Crown Audio Principles of Operation, M Series Power Amplifiers, page, Balanced Inputs, Error

M Series Power Amplifiers

5 Principles of Operation

For the sake of simplicity, only channel one of the amplifier is described.

Signal is presented to the M120 and M240 through one of three connectors when using the standard input module. Each channel is outfitted with a balanced XLR / phone jack, and a barrier strip. These connectors are wired in parallel, which allows daisy chaining when needed. The signal is then converted from bal- anced to unbalanced in the Balanced Input Stage where it also receives RFI protection. Signal then flows into the Variable Gain Stage where the front panel level controls are allowed to affect the gain.

Following this stage, the signal is put under the control of a full-time compressor circuit com- prised of a symmetrical window detector, a buffer amplifier, and the gating op amp which uses several small components to set the com- pressor’s attack and decay characteristics. The actual compressing is accomplished by an opto-isolator which affects the gain in the sig- nal path.

The signal next enters the main amplifier error amp where it is mixed with a small portion of the output signal in such a way as to control the amplifier’s overall output performance. Following the error amp is the LVA stage, which is where the low-voltage referenced signal gets translated to the output high-voltage rails. The

last voltage amplifier, in conjunction with a bootstrapped current source, drives both pre- drivers and the bias servo. The bias servo is mounted in such a way as to translate the out- put heatsink temperature into a controlled bias current to prevent thermal runaway and hold the amplifier’s notch distortion to a minimum. The predrivers provide enough signal to acti- vate the drivers, which together operate in the class AB range. For the major output current requirements, the drivers feed the various num- bers of paralleled output transistors which operate in a class B mode. This we call the Tri- ple-Deep Darlington Output Stages.

+HI-VOLTAGE BOOTSTRAP

Figure 5.1

M-Series Amplifier

Block Diagram (Shown with Stan- dard Input Module)

BALANCED

INPUTS

TO CLIP	COMPRESSOR
INDICATOR	CONTROL

				TRANSLATOR
BALANCE	VARIABLE	BUFFER	ERROR
			AMP
	GAIN STAGE			–VCC
INPUT STAGE

TO SIGNAL

PRESENCE

INDICATOR

DC PROTECT

LOGIC

TO

FAULT

CONNECTOR

				TURN-ON
				DELAY
CURRENT
SOURCE	TRIPLE-DEEP	HEATSINK
	DARLINGTON	TEMPERATURE
	NPN POSITIVE
	OUTPUT STAGE			+
	TDVI		OUTPUT
BIAS			CONTROL
	LIMITER
				OUTPUT
	TRIPLE-DEEP
			DC
	DARLINGTON
LVA	NPN NEGATIVE		PROTECT
	OUTPUT STAGE
–VCC	HEATSINK	TRANSFORMER		TO FAULT
	TEMPERATURE
		TEMPERATURE		INDICATOR
	PROPORTIONAL		+ VCC
	SPEED	POWER
			+ 15V
	FAN
	CONTROL	SUPPLY	– 15V
			– VCC
	FAN		BOOTSTRAP	+ HI-VOLTAGE
			CIRCUIT	BOOTSTRAP

MUTE

page 16 Operation Manual