Crown Audio 1200 service manual High Side HS, Low Side LS

Micro-Tech 1200 Amplifier Service Manual

Theory

	to the cutoff level under static (no signal) conditions.
High Side (HS)	This is because PNP devices require greater drive
The High Side (HS) of the bridge operates much like	current.
a conventional bipolar push-pull output configuration.
As the input drive voltage becomes more positive, the	HS bias is regulated by Q18, the Bias Servo. Q18 is a
HS NPN conducts and delivers positive voltage to the	Vbe multiplier which maintains approximately 3.3V
load. Eventually the NPN devices reach full conduc-	Vce under static conditions. The positive and negative
tion and +Vcc is across the load. At this time the HS	halves of the HS output are in parallel with this 3.3V.
PNP is biased off. When the drive signal is negative	With a full base-emitter on voltage drop across
going, the HS PNP conducts to deliver -Vcc to the load	predrivers and drivers, the balance of voltage results
and the HS NPN stage is off.	in approximately .35V drop across the bias resistors in
	the positive half, and about .5V across the bias resistor
The output of the +LVA drives the base of predriver	in the negative half. Q18 conduction (and thus bias) is
device. Together, the predriver and driver form the	adjustable.
first two parts of the three-deep Darlington and are
biased class AB. They provide output drive through	A diode string prevents excessive charge build up
the bias resistor, bypassing the output devices, at	within the high conduction output devices when off.
levels below about 100mW. An RLC network between	Flyback diodes shunt back-EMF pulses from reactive
the predriver and driver provide phase shift compen-	loads to the power supply to protect output devices
sation and limit driver base current to safe levels.	from dangerous reverse voltage levels. An output
Output devices are biased class B, just below cutoff.	terminating circuit blocks RF on output lines from
At about 100mW output they switch on to conduct high	entering the amplifier through its output connectors.
current to the load. Together with predriver and driver,
the output device provide an overall class AB+B	Low Side (LS)
output.	The Low Side (LS) operates quite differently. The
	power supply bridge rectifier is not ground refer-
The negative half of the HS is almost identical to the	enced, nor is the secondary of the main transformer.
positive half, except that the devices are PNP. One	In other words, the high voltage power supply floats
difference is that the PNP bias resistor is slightly	with respect to ground, but ±Vcc remain constant with
greater in value so that PNP output devices run closer

+

	+Vcc (Positive Rail)
Input
signal	Load
	(speaker)
	Inverting Op-amp
HIGH SIDE	-Vcc (Negative Rail)	LOW SIDE

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Figure 2. Crown Patented Grounded Bridge Topology

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