High Side (Hs); Low Side (Ls); Grounded Bridge Output Topology - Crown Macro-Tech MA-3600VZ Service Manual

130366-1 Rev. A

3.5.1 High Side (HS)

The High Side (HS) of the bridge operates much like
a conventional bipolar push-pull output confi guration.
As the input drive voltage becomes more positive,
the HS NPN conducts and delivers positive voltage
to the load. Eventually the NPN devices reach full
conduction and +Vcc is across the load. At this time
the HS PNP is biased off. When the drive signal is
negative going, the HS PNP conducts to deliver -Vcc
to the load and the HS NPN stage is off.
The output of the +LVA drives the base of predriver
device. Together, the predriver and driver form the
fi rst two parts of the three-deep Darlington and are
biased class AB. They provide output drive through
the bias resistor, bypassing the output devices,
at levels below about 100mW. An RLC network
between the predriver and driver provide phase shift
compensation and limit driver base current to safe
levels. Output devices are biased class B, just below
cutoff. At about 100mW output they switch on to
conduct high current to the load. Together with
predriver and driver, the output device provide an
overall class AB+B output.
The negative half of the HS is almost identical to the
positive half, except that the devices are PNP. One
difference is that the PNP bias resistor is slightly
greater in value so that PNP output devices run
closer to the cutoff level under static (no signal)
conditions. This is because PNP devices require
greater drive current.
+
-
©2000 Crown International, Inc.
Input
signal
HIGH SIDE
Figure 3.2 Grounded Bridge Output Topology
HS bias is regulated by Q18, the Bias Servo. Q18
is a Vbe multiplier which maintains approximately
3.3V Vce under static conditions. The positive and
negative halves of the HS output are in parallel with
this 3.3V. With a full base-emitter on voltage drop
across predrivers and drivers, the balance of voltage
results in approximately .35V drop across the bias
resistors in the positive half, and about .5V across
the bias resistor in the negative half. Q18 conduction
(and thus bias) is adjustable.
A diode string prevents excessive charge build up
within the high conduction output devices when off.
Flyback diodes shunt back-EMF pulses from reactive
loads to the power supply to protect output devices
from dangerous reverse voltage levels. An output
terminating circuit blocks RF on output lines from
entering the amplifi er through its output connectors.

3.5.2 Low Side (LS)

The Low Side (LS) operates quite differently. The
power supply bridge rectifi er is not ground referenced,
nor is the secondary of the main transformer. In other
words, the high voltage power supply floats with
respect to ground, but ±Vcc remain constant with
respect to each other. This allows the power supply to
deliver +Vcc and -Vcc from the same bridge rectifi er
and fi lter as a total difference in potential, regardless
of their voltages with respect to ground. The LS uses
inverted feedback from the HS output to control
the ground reference for the rails (±Vcc). Both LS
quadrants are arranged in a three-deep Darlington
+Vcc (Positive Rail)
Load
(speaker)
Inverting Op-amp
-Vcc (Negative Rail)
MA-3600VZ Service Manual
LOW SIDE
Theory of Operation 3-3