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.
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
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.
