Macintosh MC2200 Owner's Manual - Technical
Technical
Description
INPUT AMPLIFIER
Separate input amplifiers are used for the right and
left channels. Each input amplifier is a two transistor feed-
back amplifier which has unity gain. They are used to pro-
vide high input impedance and low output impedance for
driving the power amplifier sections. In addition, impe-
dance matching is provided for the POWER GUARD system
input attenuator.
When the MODE selector is switched to MONO, only the
right channel input, GAIN control, POWER GUARD attenua-
tor, and input amplifier is used to drive both power am-
plifiers.
POWER AMPLIFIER
There are two identical power amplifier sections. At the
input to each power amplifier is a monolithic differential
transistor selected for low noise. The monolithic design
yields closely matched transistors for low distortion. The
input and the feedback signals are applied to this differen-
tial amplifier. The second stage is a class A voltage ampli-
fier using a single transistor with feedback.
The predriver, driver, and output sections follow and are
fully complimentary with high efficiency and negligible dis-
tortion. The predriver is a complimentary transistor pair
biased to nearly class A operation for low crossover dis-
tortion. Bias is supplied by a transistor which serves as
a temperature tracking circuit so the predriver is properly
biased regardless of temperature. The predriver has a
generous amount of emitter degeneration for low distortion.
Another complimentary pair of power transistors biased
for class AB operation make up the output driver stages.
Their bias is developed across a termistor which holds the
bias current stable regardless of temperature.
The output stage consists of 6 rugged complimentary
power transistors connected in single ended push-pull
parallel. This stage is connected as a balanced emitter fol-
lower which allows great stability and low distortion. Due
to a unique arrangement of the output bias network, the
output transistors are operated class B free of crossover
distortion. Heat is not produced by these transistors when
there is no output. The bias network for the output stage
is temperature compensated assuring class B operation
at any temperature or power level.
The output transistors and drivers are mounted on gener-
ous sized black anodized aluminum heat sinks. The free
flow of room temperature air that passes through the over-
sized heat sinks provides the cooling necessary for the long
life of components.
The MC 2200 has transient free turn on and turn off
characteristics. This is because the output of the amplifier is
switched by a heavy duty relay to the output auto trans-
former. The relay is driven by a transistor switch. The con-
trol to this transistor is derived from a long time constant
capacitor charging network. The switch turns on the relay
approximately two seconds after the MC 2200 is turned on.
The same circuit has a short turn-off time constant which
turns off the relay before the amplifier's main power supply
has had a chance to drop. The output signal is fed through a
matching network to the front panel HEADPHONE jack. The
HEADPHONE output is designed to feed low impedance
dynamic stereo headphones.
The amplifier output signal is fed to the output terminals
through the autotransformer. The Mclntosh designed inter-
leaved multifilar wound autotransformer is used to properly
match the amplifier to any output impedance tap. The MC
2200 will deliver full power over the entire audio frequency
range at any of these impedances. The MC 2200 will not
overheat when driving low impedance loads {1 ohm, 2
ohms or 4 ohms). The autotransformer also protects speak-
ers from damage in the event of amplifier failure. Should a
direct current component appear in the output it is shunted
by the autotransformer and cannot damage the speaker.
The Mclntosh patented Sentry Monitoring circuit con-
stantly monitors the output signal and instantly reacts to
prevent overload of the output transistors. At signal levels
up to rated output this circuit has high impedance and has
no effect upon the output. If the power output exceeds
design maximum, the Sentry monitoring circuit oper-
ates to limit the signal to the output transistors. In the
event of a short circuit across the amplifier output or
severe impedance mismatch the Sentry Monitoring
circuit will protect the output transistors from failure.
The positive and negative halves of the output signal are
monitored independently.
LIMIT INDICATOR AND POWER GUARD
The front panel NORMAL (green) and LIMIT (red) indica-
tors are activated by a newly designed Mclntosh circuit. The
MC 2200 has a built in "waveform comparator." The wave-
form comparator electrically compares the amplifier's out-
put waveform with the input waveform. Should the wave-
form differences reach 0.5%, the red LIMIT indicator on the
front panel is turned on and the NORMAL indicator is
turned off. If the differences increase to 1% the POWER
GUARD circuit is activated. POWER GUARD provides an
unusual margin of safety for loudspeakers by the preven-
tion of amplifier clipping yet permits the amplifier to de-
liver designed maximum power.
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