McIntosh MC 2250, Руководство пользователя

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Technical Description
INPUT AMPLIFIER
Each channel contains a complete seven transis-
tor low power input amplifier. An input differential
transistor pair provides high input impedance and
low noise. The outputs of this pair are combined in a
current mirror circuit which drives a class A amplifi-
er stage. The output stage follows this amplifier and
is a complementary pair of transistors biased class
AB. The output signal drives the metering circuit,
headphone jack, and the high power output amplifi-
er. This discrete transistor amplifier design was
selected for low noise, low distortion, adequate
power output capability to drive headphones, and
freedom from turn on and turn off transients.
In the MONO mode of operation the input signal
feeds only the right input amplifier via the RIGHT/
MONO GAIN control. The right input amplifier feeds
both output power amplifier sections. When MONO
BRIDGE MODE is selected the left channel input am-
plifier is used as a phase inverter. The output chan-
nels then operate 180° out of phase with respect to
each other which is the correct arrangement for
bridge output connections. When the MONO PAR-
ALLEL MODE is selected the channels operate in
phase. The channels, of course, also operate in
phase for the STEREO MODE.
OUTPUT POWER AMPLIFIER
The input amplifiers feed the output amplifiers
through an electronic switch. The switch eliminates
turn-on and turn-off transients and is used for speak-
er switching. A junction FET and LED/light depen-
dent resistor network make up the switch. This com-
bination allows the lowest possible distortion when
the amplifier is on and the highest isolation when
the amplifier is off. The control signal to the switch
is off during the auto test countdown. Therefore,
transients that may come into the amplifier from
source equipment will not be amplified or reach the
loudspeaker. Since the HEADPHONE output and
meters are powered by the input amplifier, their op-
eration is not affected by the SPEAKER switch or the
turn on delay system.
The first stage of the output power amplifier is a
differential transistor pair biased for best linearity.
The offset to the differential pair is adjustable. Cor-
rect adjustment allows the lowest possible distor-
tion at low frequencies. A current mirror circuit com-
bines the differential outputs into one signal which
is then amplified by a following class A voltage
amplifier. Both the differential transitors and the
voltage amplifier are supplied by active current
sources. The results are lower distortion and cleaner
turn-on characteristics.
The driver stage follows the voltage amplifier. The
driver consists of a complementary pair of power
transitors biased class AB. Next, three complemen-
tary pairs of rugged power transistors make up the
power output stage. All power transistors are
mounted on conservatively sized anodized
aluminum heat sinks. Because of a unique connec-
tion of the bias network, the output transistors
operate class B and exhibit no crossover distortion
often associated with class B operation. The heat
sinks, therefore, remain cool when there is no out-
put.
The amplifier output signal is fed to the output ter-
minals through the output autotransformer. The
Mclntosh designed interleaved multifilar wound
autotransformer is used to properly match the
amplifier to stereo output load taps for 1, 2, 4 and 8
ohms. The MC 2250 will deliver full power over the
entire audio frequency range at any of these im-
pedances. 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 through the low DC
resistance of the autotransformer. DC cannot
damage the speaker.
A Mclntosh patented Sentry Monitoring circuit
constantly 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 cir-
cuit across the amplifier output or severe im-
pedance mismatch the Sentry Monitoring circuit will
protect the output transistors from failure. Both
positive and negative halves of the output signal are
monitored and protected independently.
POWER GUARD PROTECTION CIRCUIT
The Mclntosh patented Power Guard circuit
eliminates amplifier clipping due to overdrive. The
circuit also illuminates red LIMIT indicator lamps
when the amplifier is driven beyond its maximum
output capacity. Power Guard prevents loudspeaker
damage and eliminates harsh output distortion
caused by amplifier clipping.
The Power Guard circuit consists of a waveform
comparator which monitors the wave shape of the
amplifier input and output signals. Normally there is
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