Bridge-Mono Operation; Parallel-Mono Operation - Crown MA-1202 Operation Manual

7 Theory of Operation
The bridge-balanced circuit (U104-B) receives a
signal from the output of the amplifier, and differ-
ences it with the signal at the Vcc supply. The
bridge-balanced circuit then develops a voltage to
drive the bridge-balanced output stage. This results
in the Vcc supply having exactly one half of the out-
put 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
instantaneous 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 cur-
rent 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.
ONLY ONE CHANNEL SHOWN
page 20
To further protect the output stages, the patented
ODEP circuitry is used. It produces an analog out-
put proportional to the always-changing safe oper-
ating 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.
Thermal sensor S100 gives the ODEP circuits vital
information 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.

7.2.2 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 sig-
nal 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.

7.2.3 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 previ-
ously discussed, except Channel 1 also drives the
Figure 7.1 Circuit Block Diagram
MA Series Power Amplifiers
output stage of Channel 2. The Channel 2 balanced
input, error amp, translators and LVAs are discon-
nected and no longer control the Channel 2 output
stage. Disconnecting 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 output 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 Chan-
nel 2 ODEP circuit is coupled through S1, this gives
added protection 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.
Operation Manual