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13
All transistors are selected to have nearly constant
current gain over the entire current range they must
cover. Output transistors in particular,
have matched uniform current gain,
high current bandwidth product and
large active region safe operating area.
These Power Transistors are the very
latest in semiconductor technology and
incorporate a new design known as
ThermalTrak™. Refer to figure 17. This
allows for the instantaneous and accu-
rate monitoring of the Power Transistor
Temperature. The MC255 Power Output
Technical Description
Fig ure 17
Figure 18
Fig u re 16
Block Diagram of the Power Amplifier (one channel shown)
ThermalTrak
DC BIAS
MUTE
THERMAL
SENSORS
POWER GUARD
ATTENUATOR
UNBALANCED
INPUT
BALANCED
INPUT
+
-
ILUMETER
POWER OUTPUT
INDICATOR
AMP
AMP
4Ω/8Ω DISPLAY
CONTROL
AUTO SENSE
CONTROL
TO POWER
SAVE CONTROL
Figure 19
Power Output Meter
The McIntosh MC255 has a large three Meter
Wattage Output Meter that responds 95% full scale
to a single cycle tone burst at 2kHz. Refer to figure
19. Voltage and current outputs are electronically
measured, multiplied and fed to a special circuit that
accelerates the pointer movement in the upward direc-
tion. When the pointer reaches its peak it pauses only
long enough for the human eye to perceive its position,
then drops. It is almost 10 times faster than a profes-
sional VU meter.
Circuitry has a specially designed bias circuit to take
full advantage of the ThermalTrak™ Power Transis-
tors and thus precisely controls the power amplifier
operation over a wide range of music conditions with
the benefits of lower distortion and cooler operation.
Precision metal film resistors and low dielectric ab-
sorption film capacitors are used in all critical circuit
locations.
The high efficiency circuit design of the MC255
contributes to low operating temperatures. More than
2800 square inches of heat sink area keep the MC255
operating safely with convection cooling. No fans are
needed. Refer to figure 18.
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