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STEREO POWER
AMPLIFIER
MC275
MC275
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MC275 STEREO POWER AMPLIFIER
INTRODUCTION
The MC275 Commemorative issue
amplifier and owner's manual are faithful
reproductions of the originals, produced
from May, 1961 through July, 1973. A few
changes were made to the original amplifier
design to reflect technology advances and
contemporary system requirements.
Gold plated Balanced Audio Input con-
nectors are added to take advantage of the
greater noise reduction capabilities of
Balanced Cables. High current capacity
Gold Plated Output terminals are also in-
GENERAL DESCRIPTION
Over one million watts of amplifier output
power capacity have been manufactured by
Mclntosh since 1949. In this 1,000,000* watts
of audio power there are less than 10 watts
of distortion capacity and less than 1/100
watt of noise capacity!
The dramatic difference in the quality of
music reproduction when you listen through
Mclntosh instruments is directly due to low
distortion performance. Careful, devoted
research is a way of life at Mclntosh. The
world's finest amplifier is the creation of
persevering, resourceful Mclntosh engineers.
Mclntosh Laboratory is the only manufac-
turer in the entire industry to guarantee the
lowest distortion at all audio frequencies, at
full power. The U.S. Patent Office has
recognized the advanced technology of the
Mclntosh circuit by granting 6 patents.
Long life, flexibility, highest quality con-
struction are characteristic designs in every
Mclntosh instrument. Wide electrical and
cluded. Close tolerance film resistors,
polypropelene capacitors and fiberglass
printed circuit boards are used.
This amplifier is dedicated in remembrance of the late Gordon J. Gow, coinventor of the original Mclntosh Unity Coupled
circuit. Mr. Gow was Vice President of Mclntosh Laboratory Inc. from its inception in
1949 until 1977 when he was elected President. He held the position of President un-
til
1989.
thermal margins of safety for all com-
ponents and tubes, advanced engineering,
and cool operating design add to the long
life built into every Mclntosh product.
Reliability prolongs your investment without
expensive maintenance costs.
The MC275 has on one chassis two 75
watt power amplifiers. In addition to its use
as a stereo amplifier, the flexibility of the
MC275 permits it to be used as a
monophonic amplifier that delivers 150
watts, or as two separate 75 watt amplifiers
with each channel amplifying completely
separate programs, or as two amplifiers for
use with an electronic crossover network.
Such flexibility permits maximum use for
greatest return from your investment.
*1,000,000 Watts was the total power output produced in
12 years from 1949 through 1961. In 1992 alone, Mclntosh
will produce amplifiers with a total power output of more
than 5,000,000 watts with less than 1/100 watt of distortion capacity.
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U.S. Patent Nos.: 2,477,074; 2.545,788; 2,646,467; 2,654.058; 2,860,192; 2.929,028.
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TECHNICAL DESCRIPTION
The patented Mclntosh Unity Coupled circuit and output transformer have established Mclntosh amplifiers as the unchallenged leaders in the audio field.
Before 1949, low distortion at high power
and high efficiency was impossible. A completely new engineering approach resulted
in an amplifier that for the first time permitted high power with distortion below 1%.
That new engineering produced the Mclntosh Unity Coupled circuit and the Mclntosh
bifilar wound output transformer. With the
introduction of the Mclntosh amplifier, new
standards for distortion-free performance
were established.
PRIMARY NO. 1
Fig. 1 —Representation of bifilar winding.
PRIMARY NO. 2
The Mclntosh output transformer is
unique. It has two primary windings which
are wound bifilarly. In the bifilar technique
both primary wires are wound side by side.
Each turn of primary number one is next to
the same turn of primary number two. There
is almost complete magnetic coupling between the two wires. The magnetic coupling is reinforced by the capacitance be-
tween the two wires.
INPUT
Fig. 2—Representation of Mclntosh output circuit.
OUTPUT
In the Mclntosh Unity Coupled circuit one
of the bifilar primary windings is connected
through the power supply to the plate and
cathode of one of the output tubes. The
other bifilar primary winding is similarly connected to the other tube.
All low distortion high power amplifiers
use push-pull output circuits known as
Class AB1AB2 or B. Two tubes are arranged
in a balanced circuit. This permits each tube
to operate alternately somewhat over half
the time. Compared to full time operation of
the tubes, the push-pull method reduces
heating and permits more power from a
given type of tube. Despite this advantage
of the conventional push-pull circuit one problem in particular remained to be solved.
When current in each tube is cut off to begin
the idle period, distortion is produced at the
instant when current flows. This form of
distortion is known as Notch Distortion as
was well illustrated by Mr. Pen Tung Sah in
the "Proceedings of the I.R.E." Volume 24,
pp 1522-1541 in 1936.
Imperfect coupling between the primary
windings found in all conventional output
transformers produces the condition which
permits notch distortion. Trying to improve
coupling in a conventional transformer
decreases the power response at both low
and high frequencies, heating the output
tubes and lowering the available power
output.
The Mclntosh Unity Coupled output circuit and bifilar transformer is the first commercial breakthrough that eliminates notch
distortion by coupling both output tubes
almost to perfection. In the Mclntosh
transformer, the extremely close coupling of
the bifilar windings removes the condition
which permits notch distortion. Furthermore
the two output tubes are arranged as partial cathode followers. Half of the output circuit is in the cathode and half in the plate
of each tube. The output tubes now are
operating in a local feedback loop which
reduces their distortion, reduces their inter-
nal generator resistance, and reduces their
balance requirements. The Mclntosh circuit
in reality perfects push-pull high efficiency
output circuits.
The MC275 uses an advanced design of
the Mclntosh output transformer. The new
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transformer design incorporates all of the
benefits of the original Mclntosh design and
even further improves the power band width.
Leakage inductance (lack of coupling)
between the primary and secondary windings of the output transformer limits the
high frequency response of an amplifier. The
primary and secondary windings of the
Mclntosh output transformer are interleav-
ed five times to improve coupling. The interleaving is accomplished by winding 5
groups of secondary windings spread
uniformly with the primary windings and
parallel connecting the secondary windings.
Interleaving helps to extend the Mclntosh
power band width to over 100,000 cycles.
Since 1949, this new technology has been
built into every Mclntosh vacuum tube
power amplifier.
Good voltage regulation in the power
supply permits overloads without overshoot
or blocking, good transient response, and
complete stability. To improve regulation a
silicon rectifier power supply is used in the
MC275. In addition to better voltage regulation, the silicon rectifier allows even higher
operating efficiency, cooler operation, and
longer amplifier life.
To greatly extend tube and component
life, 2 thermistors in the MC275 limit current
surges produced when the equipment is
turned on. The thermistor is a special type
of resistor. Its resistance depends on its
temperature. When the amplifier is off, the
thermistor has a high resistance value
(about 25 ohms). Just after the amplifier is
turned on, the current which flows through
the thermistor heats it and causes its
resistance to decrease to a low value (less
than 0.5 ohms). Current is thus limited when
the MC275 is first turned on but is not
limited as the unit warms.
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MC275 Tube Power Amplifier
SPECIFICATIONS
Performance Limits
POWER OUTPUT, STEREO
75 watts into 16, 8 or 4 ohm loads is the
minimum sine wave continuous average
power output per channel from 20Hz to
20,000Hz.
The output RMS voltage is:
34.6 volts across 16 ohms
24.5 volts across 8 ohms
17.3 volts across 4 ohms
POWER OUTPUT, MONO PARALLEL
150 watts into 8, 4 or 2 ohm loads is the
minimum sine wave continuous average
power output from 20Hz to 20,000Hz.
The output RMS voltage is:
34.6 volts across 8 ohms
24.5 volts across 4 ohms
17.3 volts across 2 ohms
OUTPUT LOAD IMPEDANCE
16, 8 or 4 ohms stereo
8, 4 or 2 ohms mono parallel
Ratings
IHF DYNAMIC HEADROOM
1.1dB
DAMPING FACTOR
Greater than 10
INPUT IMPEDANCE
100,000 ohms unbalanced
180,000 ohms balanced
INPUT SENSITIVITY
Unbalanced, 1.0 volt to 30 volts through
gain control
Balanced, 2.0 volts fixed
General Information
POWER REQUIREMENTS
120 volts, 50/60Hz
240 watts at zero signal output
400 watts at rated output
The amplifier may be connected for 100,120,
220 or 240 volt 50/60Hz operation. It is shipped connected for 120V.
RATED POWER BAND
20Hz to 20,000Hz
TOTAL HARMONIC DISTORTION
.5% maximum harmonic distortion at any
power level from 250 milliwatts to rated
power from 20Hz to 20,000Hz.
INTERMODULATION DISTORTION
.5% maximum if instantaneous peak
power output does not exceed twice the output rating for any combination of frequencies from 20Hz to 20,000Hz.
FREQUENCY RESPONSE (at 1 watt output)
20Hz to 20,000Hz +0 -0.2dB
10Hz to 100,000Hz +0 -3dB
NOISE AND HUM (A-Weighted)
100dB below rated output
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Mechanical Information
SIZE
16" wide (40.6cm) by 7-1/2" high (19.0cm)
by 12" deep (30.5cm)
WEIGHT
67 pounds (30.5kg) net, 75 pounds (34.1kg)
in shipping carton
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PANEL FACILITIES
Fig. 3—The end panel of the MC275 showing all input and output facilities.
INPUT
The MC275 has a two-position MODE
Slide switch to permit the amplifier to be used as a 75 watt per channel stereo amplifier
or a 150 watt mono amplifier.
The amplifier also has a two position input slide switch to permit selection of unbalanced or balanced input connections.
The unbalanced input sensitivity is 1 volt.
Gain controls are provided for higher input
levels. The controls are marked for 2.5 volt
sensitivity for use with Mclntosh preamp
equipment. The unbalanced input sensitivity
is 2 volts fixed. For mono operation, the input signal is to be fed to the R (mono) input
connectors.
OUTPUT
The two barrier terminal strips marked
OUTPUT provide stereo connections for the
normal speaker impedances of 4 ohms, 8
ohms, and 16 ohms. For monophonic operation, connections for 2 ohms, 4 ohms, and
8 ohms are provided. The terminal strips may
also be connected for a constant voltage output of 25 volts in either stereo or mono.
The Common connections from the out-
put transformer secondary windings are
grounded.
FUSE
The MC275 uses a 5.0 ampere slo-blo type
fuse. The auxiliary AC socket is not fused.
AC OUTLET
The auxiliary AC outlet can be used to
supply power to other equipment in the
system. The outlet will provide a maximum
of 500 watts of power. The AC outlet is not
fused.
LINE VOLTAGE
The MC275 operates on 120 volts,
50/60Hz. The power transformer has two tap-
ped primary windings and can be connected
for 100, 120, 220, or 240 volts.
INSTALLATION
Adequate ventilation extends the troublefree life of electronic instruments. It is
generally found that each 10° centigrade
(18°F) rise in temperature reduces the life of
electrical insulation by one half. Adequate
ventilation is an inexpensive and effective
means of preventing insulation breakdown
that results from unnecessarily high
operating temperatures. The direct benefit
of adequate ventilation is longer, troublefree life.
The suggested minimum space for
mounting the MC275 is 20" long x 14" wide
x 10" high. Always allow for air flow either
by ventilation holes or space next to the bottom of the amplifier and a means for the
warm air to escape at the top.
The MC275 can be mounted in any position except upside down. If the amplifier is
to be installed on a vertical surface it is
recommended that the transformers be on
the down side. The advantage of this position is that the flow of heat from the tubes
rises vertically and does not tend to heat the
transformers.
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CONNECTING THE MC275
INPUT-STEREO
UNBALANCED CONNECTIONS
Connect shielded cables from the Left
and Right Unbalanced Outputs of a preamp-
lifier to the Land R UNBALANCED INPUTS.
BALANCED CONNECTIONS
Connect Balanced cables with XLR connectors from the Left and Right Balanced
Outputs of a preamplifier to the L and R
BALANCED INPUTS.
Balanced Jack Pin configuration
Pin 1. System Ground
Pin 2. + Input
Pin 3. - Input
INPUT-MONO
Connect a shielded cable from either an
Unbalanced or Balanced MONO output of
a preamplifier to the corresponding R
(MONO) UNBALANCED or BALANCED
INPUT.
Be sure to set the INPUTS switch to the
BALanced or UNBALanced position to correspond to the Inputs being used.
OUTPUT-STEREO
For stereo operation, it is not necessary
to use the same impedance load on each
output. Simply connect each output for the
impedance desired.
Warning: Do not parallel the amplifier out-
puts when using the amplifier as a stereo
amplifier. Damage to the output tubes may
result if parallel operation is attempted.
Speakers are connected at the barrier
strips marked OUTPUT on the left end of the
panel.
In compliance with the National Electrical
Code, Class II wiring can be used between
the speaker and the amplifier at the 4 ohm,
8 ohm, or 16 ohm connection. Class II wiring is lamp cord, bell wire, or other wire with
this type of insulation. For the normally
short distances of under 100 feet between
the amplifier and speaker, #18 wire or larger
can be used. For distances over 100 feet be-
tween the amplifier and speaker use larger
wire.
The loudspeaker impedance is usually
identified on the loudspeaker itself. Connect
one of the leads from the left loudspeaker
to the screw marked COM on the left bar-
rier strip. Connect the other lead from the
left loudspeaker to the screw marked with
the number corresponding to the speaker
impedance on the LEFT barrier strip.
If the speaker
impedance is:
4 ohms
8 ohms
16 ohms
Connect one left speaker
lead to screw LEFT-COM
and the other to:
LEFT-4
LEFT-8
LEFT-16
Connect one right speaker
lead to the screw marked
RIGHT-COM and the other to:
RIGHT-4
RIGHT-8
RIGHT-16
The only adverse effect on the operation
of a Mclntosh amplifier when it is improperly
matched is a reduction in the amount of
distortion-free power available to the
loudspeaker. Close impedance matching is
desirable for maximum distortion-free
power.
Use this table to determine proper
speaker connections:
Connect the
If the speaker impedance
is between:
3.2 to 6.5 ohms
6.5 to 13 ohms
13 to 26 ohms
speaker leads
between COM and:
4 ohms
8 ohms
16 ohms
OUTPUT-MONOPHONIC
When the MC275 is to operate as a 150
watt monophonic amplifier, the outputs of
the two channels combine to produce a
single 150 watt output. This chart lists the
proper connections and interconnections
for monophonic operation.
If the
speaker
impedance
2 ohms
4 ohms
8 ohms
Connect one speaker
is:
lead to the screw
marked LEFT-COM
and the other to:
LEFT-4
LEFT-8
LEFT-16
Connect a
wire between:
LEFT-COM and
RIGHT-COM
LEFT-COM and
RIGHT-COM
LEFT-COM and
RIGHT-COM
Connect another
wire between:
LEFT-4 and
RIGHT-4
LEFT-8 and
RIGHT-8
LEFT-16 and
RIGHT-16
When connected as outlined, the MC275
operates as a 150 watt monophonic
amplifier.
LINE OUTPUT-STEREO
For 25 volt line operation, connect one of
the left leads to the screw marked COM on
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the LEFT barrier strip. The other left lead is
connected to the screw marked 8 on the
LEFT barrier strip. Connect the right leads
in the same manner on the RIGHT barrrier
strip.
LINE OUTPUT-MONOPHONIC
To feed a 25 volt line, connect one side to
the screw marked COM on the LEFT barrier
strip. Connect the other side to the screw
marked 8 on the LEFT barrier strip. Then
connect with a wire the screw marked COM
on the LEFT barrier strip to the screw mark-
OPERATING THE MC275
ed COM on the RIGHT barrier strip. Connect,
with another wire, the screw marked 8 on the
LEFT barrier strip to the screw marked 8 on
the RIGHT barrier strip.
AC POWER
The MC275 operates on 120 volt, 50 to 60
cycle power. The amplifier will be turned on
and off if its power cord is plugged in one
of the auxiliary AC outlets of an accessory
component.
Before turning the MC275 on, check all
connections and plugs to see that they are
firmly and correctly connected. Check to
make sure that the tubes are firmly seated
ADJUSTMENTS
MODE SWITCH
With the MODE switch in the Stereo position, the input signals are to be fed to either
the L and R unbalanced jacks or to the
balanced XLR, L and R connectors. With the
switch in the Mono position, the input signal
is to be fed to either the R (MONO) unbalanced jack or to the balanced XLR, R (MONO)
connector.
INPUTS SWITCH
Place this switch in the Unbalanced posi-
tion to use the Unbalanced inputs or in the
Balanced position to use the Balanced
inputs.
UNBALANCED GAIN
For stereo operation, position the L and
R gain controls for decired sensitivity. When
using Mclntosh preamplifiers, we suggest
setting the gain controls to the 2.5V position. For mono operation, use the R MONO
control.
in the proper sockets. After the following ad-
justments have been completed, the MC275
operate without any further attention.
IMPORTANT
The excellent performance that is inherent in all Mclntosh amplifiers does not
depend on the critical adjustment of bias or
balance controls in the output circuit. The
patented Mclntosh circuit delivers its advertised specifications without any need for
these controls and is not dependent on
carefully balanced tubes for its performance. With Mclntosh you can install the
amplifier and forget it.
The MC275 you have purchased will give
you years of pleasant and satisfactory performance. If you have any questions concerning the operation or maintenance of this
amplifier, please contact:
Mclntosh Laboratory Inc.
2 Chambers Street
Binghamton, NY 13903-2699
Our telephone number is 607-723-3512
Design subject to change without notice.
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LABORATORY INC.
2 CHAMBERS STREET, BINGHAMTON, NY 13903-2699
Made in U.S.A.
Phone 607-723-3512
039952
BE032003
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