanced gain stage (U104-C and U104-D) causes
balanced to single-ended conversion using a difference
amplifier. From there, gain can be controlled with a po-
tentiometer. The error amp (U104-A) amplifies the dif-
ference between the output signal and the input signal
from the gain pot, and drives the voltage translator
stage.
From the error amp U104-A, the voltage translator stage
channels the signal to the Last Voltage Amplifiers (LVAs)
depending on the signal polarity. The +LVA (Q104 and
Q105) and the –LVA (Q110 and Q111), with their push-
pull effect through the bias servo Q318, drive the fully
complementary 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. Depending on the polarity of the out-
put signal, D301, D302, D303 and D304 are used to
remove the charge on the unused portion of the output
stage.
With the voltage swing provided by the LVAs, the signal
then gains current amplification through the Darlington
emitter-follower output stage.
The bridge-balanced circuit (U104-B) receives a signal
from the output of the amplifier, and differences it with
the signal at the Vcc supply. The bridge-balanced cir-
cuit then develops a voltage to drive the bridge-bal-
anced output stage. This results in the Vcc supply
having exactly one half of the output voltage added to
their quiescent voltage. D309, D310, D311 and a trim-
mer 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
device operation outside safe conditions.
Q107 and Q108 sense current in the output stage and
act as a conventional current limiter. 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 propor-
tional 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.
Reference Manual
Thermal sensor S100 gives the ODEP circuits vital infor-
mation on the operating temperature of the heat sink on
which the output devices are mounted.
Should the amplifier fail in such a way that would cause
DC across the output leads, 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 back panel stereo/mono switch to Bridge-
Mono, the user can convert the Macro-Tech into a
bridged, single-channel amplifier. With a signal applied
to the Channel 1 input jack and the load connected
across the red (+) back panel 5-way binding posts,
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 stereo/mono switch set to Parallel-Mono, the
output of Channel 2 is paralleled with the output of
Channel 1. A suitable jumper capable of handling high
current levels must be connected across the red (+)
5-way binding posts to gain the benefits of this mode of
operation.
The signal path for Channel 1 is the same as previously
discussed, except Channel 1 also drives the output
stage of Channel 2. The Channel 2 balanced input, er-
ror amp, translators and LVAs are disconnected and no
longer control the Channel 2 output stage. Disconnect-
ing the front-end stages from the Channel 2 output
causes the Channel 2 IOC circuit to note that the input
waveform (which is not present) does not match the out-
put waveform (which is driven by the Channel 1 input
signal). This activates the Channel 2 IOC indicator any
time the amplifier is switched to Parallel-Mono. 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 one channel
alone can be obtained. Because the Channel 2 ODEP
circuit is coupled through S1, this gives added protec-
tion if a fault occurs in the Channel 2 output stage. The
ODEP circuit of Channel 2 will limit the output of both
output stages by removing the drive from the Channel 1
translator stages.
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