Manual

Manual
IMPORTANT SAFETY INSTRUCTIONS
THESE INSTRUCTIONS ARE TO
PROTECT YOU AND THE MclNTOSH
INSTRUMENT. BE SURE TO
FAMILIARIZE YOURSELF WITH THEM.
1. Read all instructions - Read the safety and
operating instructions before operating the
instrument.
2. Retain Instructions - Retain the safety and
operating instructions for future reference.
3. Heed warnings - Adhere to warnings and
operating instructions.
4. Follow Instructions - Follow all operating and use
instructions.
WARNING: TO REDUCE RISK OF FIRE OR
ELECTRICAL SHOCK, DO NOT EXPOSE THIS
INSTRUMENT TO RAIN OR MOISTURE.
5. Power Sources - Connect the power supply only to
the type described in the operating instructions or
as marked on the unit.
6. Power-Cord Protection - Route power-supply cords
so that they are not likely to be walked on or
pinched by items placed upon or against them,
paying particular attention to cords at plugs,
convenience receptacles, and the point where they
exit from the instrument.
7. Ventilation - Locate the instrument for proper
ventilation. For example, the instrument should
not be placed on a bed, sofa, rug, or similar
surface that may block ventilation openings; or,
placed in a built-in installation, such as a bookcase
or cabinet, that may impede the flow of air
through the ventilation openings.
8. Heat - Locate the instrument away from heat
sources such as radiators, heat registers, stoves, or
other appliance (including amplifiers) that produce
heat.
9. Wall or Cabinet Mounting - Mount the instrument
in a wall or cabinet only as described in the
owners manual.
10. Water and Moisture - Do not use the instrument
near water - for example, near a bathtub,
washbowl, kitchen sink, laundry tub, in a wet
basement, or near a swimming pool, etc.
11. Cleaning - Clean the instrument by dusting with a
dry cloth. Clean the panel with a cloth moistened
with a window cleaner.
12. Object and Liquid Entry - Do not permit objects to
fall and liquids to spill into the instrument through
enclosure openings.
13. Power Lines - Locate any outdoor antenna away
from power lines.
14. Outdoor Antenna Grounding - If an outdoor
antenna is connected to the antenna terminal, be
sure the antenna system is grounded to provide
some protection against voltage surges and built
up static charge.
In the U.S.A., section 810 of the National Electrical
Code, ANSI/NFPA No. 70-1987, provides
information on the proper ground for the mast
and supporting structure, ground for the lead-in
wire to an antenna discharge unit, and size of
ground conductors, location of antenna-discharge
unit, connection to grounding electrodes, and
requirements for the grounding electrode.
For ground wire:
a) Use No. 10 AWG (5.3 mm2) copper No. 8 AWG
(8.4 mm2) aluminum, No. 17 AWG (1.0 mm2)
copper-clad steel, bronze wire, or larger as ground
wire.
b) Secure antenna lead-in and ground wires to
house with stand-off insulators spaced from 4 feet
(1.22 meters) to 6 feet (1.83 meters) apart.
c) Mount antenna discharge unit as closely as
possible to where lead-in enters house.
d) Use jumper wire not smaller than No. 6 AWG
(13.3 mm2) copper or equivalent when separate
antenna grounding electrode is used.
15. Nonuse Periods - Unplug the power cord from the
AC power outlet when (eft unused for a long
period of time.
16. Damage Requiring Service - Service must be
performed by qualified service personnel when:
A. The power supply cord or the plug has been
damaged; or
B. Objects have fallen, or liquid has been spilled
into the instrument; or
C. The instrument has been exposed to rain; or
D. The instrument does not appear to operate
normally or exhibits a marked change in
performance; or
E. The instrument has been dropped, or the
enclosure damaged.
17. Servicing - Do not attempt to service beyond that
described in the operating instructions. All other
service should be referred to qualified service
personnel.
18. Grounding or Polarization - Do not defeat the
inherent design features of the polarized plug,
Non-polarized line cord adaptors will defeat the
safety provided by the polarized AC plug.
The lightning flash with arrowhead, within an
equilateral triangle, is intended to alert the user to
the presence of uninsulated "dangerous voltage"
within the product's enclosure that may be of
sufficient magnitude to constitute a risk of electric
shock to persons.
19. CAUTION: TO PREVENT ELECTRICAL SHOCK DO
NOT USE THIS (POLARIZED) PLUG WITH AN
EXTENSION CORD, RECEPTACLE OR OTHER
OUTLET UNLESS THE BLADES CAN BE FULLY
INSERTED TO PREVENT BLADE EXPOSURE.
Note to CATV system installer:
This reminder is provided to call the CATV system
installer's attention to Article 820-22 of the NEC
that provides guidelines for proper grounding
and, in particular, specifies that the cable ground
shall be connected to the grounding system of
the building, as close to the point of cable entry
as practical.
ATTENTION: POUR PREVENIR LES CHOCS
ELECTRIQUES PAS UTILISER CETTE FICHE
POLARISEE AVEC UN PROLONGATEUR, UNE PRISE
DE COURANT OU UNE AUTRE SORTIE DE
COURANT, SAUF SI LES LAMES PEUVENT ETRE
INSEREES A FOND SANS EN LAISSER AUCUNE
PARTIE A DECOUVERT.
The serial number, purchase date, and Mclntosh
Laboratory Service Contract number are important to
you for possible insurance claim or future service.
Record this information here.
Serial Number
Purchase Date
CAUTION: TO PREVENT THE RISK OF ELECTRIC
SHOCK, DO NOT REMOVE COVER (OR BACK). NO
USER-SERVICABLE PARTS INSIDE. REFER SERVICING
TO QUALIFIED PERSONNEL.
Service Contract Number
The exclamation point within an equilateral triangle
is intended to alert the user to the presence of
important operating and maintenance (servicing)
instructions in the literature accompanying the
appliance.
Upon application, Mclntosh Laboratory
provides a Service Contract to the original
purchaser. Your Mclntosh Authorized
Service Agency can expedite repairs when
you provide the Service Contract with the
instrument for repair.
Copyright 1986 © by Mclntosh Laboratory Inc.
1
Contents
INTRODUCTION
INSTALLATION
HOW TO CONNECT
CONNECTIONS
FRONT PANEL CONTROLS
3, 4, 5
6
1, 8
9
10, 11, 12
REAR PANEL
INFORMATION
13
PERFORMANCE LIMITS
14
PERFORMANCE CHARTS
15, 16
TECHNICAL DESCRIPTION
17, 18
BLOCK DIAGRAM
Your MC 7270 Stereo Power Amplifier will give you
many years of satisfactory performance.
If you have any questions, please contact:
CUSTOMER SERVICE
Mclntosh Laboratory Inc.
2 Chambers Street
Binghamton, New York 13903-9990
Phone: 607-723-3512
19
Take Advantage of 3 years
of Contract Service...
Fill in the Application NOW.
MclNTOSH THREE YEAR SERVICE CONTRACT
An application for A THREE YEAR SERVICE CONTRACT is included with this manual.
The terms of the contract are:
1. If the instrument covered by this contract
becomes defective, Mclntosh will provide all parts,
materials, and labor needed to return the
measured performance of the instrument to the
original performance limits free of any charge. The
service contract does not cover any shipping costs
to and from the authorized service agency or the
factory.
2. Any Mclntosh authorized service agency will
repair all Mclntosh instruments at normal service
rates. To receive the free service under the terms
of the service contract, the service contract certificate must accompany the instrument when
taken to the service agency.
3. Always have service done by a Mclntosh
authorized service agency. If the instrument is
2
modified or damaged as a result of unauthorized
repair the service contract will be cancelled.
Damage by improper use or mishandling is not
covered by the service contract.
4. The service contract is issued to you as the
original purchaser. To protect you from
misrepresentation this contract cannot be
transferred to a second owner.
5. Units in operation outside the United States and
Canada are not covered by the Mclntosh Factory
Service Contract, irrespective of the place of
purchase. Nor are units acquired outside the USA
and Canada, the purchasers of which should
consult with their dealer to ascertain what, if any,
service contract or warranty may be available
locally.
The Mclntosh MC 7270 is the first and only amplifier
specially designed to fulfill Digital Dynamic Range
demands. It outperforms all others when listening to
sound derived from digitally recorded tapes, records and
compact discs. The MC 7270 has been designed to perform flawlessly because of this capacity for overload: 10
decibels of overstress at less than an average of 0.3% of
distortion! Mclntosh amplifiers with POWER GUARD are
the only amplifiers which can tolerate 10 decibels of
dynamic overload, without severe distortion breakup.
The noise level of digitally recorded sound is 30
decibels below that of conventional analog recordings.
Thirty decibels reduction of noise level means that the
digital disc noise power is 1000 times less than that on
the best analog records. The compact disc is capable of
real life dynamic range while noise generated from compact discs is inaudible. With the noise restraint removed it
is both easier and dramatically more enjoyable to listen to
music at much louder levels. To fully enjoy this new
capability your amplifier must be able to receive three to
ten decibels of over stress from music, and it must do this
without severely distorting the sound!
For an amplifier to handle a three-decibel overload, it
must have a full time capacity of twice its full power. An
over stress demand of 10 decibels is a demand for 10
times the full power capacity of an amplifier. To provide
2500 watts of overload for a 250 watt amplifier is expensive both in the amplifier and in the loudspeaker system
as well. This is the real world of Digital Dynamics Range
demand. How to achieve the performance demanded,
which often lasts from minutes to only a few thousandths
of a second, and to achieve the goal economically, is a
real achievement. In each Mclntosh MC 7270, there is a
unique, patented*, digital sonic overload corrector which
prevents the amplifier from exceeding an average of 0.3%
distortion for overload stresses up to 10 decibels!
Mclntosh has developed a new method of measuring
amplifiers which shows this capability very dramatically.
This is a test of the Spectral Fidelity of an amplifier under
stress.
SIMULATING COMPLEX CRESCENDO DEMANDS
Two tones, 14 kHz and 15 kHz, are fed to the input of
the amplifier under test. The output of the amplifier is fed
to a resistive dummy load across which is an A.C.
voltmeter, (to allow computation of the amplifier's output
power), and to a spectrum analyzer which displays the
magnitude and frequencies of signals at the amplifier output.
Ideally, only the two test tones should be reproduced at
the amplifier output. Some amplifiers generate a broad intermodulation spectrum of tones corresponding to the
sum and difference of the test tones. These spurious
tones are called Intermodulation Distortion.
All amplifiers (Mclntosh amplifiers with Power Guard are
the only exception), produce such a spectrum when they
are driven beyond their output capacity. They produce Intermodulation and Harmonic Distortion, the severity of
which depends on the amount of overdrive and the
design of the amplifier. Distortion of 30% or greater is not
uncommon. Most of the spurious frequency components
are at high frequencies and can easily destroy the tweeter
sections of loudspeakers, while severely distorting the
sound.
*U.S. Patent #4048573
INTRODUCTION 3
In these oscillograms, you can see the difference in
Spectral Fidelity between a stressed Mclntosh, and other
amplifiers.
1. The Mclntosh stressed 10 dB above full power
2. An imported amplifier stressed 10 dB above full power
3. A domestically manufactured amplifier which had to be
tested "under-stressed" since it would not perform with
10 dB of overdrive.
The Mclntosh shows only 3 components, which are 44
for one and 50 dB down for all others, roughly equivalent
to 0.3% distortion. The other amplifier shows 17 discords,
some of which are only 10 dB down, or 30% distortion,
with many less than 30 dB down, or 3% distortion.
This is why Mclntosh customers consistently find that
their Mclntosh alone meets the test for accuracy, the test
for clarity, the test for musicality even when a peak of ten
times the power demand suddenly smashes into the
power amplifier.
When other amplifiers are similarly stressed they
generate large quantities of discordant sounds destroying
the real musicality of the reproduced instruments.
Note in oscillogram number 3 the complete failure of
one of the most popular of American amplifiers. If 14 and
15 kHz are amplified at the same time the amplifier shuts
down by "motor boating". It was necessary to lower the
frequency of the test signals to 10 and 11 kHz to even
complete the test or to perform the test at less than half
rated power on this domestically manufactured amplifier.
It is no accident that Mclntosh amplifiers sound better.
It is no accident that a Mclntosh is a better investment.
•
•
•
•
It sounds better
It is more reliable
It lasts longer
Its resale value is the highest
If good enough will do, there are at least 100 answers
for you. But if the best is what you need then there is only one real answer.
Mclntosh—the amplifier that in 40 years has outlived 60
others who have simply faded away.
4 INTRODUCTION
A NEW AMPLIFIER DESIGNED FOR
A NEW AGE OF HOME ENTERTAINMENT
The MC 7270 stereo power amplifier is designed to
operate with loudspeakers having a nominal impedance
of 1, 2, 4, or 8 ohms. The amplifier is rugged and reliable.
The mechanical and electrical design of the MC 7270 is
the result of the many years of engineering and manufacturing experience by the designers at Mclntosh. This
"know how", combined with meticulous attention to
design and production details, makes the MC 7270 one of
the finest products produced by Mclntosh Laboratory.
Some manufacturers of power amplifiers advertise that
their products do not use protection circuits and that such
circuits compromise performance. It is indeed possible for
such circuits to cause substantial amounts of distortion
and undesirable listening effects which is true of almost
any protective circuit design. The real genius of good
design recognizes these problems and circumvents them
while retaining the real merits of the protective circuits.
These are just some of the extra values you receive when
you invest in Mclntosh equipment. It is precisely for this
reason that it takes longer to complete a Mclntosh
engineering design task. From such engineering dedication
comes the Mclntosh reputation for highest sound quality
with greatest long term reliability.
The MC 7270 incorporates seven protection circuits
which insure its total reliability, seven protection circuits
that protect the music and your listening, not interfere
with them. These circuits are described in the Technical
Description.
INTRODUCTION 5
The MC 7270 may be installed in a Mclntosh cabinet or
custom installed in furniture of your choice. Always provide adequate ventilation. Make sure there is 1½ inches
(3.8 cm) of space above the amplifier so as not to interfere with a cooling flow of air.
CUSTOM INSTALLATION
The PANLOC system of installing equipment conveniently and securely, is a product of Mclntosh research. Turned
clockwise, the PANLOC buttons on the front panel lock
the chassis firmly in place. A counterclockwise turn of the
PANLOC buttons unlocks the chassis from its mounting.
To install the instrument in a Mclntosh cabinet, follow
the instructions that are enclosed with the cabinet. For
any other type of installation follow these instructions:
1. Unpack from Carton
Open the carton and remove the PANLOC brackets, hardware package, and mounting template. Remove the instrument from its plastic bag and place it upside down on
the shipping pallet. Unscrew the four plastic feet from the
bottom of the chassis.
2. Mark the Cabinet Panel.
Tape the mounting template in position on the cabinet
panel where the instrument is to be installed. The broken
lines that represent the outline of the rectangular cutout
also represent the outside dimensions of the chassis.
Make sure these lines clear shelves, partitions, or any
equipment. With the template in place, first mark the six
A and B holes and the four small holes that locate the
corners of the cutout. Then, join the four corner markings
with pencil lines, using the edge of the template as a
straightedge.
3. Drill Holes
Use a drill with a 3/16 inch (5 mm) bit held perpendicular
to the panel and drill the six A and B holes. Then, using a
drill bit slightly larger than the tip of your saw blade, drill
one hole at each of two diagonally opposite corners. The
holes should barely touch the inside edge of the penciled
outline. Before taking the next step, make sure that the six
A and B holes have been drilled.
4. Saw the Panel Cutout
Saw, carefully, on the inside of the penciled lines. First
make the two long cuts and then the two short cuts.
After the rectangular opening has been cut out, use a file
to square the corners and smooth any irregularities in the
cut edges.
5. Install the Mounting Strips
In the hardware package you will find two mounting
strips, and two sets of machine screws. For panels that
are less than 1/2 inch (12.7 mm) thick, use the 3/4 inch
6 INSTALLATION
(19.1 mm) screws; for panels that are more than 1/2 inch
(12.7 mm) thick, use the 1-1/4 inch (31.8 mm) screws.
Starting at the right-hand side of the panel, insert a
screw of the proper length into the center hole in the
panel, marked B on the template. On the back of the
panel, align a mounting strip with the holes in the panel
and tighten the screw until the screwhead is pulled into
the wood.
Repeat this procedure to attach the mounting strip to
the left side of the panel.
6. Attach the PANLOC Brackets
Using two screws of the proper length in the A holes on
each side, attach the PANLOC brackets to the cabinet
panel; the short flange is mounted against the front (face)
of the cabinet panel. The screws pass through the
PANLOC bracket flange, the cabinet panel, and then
through the mounting strips previously mounted.
7. Install the Instrument
Guide the AC power cord through the panel opening to
the back of the cabinet; then, slide the instrument into
the opening carefully so that the rails on the bottom of
each side of the chassis engage the tracks on the mounting brackets. Continue to slide the instrument into the
cabinet until the front panel is flush with the cabinet
panel. Turn the PANLOC buttons at the lower left and
right corners of the instrument panel clockwise to lock
the unit firmly in the cabinet. Turn the PANLOC buttons
counterclockwise to unlock the instrument. It can then
slide outward to permit the removal of the instrument
from the cabinet.
MOUNTING TEMPLATE
Use shielded cables to connect the signal from the
preamplifier or signal source to the power amplifier. To
minimize the possibility of hum, the shielded cables
should be of parallel construction or loosely twisted
together, located away from speaker connecting cables
and AC power cords. Be certain to use good quality
shielded cables for all interconnections. Your dealer can
advise you on the kind and length of cable that will best
suit your installation.
The appropriate length and size of loudspeaker cable for
your installation will help to preserve the quality of sound
for which the loudspeakers have been designed. If undersize wire is use, resistance is added to the amplifier/
loudspeakers combination which adversely affects the performance. Added resistance reduces the damping factor,
modifies the frequency response and reduces the power
output. Your dealer's advice will serve you best for your
installation. The cables to and from the speaker should be
of parallel construction or be loosely twisted together.
The chart shows the recommended minimum wire size
for the length of wire between the amplifier and the
loudspeakers.
SPEAKER CABLE LENGTHS
AMPLIFIER TO SPEAKER
For 4 Ohm Load
Feet
15
25
40
60
100
Meters
4.6
7.6
12.2
18.3
30.48
For 8 Ohm Load
Wire
Feet
30
50
80
Meters
9.1
15.2
Gauge
18
16
24.4
14
120
36.6
60.0
10
200
12
These speaker cable lengths represent a wire
resistance equal to 5% of the speaker impedance.
CONNECTING FOR STEREO OPERATION
Plug the left output of the preamplifier into the Left input
jack of the power amplifier. Plug the right output of the
preamplifier into the Right (Mono) input jack of the power
amplifier.
Set the MODE switch to the STEREO position.
Connect a speaker cable from the common terminal of
the left speaker to the amplifier LEFT OUTPUT terminal
strip COMmon screw. Connect another speaker cable
from the other loudspeaker terminal to the correct
speaker impedance terminal on the LEFT OUTPUT terminal strip. The right channel speaker is connected in the
same manner to the RIGHT OUTPUT terminal strip.
When multiple speakers are to be connected to either
or both outputs, the combined impedance must be
calculated. The combined impedance must be connected
to the appropriate impedance tap. Use this table to aid in
selecting the correct impedance match:
Combined impedance in ohms:
Connect for:
0.8 to 1.6
1 ohm output
1.6 to 3.2
2 ohm output
3.2 to 6.4
4 ohm output
6.4 and up
8 ohm output
If the load impedance used is lower than the output impedance tap, reduced power and possible distortion will
result. If the load impedance used is higher than the output impedance tap, neither the quality of sound nor the
amplifier will be harmed, only the available power is
reduced. For multiple speaker operation, run separate
speaker cables from the amplifier to each speaker.
The MC 7270 will feed a constant voltage line as is often
used in background music applications, paging systems and
the like. For constant voltage of 25 volts connect output leads
to the 4 ohm connectors on each channel.
Because the crosstalk between channels is almost non
existent, each channel can be used as a separate
amplifier. (Example; use one channel for mono
background program in one area and the other channel
for paging in a separate area.)
CONNECTING FOR MONOPHONIC BRIDGE OR SINGLE
CHANNEL OPERATION
When the MC 7270 is used as a monophonic or single
channel power amplifier, the two channels are added in a
bridge configuration to produce output up to 540 watts.
For monophonic operation using the MONO-BRIDGE
mode, the amplifier can supply full power to output impedances of 2, 4, 8 and 16 ohms.
Plug a shielded cable from the signal source or
preamplifier to the RIGHT (MONO) input jack only.
Change the MODE switch on the back panel of the
amplifier to the MONO-BRIDGE position. The MONOBRIDGE switch internally connects the output of the right
channel input amplifier to both left and right channel
power amplifiers with the phase of the left channel inverted to achieve bridge operation and increased power
output.
BE CERTAIN THE MC 7270 IS NEVER OPERATED WITH THE
BACK PANEL MODE SWITCH IN THE STEREO POSITION
WITH THE AMPLIFIER CONNECTED FOR MONO BRIDGE.
Operated with the MODE switch in the MONO-BRIDGE
position, the circuit subtracts the left channel from the
right channel L-R). The remaining difference signal is all
that is then fed to the speaker.
HOW TO CONNECT 7
The MONO-BRIDGE operation of the MC 7270 amplifier
always requires the speaker load to be connected between two opposite channel impedance terminals. The
common terminals are not used. Do not connect leads independently between left and right channel impedance
taps as this will cause high circulating currents and
overheating. Connect the speaker leads to the output terminals as listed in this chart:
Load Impedance
In Ohms
Connect the
-Speaker Cables to:
2
4
8
16
Left 1
Left 2
Left 4
Left 8
Ohm
Ohm
Ohm
Ohm
Terminal
Terminal
Terminal
Terminal
Connect the
+Speaker
Cables to:
Right
Right
Right
Right
1
2
4
8
Ohm
Ohm
Ohm
Ohm
Terminal
Terminal
Terminal
Terminal
Neither output terminal is at ground potential.
To use the MC 7270 to feed a line whose constant
voltage is 25 volts, connect the speaker cables for 2 ohm
MONO-BRIDGE output.
8 HOW TO CONNECT
PROGRAM SOURCE
STEREOPHONIC
CONNECTIONS
LEFT
SPEAKER
MUST BE IN
S T E R E O POSITION
FOR S T E R E O PROGRAMS
RIGHT
SPEAKER
PROGRAM SOURCE
MONO-BRIDGED
CONNECTIONS
(CONNECTIONS FOR
8 OHM S P E A K E R ILLUSTRATED)
SPEAKER
CONNECTIONS SHOWN FOR MONO
WHEN MODE SWITCH
IS IN MONO-BRIDGE POSITION
CONNECTIONS 9
METERS
Output power monitor meters calibrated in both watts
and decibels, indicate the output power of each channel.
The upper scale on the meter has been calibrated to
show average watts and the lower scale, decibels.
Reading from right to left, the marks between the
numbers indicating watts are (from the indicated 270
watts): first mark, 108 watts. Second 54 watts, the indicated 27 watts, 10.8 watts, 5.4 watts, the indicated 2.7
watts, 1.08 watts, 0.54 watts, the indicated 0.027 watts,
0.0108 watts, .0054 watts, the indicated 0.0027 watts,
0.00108 watts and 0.00054 watts.
The meters respond to the peak output of each channel.
Ordinary meters lack the capability of indicating the short
interval information in a sound wave. The mass of the
meter movement is too great to respond to the nearly instantaneous changes in music program material. Short interval information can have a duration as short as half a
thousandth of a second. Ordinarily, a meter pointer moving over its scale in such a short time could not be seen.
Mclntosh has developed circuits that drive the meters to
respond to the short interval information in a sound wave
to an accuracy of 90%. The electrical pulse that drives the
meter pointer is time stretched so that the peak position
of the pointer can register in the persistence of vision
characteristic of the retina of the human eye. The pointer
can be locked at the peak indication by switching to the
HOLD position of the METER range switch.
LEFT CAIN
Use the LEFT GAIN control to adjust the output in the
left channel to the desired listening level. Turn the control
clockwise to increase the output.
RIGHT/MONO GAIN
Use the RIGHT/MONO GAIN control to adjust the output in the right channel to the desired listening level. Turn
the control clockwise to increase the output.
When connected for monophonic operation use the
RIGHT/MONO GAIN to adjust the total output to the
desired listening level. The rear panel MODE SWITCH
must be in the MONO-BRIDGE position.
10 FRONT PANEL CONTROLS
METER RANGE
The METER RANGE switch has two positions: WATTS
and HOLD.
WATTS
In the WATTS position, the meter needle indicates the
variations in program loudness. Although the primary output calibration of the meters is from 0.0027 watts (2.7
milliwatts) up to 270 watts, the rated power output of the
MC 7270, the additional indication to the right of the 270
watts mark, is 540 watts. While the MC 7270 cannot
reach this power level continuously it is possible for short
interval peaks to considerably exceed the 270 watts continuous rating.
HOLD
In the HOLD position, the meter needle locks to the
highest power peak in a sequence of peaks. The meter is
driven to maximum power, electronically held there until
a higher peak passes through the amplifier, which moves
the meter needle to a new indication. If no further peaks
are reached the meter needle will very slowly return to
its rest position (decay rate: 6 dB per minute).
POWER
The power switch turns the MC 7270 ON or OFF. The
switch does not control the power outlet on the back
panel. If you wish to control the AC power from a
preamplifier control center, leave the switch in the ON
position. Be sure the AC cord of the MC 7270 is plugged
into the controlled outlets on the rear of the preamplifier
control center.
OFF: In the OFF position the AC power to the amplifier
is turned off.
FRONT PANEL CONTROLS 11
THE MclNTOSH (EXCLUSIVE) POWER GUARD
DIGITAL DYNAMICS PROTECTION CIRCUIT
Improved recordings and recording techniques have imposed higher power demands on today's amplifiers. Poorly designed amplifiers can present music listeners with a
form of harsh unpleasant distortion due to amplifier
overload (hard clipping). Clipping, which looks and acts
like non musical square waves, is caused when the
amplifier is asked to produce more power output with
low distortion than it is capable of or designed to deliver.
Amplifiers, when driven to clipping, can deliver up to 40%
harmonic and intermodulation distortion that decreases
the pleasure and enjoyment you get from listening. This
form of distortion (clipped signal) also produces extra heat
energy which will damage most speakers. Mclntosh
leadership in engineering has developed the Power Guard
circuit which - (1) dynamically prevents power amplifiers
from being overdriven into hard clipping - (2) assures that
the amplifier will produce its maximum output without increased distortion - (3) protects your speaker from excessive heating. Power Guard is a patented Mclntosh
design (U.S. patent #4048573).
The MC 7270 has a circuit that compares the wave
shape of the output signal to the input signal. If the
disparity between the two signals, due to overdrive, exceeds an average of 0.3% (equivalent to 0.3% total harmonic distortion) an amber POWER GUARD indicator illuminates. With any further increase in distortion the
POWER GUARD circuit operates to limit the amplifier input dynamically so that the amplifier cannot be overdriven. POWER GUARD eliminates amplifier output clipping. POWER GUARD only operates when the amplifier is
asked to deliver more power than that for which it was
designed. While the power output remains within these
limits the POWER GUARD indicators do not illuminate.
12 FRONT PANEL CONTROLS
MODE SWITCH
The MC 7270 will operate in two modes, Stereo and
Mono Bridge.
LEFT AND RIGHT OUTPUT TERMINALS
For stereo operation, output impedances of 1, 2, 4 and
8 ohms have been provided on a secure, screw type barrier strip. For rnonophonic operation proper interconnection provides 2, 4, 8, and 16 ohms at the same barrier
strips. See page 7 for connecting instructions.
INPUT JACKS
In the stereo mode of operation, both Left and Right input jacks function. In the mono mode of operation only
the Right (MONO) channel input jack accepts signal. In
mono, the Left channel input jack is disconnected.
INPUT LEVEL
The input sensitivity of the MC 7270 is 0.75 volts or
2.5V depending on the position of the INPUT LEVEL
switch. For the best signal to noise ratio when using Mclntosh source equipment, place the INPUT LEVEL switch in
the 2.5V position and the front panel LEFT and
RIGHT/MONO GAIN controls in the fully clockwise position. If more gain is desired the 0.75V position may be
used. All Mclntosh preamplifiers have been designed to
deliver 2.5 volts output with rated input. For source
equipment other than Mclntosh, set the switch in the
position nearest to the stated output rating of the source
equipment.
AC POWER
The input to the MC 7270 is 120 volts 50/60 Hz at up to
12 amps. The primary circuit is protected by a 15 Amp
Fuse.
REAR PANEL INFORMATION 13
PERFORMANCE GUARANTEE
Performance Limits are the maximum deviation from
perfection permitted for a Mclntosh instrument. We promise you that when you purchase a new MC 7270 from a
Mclntosh franchised dealer, it will be capable of or can be
made capable of performance at or exceeding these limits
or you can return the unit and get your money back.
Mclntosh is the only manufacturer that makes this statement.
PERFORMANCE
Mclntosh audio power ratings are in accordance with
the Federal Trade Commission Regulation of November 4,
1974 concerning power output claims for amplifiers used
in home entertainment products.
POWER OUTPUT
STEREO
270 watts minimum sine wave continuous average power
output, per channel, both channels operating into 1 ohm,
2 ohms, 4 ohms, or 8 ohms load impedance, which is:
16.4 volts RMS across 1 ohm
23.2 volts RMS across 2 ohms
32.9 volts RMS across 4 ohms
46.5 volts RMS across 8 ohms
MONO-BRIDGE
540 watts minimum sine wave continuous average
power output into 2 ohms, 4 ohms, 8 ohms, or 16 ohms
load impedance, which is:
32.9 volts RMS across 2 ohms
46.5 volts RMS across 4 ohms
65.7 volts RMS across 8 ohms
93.0 volts RMS across 16 ohms
OUTPUT LOAD IMPEDANCE
STEREO
1 ohm, 2 ohms, 4 ohms, and 8 ohms; separate terminals
are provided for each output.
MONO-BRIDGED
2 ohms, 4 ohms, 8 ohms, or 16 ohms, balanced to
ground.
RATED POWER BAND
20 Hz to 20,000 Hz
TOTAL HARMONIC DISTORTION
STEREO
0.02% maximum harmonic distortion at any power level
from 250 milliwatts to 270 watts per channel from 20 Hz
to 20,000 Hz, both channels operating.
MONO
0.02% maximum harmonic distortion at any power level
from 250 milliwatts to 540 watts from 20 Hz to 20,000
Hz.
INTERMODULATION DISTORTION
STEREO
0.02% maximum if instantaneous peak power
output is 540 watts or less per channel with both
channels operating for any combination of frequencies, 20 Hz to 20,000 Hz,
MONO
0.02% maximum if instantaneous peak power
output is 1080 watts or less for any combination of
frequencies, 20 Hz to 20,000 Hz.
FREQUENCY RESPONSE (AT ONE WATT OUTPUT)
20 Hz to 20,000 Hz +0 -0.25 dB.
10 Hz to 100,00 Hz +0 - 2 dB.
NOISE AND HUM
100 dB below rated output.
RATINGS
DAMPING FACTOR
Greater than 30
INPUT IMPEDANCE
20,000 ohms.
INPUT SENSITIVITY
Switchable: 0.75 volt or 2.5 volts-level control provided
for higher input voltages.
POWER GUARD
Clipping is prevented and THD does not exceed 2% with
up to 20 dB overdrive a 1 kHz.
GENERAL INFORMATION
POWER REQUIREMENTS
120 volts 50/60 Hz; 13 amps.
SEMICONDUCTOR COMPLEMENT
79 silicon transistors
31 silicon rectifiers and diodes
7 integrated circuits
MECHANICAL INFORMATION
SIZE
16 3/16 inches wide (41.1 cm) by 7 1/8 inches high (18.1
cm) by 14 1/2 inches deep (36.8 cm), including connectors. Knob clearance required is 1 1/4 inches (3.2 cm) in
front of mounting panel.
FINISH
The front panel is a combination of glass and black
anodized aluminum. The chassis is black.
WEIGHT
82 pounds (37.2 kg) net, 96 pounds (43.5 kg) in shipping
carton.
14 PERFORMANCE LIMITS
HARMONIC DISTORTION IN %
0.05
MC 7270
0.04
HARMONIC DISTORTION VS FREQUENCY
AT RATED POWER
0.03
0.02
0.01
0
20
100
10k
1k
FREQUENCY IN Hz
20k
0
-1
OUTPUT IN dB
-2
-3
-4
-5
-6
-7
MC 7270
FREQUENCY RESPONSE AT 1 WATT OUTPUT
-8
-9
-10
10
100
1k
10k
100k
FREQUENCY IN Hz
OUTPUT IN dB
0
-10
MC 7270
CHANNEL SEPARATION VS FREQUENCY
-20
OUTPUT IMPEDANCE: 8
INPUT IMPEDANCE: 1k
-30
-40
-50
-60
-70
-80
-90
20
100
1k
10k
20k
FREQUENCY IN Hz
PERFORMANCE CHARTS 15
HARMONIC DISTORTION IN %
0.05
MC 7270
HARMONIC DISTORTION VS POWER OUTPUT
0.04
BOTH CHANNELS OPERATING INTO 8 OHMS
20kHz
1kHz
20Hz
0.03
0.02
0.01
0
1
0.1
10
100
1000
POWER OUTPUT IN WATTS
POWER OUTPUT IN WATTS
300
400
300
200
MC 7270 POWER OUTPUT VS FREQUENCY
0.02% HARMONIC DISTORTION
100
BOTH CHANNELS OPERATING INTO 8 OHMS
0
100
10
1k
10k
100k
100
1000
FREQUENCY IN Hz
0,06
MC 7270
IM DISTORTION IN %
0.05
INTERMODULATION DISTORTION VS POWER OUTPUT
BOTH CHANNELS DRIVEN
0.04
0.03
0.02
0.01
0
0-1
1.0
10
POWER OUTPUT IN WATTS
16 PERFORMANCE CHARTS
INPUT AMPLIFIER
Each channel input contains a complete seven transistor
low power amplifier. A differential transistor pair provides
high input impedance with low noise. The differential
signals are combined in a current mirror circuit which
drives a class A amplifier stage. The following output
stage is a complimentary pair of transistors with class AB
biasing. The output signal drives the metering circuit and
the high power output amplifier. This discrete transistor
input amplifier design has low noise, low distortion and
freedom from turn-on/turn-off transients.
The INPUT LEVEL selector and GAIN controls (as passive
attenuators) which precede the input amplifier, prevent input overloaded when the controls are correctly set.
In the MONO BRIDGE mode of operation the input
signal feeds only the right input amplifier using the
RIGHT/MONO GAIN control. The output of the right input
amplifier feeds both output power amplifier sections with
the left input amplifier as a phase inverter to the left output amplifier. The phase inversion puts the output
amplifiers 180° out of phase with respect to each other
to provide a bridge output. The channels, of course,
operate in phase for the STEREO mode.
OUTPUT POWER AMPLIFIER
The output of the input amplifier is fed to the power
amplifier input through a junction FET electronic switch.
The control signal to the switch is held for about one second to eliminate turn-on/turn-off transients and prevent
transients that may originate in the source equipment
from being amplified and fed to the loudspeakers.
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. Correct adjustment
assures the lowest possible distortion at low frequencies.
In each channel, current mirror circuit combines the differential outputs into one signal which is amplified by a
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 turnon characteristics.
The driver stage consists of a complimentary pair of
power transitors biased class AB. Three complimentary
pairs of rugged power transistors mounted on over-sized
anodized aluminum heat sinks, make up the power output stage. A unique design of the bias network permits
the output transistors to operate class B but prevents the
crossover distortion associated with class B operation. The
heat sinks remain cool when there is no output.
The amplifier output signal is fed to the output terminals
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 7270 will deliver full
power over the entire audio frequency range at any of
these impedances. 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 to ground by 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
design maximum the circuit has high impedance and has
no effect upon the output. Should the power output exceed 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 impedance mismatch the Sentry Monitoring circuit
will protect the output transistors from failure. Positive
and negative halves of the output signal are independently monitored and protected.
POWER GUARD PROTECTION CIRCUIT
The Mclntosh patented Power Guard circuit eliminates
amplifier clipping due to overdrive. The circuit illuminates
amber POWER GUARD indicator lamps when the
amplifier drive trys to exceed the maximum output capacity. Power Guard prevents loudspeaker damage and
eliminates harsh output distortion caused by amplifier clipping.
In the Power Guard circuit, the output waveform is compared to the input waveform. As long as there isn't any
disparity between these signals, the circuit hasn't any influence on the performance of the amplifier. Should the
amplifier drive try to exceed the amplifier's maximum
power capacity, a difference will develop. If the disparity
exceeds 0.03% on the average, (equivalent to 0.3% total
harmonic distortion) the difference causes the amber
POWER GUARD indicators to light. A further increase in
the disparity, controls an electronic attenuator at the
amplifier input to reduce the amplifier gain, thus holding
the amplifier output to its maximum undistorted value
regardless of the degree of overdrive to the amplifier.
The difference signal is fed to a specially compensated
operational amplifier integrated circuit. Its output is
detected by a full wave bridge that feeds signals to the
control circuitry for the POWER GUARD indicators and to
the electronic attenuator at the amplifier input. The attenuator is a light emitting diode/light dependent resistor
network selected specially for its low distortion and time
constant characteristics.
TECHNICAL DESCRIPTION 17
METER CIRCUIT
The meter circuit has three basic sections: a logarithmic
amplifier, a full wave rectifier, and a DC amplifier.
The logarithmic amplifier consists of a high gain operational amplifier with a bipolar connected silicon diode
pair as feedback elements. These diodes have a uniform
logarithmic characteristic over an 80 dB range. Only 60 dB
of this logarithmic range is used in the MC 7270.
The full wave rectifier circuit uses an operational amplifier with silicon diode feedback networks. This amplified
diode circuit has nearly perfect rectification characteristics.
One rectifier detects only positive signals. The other
responds only to negative signals and produces a positive
output. The outputs of the rectifiers are combined at the
operational amplifier output, so the highest signal, either
positive or negative, is the one that is indicated by the
meters. Gate diodes are used to charge a low leakage
capacitor which attains and holds a charge during signal
peaks. The operational amplifier provides a large amount
of current so the peak holding capacitor can charge suddenly. The charge on the peak holding capacitor is
amplified in a two transistor DC amplifier which is used to
drive the meter. The output of the amplifier has a DC
feedback network connected to the detector to assure excellent overall linearity and frequency response. The current drive to the meters has a peaking capacitor to accelerate the upscale response of the meter needle. The
meters also have a parallel shunt resistor to correctly
damp their action. In the WATTS mode the discharge of
the peak holding capacitor is controlled by a resistor current source. In WATTS HOLD, the resistor is disconnected
so the peak reading is retained. The rate of decay in
WATTS HOLD is about 6 dB per minute.
POWER SUPPLY
The power supply is a full wave bridge rectifier arrangement providing plus and minus 52 volts DC. Electronic
regulators step down and regulate plus and minus 15 volt
sources for low level circuits and plus 5 volts for the logic
circuits. Thermistors are used in the power transformer
primary circuit to limit the turn-on current.
18 TECHNICAL DESCRIPTION
BLOCK DIAGRAM 19
MclNTOSH MC 7270 STEREO POWER AMPLIFIER
"...Proves out
what my grandfather
told me, years ago:
'BUY THE BEST...
QUALITY LASTS"
Mclntosh owner
J. C. Mc. Orchard Lake, Ml
Music reproducing instruments
that carry the Mclntosh name
have always been designed to
maintain the Mclntosh reputation
for best sound, for durability and
for long life. Mclntosh has, since
1949, lead the industry in technological advancement. Mclntosh
has always earned the foremost
reputation for quality performance, Mclntosh has provided user
oriented facilities and appearance
and Mclntosh design always provides ease of maintenance or
repair. Regardless of the
Mclntosh combination you
choose you are always assured
you have chosen the best, with
the latest technology that leads to
superior sound reproduction...
technology whose integrity
has been proven by time.
XRT 18
Isoplanar Loudspeaker System
with optional base stand
MR 7082
AM/FM Tuner
C 31V
Home Entertainment Control Center
Handcrafted with pride in the United States by dedicated, highly trained craftspeople
20
MC 7270
Power Amplifier
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