Crown Macro-Tech 2402, Service Manual, Page: 17 / 90

Circuit Theory 4-3
130445-1 Rev. A MA-2402 Service Manual
©2000 Crown International, Inc.
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 out-
put 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 dif-
ference is that the PNP bias resistor is slightly greater in
value so that PNP output devices run closer to the cut-
off level under static (no signal) conditions. This is be-
cause PNP devices require greater drive current.
HS bias is regulated by Q18, the Bias Servo. Q18 is a
Vbe multiplier which maintains approximately 3.2V Vce
under static conditions. The positive and negative halves
of the HS output are in parallel with this 3.2V. 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 dan-
gerous reverse voltage levels. An output terminating
circuit blocks RF on output lines from entering the am-
plifier through its output connectors.
4.5.2 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 respect to
ground, but ±Vcc remain constant with respect 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 quadrants are arranged in a three-
deep Darlington and are biased AB+B in the same
manner as the HS.
When the amplifier output swings positive, the audio is
fed to an op-amp stage where it is inverted. This in-
verted signal is delivered directly to the bases of the
positive (NPN) and negative (PNP) LS predrivers. The
negative drive forces the LS PNP devices on (NPN off).
As the PNP devices conduct, Vce of the PNP Darlington
drops. With LS device emitters tied to ground, –Vcc is
pulled toward ground reference. Since the power sup-
ply is not ground referenced (and the total voltage from
+Vcc to –Vcc is constant) +Vcc is forced higher above
ground potential. This continues until, at the positive
amplifier output peak, -Vcc = 0V and +Vcc equals the
total power supply potential with a positive polarity. If,
+
-
+Vcc (Positive Rail)
-Vcc (Negative Rail)
Load
(speaker)
Input
signal
HIGH SIDE LOW SIDE
Inverting Op-amp
Figure 4.2 Grounded Bridge Output Topology