Ab international 900 owners manual

Page 1

600/900

General

AB International amplifier products are designed to deliver uncompromised
performance in continuous-duty commercial and professional audio applications.
The following operating instructions cover the installation and operation of the
model 600. New owners are encouraged to read the entire contents prior to placing
amplifiers into service.

Page 2

Circuit Description MODEL 900 ONLY
The output section of each channel incorporates 12 RCA Multiple Emitter
Power Transistors, which provide 1800 watts of power dissipation per
channel - a 4:1 safety margin. The output stage is arranged in a quasi-
cmplimentary format and biased for class AB/Z operation, with the bias
current evenly distributed among all output devices. Bias thermal compen-
sation is accomplished by thermally mating a bipolar semiconductor junction
to the heat-producing output devices. Triple diffused high pmver driver
transistors are employed along with high speed, high voltage silicon
annular devices for the pace-driver and inverter stages. Utilization

of these components provides the required separation of ft break points

for absolute stability. Fully carpljnmtary current source drive and
loading is utilized throughout. Only ZOdB of negative feedback is used

to reduce forward transfer distortim to nfinimrn levels, which accounts

for the characteristic 'open' sound of the 900. VI type energy limiters
are incorporated for short-circuit protection of the amplifier. Due to
the unusually large safe operating area of the output stage, the limiters
do not actuate until driving a forty-five degree reactive load of 1.4 ohms
at full power.

Under normal operation, the power supply provides plus and minus 70 volt
rails for the output devices. Should the input signal envelope exceed
the preset threshold, the output rails will automatically switch to plus
and minus 90 volts, enabling the full output capability of the instrument
to be realized.

Certain conditions of operatim (restricted cooling airflow, sustained
high power operatim in low impedance loads) can result in a rise in
output device case tamerature sufficient to affect any amplifier's perfor-
mance. To protect the output stage frcm thermal overload, the lower set
of power supply railsflwill automatically activate when the heatsink reaches
65°C. This action is inaudible, and reduces the operating (voltage)
headroom by 3dB, while substantially reducing the case temperature of

the output devices.