User's Manual

Technical Description
A two stage preamplifier with three transistors in
each channel increases the input voltage 16 dB.
There are 13 transistors in each power amplifier
section. The two stage preamplifier is fed to a pair of
matched transistors arranged as an emitter coupled
amplifier with two inputs and one output. The signal
from the preamplifier section connects to one of
these inputs. Both AC and DC negative feedback are
applied to the other input. This large quantity of
feedback is used to reduce noise and distortion. The
signal is then fed to a voltage amplifier. The voltage
amplifier is followed by two driver transistors.
The output section is arranged as a series push-
pull amplifier. The power transistors used in the out-
put section of your MC 250 are selected for their high
power dissipation capability, wide frequency re-
sponse, and large "safe operating area." In addition,
each power transistor is given four separate tests
before it is put in your MC 250. This additional test-
ing makes sure your MC 250 will deliver its rated
power from 20 Hz to 20,000 Hz with low distortion
and complete reliability.
The power transistors are mounted on oversized
anodized heat sinks. The heat sinks assure that un-
der normal operation the transistors will operate at
a low temperature. If temperatures increase due to a
shorted speaker, or restricted ventilation, an auto-
matic temperature sensing device turns off the MC
250. The device operates automatically at a preset
temperature. The MC 250 will turn on again when the
temperature has returned to normal limits. This ad-
ditional feature gives your MC 250 complete relia-
bility under the most extreme operating conditions.
The output stages are matched to the load by the
Mclntosh autoformer. The Mclntosh autoformer is
carefully wound using Mclntosh trifilar winding and
interleaving techniques. Trifilar winding and inter-
leaving gives the autoformers exceptional band-
width. The autoformers properly match the power
transistors to 4, 8, and 16 ohm loads at all audio
frequencies.
The use of the Mclntosh designed trifilar auto-
former makes the Mclntosh solid state amplifiers the
only amplifiers that deliver FULL POWER AT ALL
SPEAKER IMPEDANCES. You have not been power
penalized for your choice of loudspeakers when
using the Mclntosh MC 250.
Another of the advantages of the autoformers is
the 25 volt output for a constant voltage distribution
system. With the MC 250 several sets of speakers
can be operated independently throughout your
home.
To further insure reliability a special power output
SENTRY MONITORING CIRCUIT prevents failure of
the power output transistors due to excessive mis-
match of the output. When your MC 250 operates
normally the SENTRY MONITORING CIRCUIT has no
effect on signals passing through the power ampli-
fier. If the power dissipation should rise above nor-
mal operation, the SENTRY MONITORING CIRCUIT
restricts the drive to the output transistors. The
SENTRY MONITORING CIRCUIT acts instantaneous-
ly for any input signal or load combination. This ar-
rangement assures complete circuit reliability. Only
Mclntosh gives you this degree of protection.
There are three separate power supply sections,
One positive and one negative high current supply
is used for the output stages. The other positive sup-
ply is used for the driving amplifier stages. All sup-
plies are full wave and use silicon rectifiers. Ade-
quate filtering is used to assure an absolute minimum
of hum. The power output stage filter capacitors have
very high capacity, which allows full power output
below 20 Hz. The power transformer is generous in
size and runs cool, even under heavy use.
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