Com-Tech 200 Amplifier Service Manual
9
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 first
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.
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
Theory
+
-
+Vcc (Positive Rail)
-Vcc (Negative Rail)
Load
(speaker)
Input
signal
HIGH SIDE
LOW SIDE
Inverting Op-amp
Figure 2. Crown Patented Grounded Bridge Topology
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 amplifier through its output connectors.
Low Side (LS)
The Low Side (LS) operates quite differently. The
power supply bridge rectifier is not ground referenced,
nor is the secondary of the main transformer. In other
words, the high voltage power supply floats with re-
spect to ground, but ±Vcc remain constant with re-
spect to each other. This allows the power supply to
dVcc and -Vcc from the same bridge rectifier
and filter 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 quad-
rants are arranged in a three-deep Darlington and are
biased AB+B in the same manner as the HS.
