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 ampli-
fiers.
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 distor-
tion. Bias is supplied by a transistor which serves as a tem-
perature 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 thermistor which holds the
bias current stable regardless of temperature.
The output stage consists of 4 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 s i n k s provides the cooling necessary fur the
long life of components.
The MC 2120 has transient free turn on and turn off char-
acteristics. This is because the output of the amplifier is
switched by a heavy duty relay to the output autotrans-
former. The relay is driven by a transistor switch. The con-
trol signal to this transistor is derived from a long time con-
stant capacitor charging network. The switch turns on the
relay approximately two seconds after the MC 2120 is
turned on. The same circuit has a short turn-off time con-
stant 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
2120 will deliver full power over the entire audio frequency
range at any of these impedances. The MC 2120 will not
overheat when driving low impedance loads (2 ohms or
4 ohms). The autotransformer also protects speakers 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 operates
to limit the signal to the output transistors. In the event
of a short circuit across the amplifier output or severe im-
pedance mismatch the Sentry Monitoring circuit will
protect the output transistors from failure. The positive and
negative halves of the output signal are monitored inde-
pendently.
LIMIT INDICATOR AND POWER GUARD
The front panel NORMAL (green) and LIMIT (red) indi-
cators are activated by a newly designed Mclntosh circuit.
The MC 2120 has a built in "waveform comparator." The
waveform comparator electrically compares the amplifier's
output w a v e f o r m with the input waveform. Should the
waveform 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.
14
www.freeservicemanuals.info
5/10/2017
Digitized in Heiloo Netherland
