IMPORTANT SAFETY INSTRUCTIONSTHESE INSTRUCTIONS ARE TOPROTECT YOU AND THE MclNTOSHINSTRUMENT. BE SURE TOFAMILIARIZE YOURSELF WITH THEM.

1. Read all instructions - Read the safety andoperating instructions before operating theinstrument.

2. Retain Instructions - Retain the safety andoperating instructions for future reference.

3. Heed warnings - Adhere to warnings andoperating instructions.

4. Follow Instructions - Follow all operating and useinstructions.

WARNING: TO REDUCE RISK OF FIRE ORELECTRICAL SHOCK, DO NOT EXPOSE THISINSTRUMENT TO RAIN OR MOISTURE.

5. Power Sources - Connect the power supply only tothe type described in the operating instructions oras marked on the unit.

6. Power-Cord Protection - Route power-supply cordsso that they are not likely to be walked on orpinched by items placed upon or against them,paying particular attention to cords at plugs,convenience receptacles, and the point where theyexit from the instrument.

7. Ventilation - Locate the instrument for properventilation. For example, the instrument shouldnot be placed on a bed, sofa, rug, or similarsurface that may block ventilation openings; or,placed in a built-in installation, such as a bookcaseor cabinet, that may impede the flow of airthrough the ventilation openings.

8. Heat - Locate the instrument away from heatsources such as radiators, heat registers, stoves, orother appliance (including amplifiers) that produceheat.

9. Wall or Cabinet Mounting - Mount the instrumentin a wall or cabinet only as described in theowners manual.

10. Water and Moisture - Do not use the instrumentnear water - for example, near a bathtub,washbowl, kitchen sink, laundry tub, in a wetbasement, or near a swimming pool, etc.

11. Cleaning - Clean the instrument by dusting with adry cloth. Clean the panel with a cloth moistenedwith a window cleaner.

12. Object and Liquid Entry - Do not permit objects tofall and liquids to spill into the instrument throughenclosure openings.

13. Power Lines - Locate any outdoor antenna awayfrom power lines.

14. Outdoor Antenna Grounding - If an outdoorantenna is connected to the antenna terminal, besure the antenna system is grounded to providesome protection against voltage surges and builtup static charge.In the U.S.A., section 810 of the National ElectricalCode, ANSI/NFPA No. 70-1987, providesinformation on the proper ground for the mastand supporting structure, ground for the lead-inwire to an antenna discharge unit, and size ofground conductors, location of antenna-dischargeunit, connection to grounding electrodes, andrequirements 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 groundwire.b) Secure antenna lead-in and ground wires tohouse 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 aspossible to where lead-in enters house.d) Use jumper wire not smaller than No. 6 AWG(13.3 mm2) copper or equivalent when separateantenna grounding electrode is used.

15. Nonuse Periods - Unplug the power cord from theAC power outlet when (eft unused for a longperiod of time.

16. Damage Requiring Service - Service must beperformed by qualified service personnel when:A. The power supply cord or the plug has beendamaged; orB. Objects have fallen, or liquid has been spilledinto the instrument; orC. The instrument has been exposed to rain; orD. The instrument does not appear to operatenormally or exhibits a marked change inperformance; orE. The instrument has been dropped, or theenclosure damaged.

17. Servicing - Do not attempt to service beyond thatdescribed in the operating instructions. All otherservice should be referred to qualified servicepersonnel.

18. Grounding or Polarization - Do not defeat theinherent design features of the polarized plug,Non-polarized line cord adaptors will defeat thesafety provided by the polarized AC plug.

19. CAUTION: TO PREVENT ELECTRICAL SHOCK DONOT USE THIS (POLARIZED) PLUG WITH ANEXTENSION CORD, RECEPTACLE OR OTHEROUTLET UNLESS THE BLADES CAN BE FULLYINSERTED TO PREVENT BLADE EXPOSURE.

Note to CATV system installer:This reminder is provided to call the CATV systeminstaller's attention to Article 820-22 of the NECthat provides guidelines for proper groundingand, in particular, specifies that the cable groundshall be connected to the grounding system ofthe building, as close to the point of cable entryas practical.

ATTENTION: POUR PREVENIR LES CHOCSELECTRIQUES PAS UTILISER CETTE FICHEPOLARISEE AVEC UN PROLONGATEUR, UNE PRISEDE COURANT OU UNE AUTRE SORTIE DECOURANT, SAUF SI LES LAMES PEUVENT ETREINSEREES A FOND SANS EN LAISSER AUCUNEPARTIE A DECOUVERT.

The lightning flash with arrowhead, within anequilateral triangle, is intended to alert the user tothe presence of uninsulated "dangerous voltage"within the product's enclosure that may be ofsufficient magnitude to constitute a risk of electricshock to persons.

CAUTION: TO PREVENT THE RISK OF ELECTRICSHOCK, DO NOT REMOVE COVER (OR BACK). NOUSER-SERVICABLE PARTS INSIDE. REFER SERVICINGTO QUALIFIED PERSONNEL.

The exclamation point within an equilateral triangleis intended to alert the user to the presence ofimportant operating and maintenance (servicing)instructions in the literature accompanying theappliance.

The serial number, purchase date, and MclntoshLaboratory Service Contract number are important toyou for possible insurance claim or future service.Record this information here.

Serial Number

Purchase Date

Service Contract Number

Upon application, Mclntosh Laboratoryprovides a Service Contract to the originalpurchaser. Your Mclntosh AuthorizedService Agency can expedite repairs whenyou provide the Service Contract with theinstrument for repair.

Copyright 1986 © by Mclntosh Laboratory Inc.

1

ContentsINTRODUCTION 3, 4, 5

INSTALLATION 6

HOW TO CONNECT 1, 8

CONNECTIONS 9

FRONT PANEL CONTROLS 10, 11, 12

REAR PANELINFORMATION 13

PERFORMANCE LIMITS 14

PERFORMANCE CHARTS 15, 16

TECHNICAL DESCRIPTION 17, 18

BLOCK DIAGRAM 19

Your MC 7270 Stereo Power Amplifier will give youmany years of satisfactory performance.If you have any questions, please contact:

CUSTOMER SERVICE

Mclntosh Laboratory Inc.2 Chambers StreetBinghamton, New York 13903-9990Phone: 607-723-3512

Take Advantage of 3 yearsof Contract Service...Fill in the Application NOW.

MclNTOSH THREE YEAR SERVICE CONTRACTAn 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 themeasured performance of the instrument to theoriginal performance limits free of any charge. Theservice contract does not cover any shipping coststo and from the authorized service agency or thefactory.

2. Any Mclntosh authorized service agency willrepair all Mclntosh instruments at normal servicerates. To receive the free service under the termsof the service contract, the service contract cer-tificate must accompany the instrument whentaken to the service agency.

3. Always have service done by a Mclntoshauthorized service agency. If the instrument is

modified or damaged as a result of unauthorizedrepair the service contract will be cancelled.Damage by improper use or mishandling is notcovered by the service contract.

4. The service contract is issued to you as theoriginal purchaser. To protect you frommisrepresentation this contract cannot betransferred to a second owner.

5. Units in operation outside the United States andCanada are not covered by the Mclntosh FactoryService Contract, irrespective of the place ofpurchase. Nor are units acquired outside the USAand Canada, the purchasers of which shouldconsult with their dealer to ascertain what, if any,service contract or warranty may be availablelocally.

2

The Mclntosh MC 7270 is the first and only amplifierspecially designed to fulfill Digital Dynamic Rangedemands. It outperforms all others when listening tosound derived from digitally recorded tapes, records andcompact discs. The MC 7270 has been designed to per-form flawlessly because of this capacity for overload: 10decibels of overstress at less than an average of 0.3% ofdistortion! Mclntosh amplifiers with POWER GUARD arethe only amplifiers which can tolerate 10 decibels ofdynamic overload, without severe distortion breakup.

The noise level of digitally recorded sound is 30decibels below that of conventional analog recordings.Thirty decibels reduction of noise level means that thedigital disc noise power is 1000 times less than that onthe best analog records. The compact disc is capable ofreal life dynamic range while noise generated from com-pact discs is inaudible. With the noise restraint removed itis both easier and dramatically more enjoyable to listen tomusic at much louder levels. To fully enjoy this newcapability your amplifier must be able to receive three toten decibels of over stress from music, and it must do thiswithout severely distorting the sound!

For an amplifier to handle a three-decibel overload, itmust have a full time capacity of twice its full power. Anover stress demand of 10 decibels is a demand for 10times the full power capacity of an amplifier. To provide2500 watts of overload for a 250 watt amplifier is expen-sive both in the amplifier and in the loudspeaker systemas well. This is the real world of Digital Dynamics Rangedemand. How to achieve the performance demanded,which often lasts from minutes to only a few thousandthsof a second, and to achieve the goal economically, is areal achievement. In each Mclntosh MC 7270, there is aunique, patented*, digital sonic overload corrector whichprevents the amplifier from exceeding an average of 0.3%distortion for overload stresses up to 10 decibels!

Mclntosh has developed a new method of measuringamplifiers which shows this capability very dramatically.This is a test of the Spectral Fidelity of an amplifier understress.

SIMULATING COMPLEX CRESCENDO DEMANDSTwo tones, 14 kHz and 15 kHz, are fed to the input of

the amplifier under test. The output of the amplifier is fedto a resistive dummy load across which is an A.C.voltmeter, (to allow computation of the amplifier's outputpower), and to a spectrum analyzer which displays themagnitude and frequencies of signals at the amplifier out-put.

Ideally, only the two test tones should be reproduced atthe amplifier output. Some amplifiers generate a broad in-termodulation spectrum of tones corresponding to the

sum and difference of the test tones. These spurioustones are called Intermodulation Distortion.

All amplifiers (Mclntosh amplifiers with Power Guard arethe only exception), produce such a spectrum when theyare driven beyond their output capacity. They produce In-termodulation and Harmonic Distortion, the severity ofwhich depends on the amount of overdrive and thedesign of the amplifier. Distortion of 30% or greater is notuncommon. Most of the spurious frequency componentsare at high frequencies and can easily destroy the tweetersections of loudspeakers, while severely distorting thesound.

*U.S. Patent #4048573

INTRODUCTION 3

In these oscillograms, you can see the difference inSpectral Fidelity between a stressed Mclntosh, and otheramplifiers.1. The Mclntosh stressed 10 dB above full power2. An imported amplifier stressed 10 dB above full power3. A domestically manufactured amplifier which had to betested "under-stressed" since it would not perform with

10 dB of overdrive.

The Mclntosh shows only 3 components, which are 44for one and 50 dB down for all others, roughly equivalentto 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 thattheir Mclntosh alone meets the test for accuracy, the testfor clarity, the test for musicality even when a peak of tentimes the power demand suddenly smashes into thepower amplifier.

When other amplifiers are similarly stressed theygenerate large quantities of discordant sounds destroyingthe real musicality of the reproduced instruments.

Note in oscillogram number 3 the complete failure ofone of the most popular of American amplifiers. If 14 and15 kHz are amplified at the same time the amplifier shutsdown by "motor boating". It was necessary to lower thefrequency of the test signals to 10 and 11 kHz to evencomplete the test or to perform the test at less than halfrated 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 answersfor you. But if the best is what you need then there is on-ly one real answer.

Mclntosh—the amplifier that in 40 years has outlived 60others who have simply faded away.

4 INTRODUCTION

A NEW AMPLIFIER DESIGNED FORA NEW AGE OF HOME ENTERTAINMENT

The MC 7270 stereo power amplifier is designed tooperate with loudspeakers having a nominal impedanceof 1, 2, 4, or 8 ohms. The amplifier is rugged and reliable.

The mechanical and electrical design of the MC 7270 isthe result of the many years of engineering and manufac-turing experience by the designers at Mclntosh. This"know how", combined with meticulous attention todesign and production details, makes the MC 7270 one ofthe finest products produced by Mclntosh Laboratory.

Some manufacturers of power amplifiers advertise thattheir products do not use protection circuits and that suchcircuits compromise performance. It is indeed possible forsuch circuits to cause substantial amounts of distortionand undesirable listening effects which is true of almostany protective circuit design. The real genius of gooddesign recognizes these problems and circumvents themwhile retaining the real merits of the protective circuits.These are just some of the extra values you receive whenyou invest in Mclntosh equipment. It is precisely for thisreason that it takes longer to complete a Mclntoshengineering design task. From such engineering dedicationcomes the Mclntosh reputation for highest sound qualitywith greatest long term reliability.

The MC 7270 incorporates seven protection circuitswhich insure its total reliability, seven protection circuitsthat protect the music and your listening, not interferewith them. These circuits are described in the TechnicalDescription.

INTRODUCTION 5

The MC 7270 may be installed in a Mclntosh cabinet orcustom installed in furniture of your choice. Always pro-vide adequate ventilation. Make sure there is 1½ inches(3.8 cm) of space above the amplifier so as not to in-terfere with a cooling flow of air.

CUSTOM INSTALLATIONThe PANLOC system of installing equipment convenient-

ly and securely, is a product of Mclntosh research. Turnedclockwise, the PANLOC buttons on the front panel lockthe chassis firmly in place. A counterclockwise turn of thePANLOC buttons unlocks the chassis from its mounting.

To install the instrument in a Mclntosh cabinet, followthe instructions that are enclosed with the cabinet. Forany other type of installation follow these instructions:

1. Unpack from CartonOpen the carton and remove the PANLOC brackets, hard-ware package, and mounting template. Remove the in-strument from its plastic bag and place it upside down onthe shipping pallet. Unscrew the four plastic feet from thebottom of the chassis.

2. Mark the Cabinet Panel.Tape the mounting template in position on the cabinetpanel where the instrument is to be installed. The brokenlines that represent the outline of the rectangular cutoutalso represent the outside dimensions of the chassis.Make sure these lines clear shelves, partitions, or anyequipment. With the template in place, first mark the sixA and B holes and the four small holes that locate thecorners of the cutout. Then, join the four corner markingswith pencil lines, using the edge of the template as astraightedge.

3. Drill HolesUse a drill with a 3/16 inch (5 mm) bit held perpendicularto the panel and drill the six A and B holes. Then, using adrill bit slightly larger than the tip of your saw blade, drillone hole at each of two diagonally opposite corners. Theholes should barely touch the inside edge of the penciledoutline. Before taking the next step, make sure that the sixA and B holes have been drilled.

4. Saw the Panel CutoutSaw, carefully, on the inside of the penciled lines. Firstmake the two long cuts and then the two short cuts.After the rectangular opening has been cut out, use a fileto square the corners and smooth any irregularities in thecut edges.

5. Install the Mounting StripsIn the hardware package you will find two mountingstrips, and two sets of machine screws. For panels thatare less than 1/2 inch (12.7 mm) thick, use the 3/4 inch

(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 ascrew of the proper length into the center hole in thepanel, marked B on the template. On the back of thepanel, align a mounting strip with the holes in the paneland tighten the screw until the screwhead is pulled intothe wood.

Repeat this procedure to attach the mounting strip tothe left side of the panel.

6. Attach the PANLOC BracketsUsing two screws of the proper length in the A holes oneach side, attach the PANLOC brackets to the cabinetpanel; the short flange is mounted against the front (face)of the cabinet panel. The screws pass through thePANLOC bracket flange, the cabinet panel, and thenthrough the mounting strips previously mounted.

7. Install the InstrumentGuide the AC power cord through the panel opening tothe back of the cabinet; then, slide the instrument intothe opening carefully so that the rails on the bottom ofeach side of the chassis engage the tracks on the moun-ting brackets. Continue to slide the instrument into thecabinet until the front panel is flush with the cabinetpanel. Turn the PANLOC buttons at the lower left andright corners of the instrument panel clockwise to lockthe unit firmly in the cabinet. Turn the PANLOC buttonscounterclockwise to unlock the instrument. It can thenslide outward to permit the removal of the instrumentfrom the cabinet.

6 INSTALLATION

MOUNTING TEMPLATE

Use shielded cables to connect the signal from thepreamplifier or signal source to the power amplifier. Tominimize the possibility of hum, the shielded cablesshould be of parallel construction or loosely twistedtogether, located away from speaker connecting cablesand AC power cords. Be certain to use good qualityshielded cables for all interconnections. Your dealer canadvise you on the kind and length of cable that will bestsuit your installation.

The appropriate length and size of loudspeaker cable foryour installation will help to preserve the quality of soundfor which the loudspeakers have been designed. If under-size wire is use, resistance is added to the amplifier/loudspeakers combination which adversely affects the per-formance. Added resistance reduces the damping factor,modifies the frequency response and reduces the poweroutput. Your dealer's advice will serve you best for yourinstallation. The cables to and from the speaker should beof parallel construction or be loosely twisted together.The chart shows the recommended minimum wire sizefor the length of wire between the amplifier and theloudspeakers.

SPEAKER CABLE LENGTHSAMPLIFIER TO SPEAKER

For 4 Ohm Load

Feet152540

60100

Meters4.67.612.218.3

30.48

For 8 Ohm Load

Feet305080120

200

Meters9.115.224.4

36.660.0

Wire

Gauge181 614

12

10

These speaker cable lengths represent a wireresistance equal to 5% of the speaker impedance.

CONNECTING FOR STEREO OPERATIONPlug the left output of the preamplifier into the Left inputjack of the power amplifier. Plug the right output of thepreamplifier into the Right (Mono) input jack of the poweramplifier.

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 terminalstrip COMmon screw. Connect another speaker cablefrom the other loudspeaker terminal to the correctspeaker impedance terminal on the LEFT OUTPUT ter-minal strip. The right channel speaker is connected in thesame manner to the RIGHT OUTPUT terminal strip.

When multiple speakers are to be connected to eitheror both outputs, the combined impedance must becalculated. The combined impedance must be connected

to the appropriate impedance tap. Use this table to aid inselecting 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 im-pedance tap, reduced power and possible distortion willresult. If the load impedance used is higher than the out-put impedance tap, neither the quality of sound nor theamplifier will be harmed, only the available power isreduced. For multiple speaker operation, run separatespeaker cables from the amplifier to each speaker.

The MC 7270 will feed a constant voltage line as is oftenused in background music applications, paging systems andthe like. For constant voltage of 25 volts connect output leadsto the 4 ohm connectors on each channel.

Because the crosstalk between channels is almost nonexistent, each channel can be used as a separateamplifier. (Example; use one channel for monobackground program in one area and the other channelfor paging in a separate area.)

CONNECTING FOR MONOPHONIC BRIDGE OR SINGLECHANNEL OPERATION

When the MC 7270 is used as a monophonic or singlechannel power amplifier, the two channels are added in abridge configuration to produce output up to 540 watts.For monophonic operation using the MONO-BRIDGEmode, the amplifier can supply full power to output im-pedances of 2, 4, 8 and 16 ohms.

Plug a shielded cable from the signal source orpreamplifier to the RIGHT (MONO) input jack only.Change the MODE switch on the back panel of theamplifier to the MONO-BRIDGE position. The MONO-BRIDGE switch internally connects the output of the rightchannel input amplifier to both left and right channelpower amplifiers with the phase of the left channel in-verted to achieve bridge operation and increased poweroutput.

BE CERTAIN THE MC 7270 IS NEVER OPERATED WITH THEBACK PANEL MODE SWITCH IN THE STEREO POSITIONWITH THE AMPLIFIER CONNECTED FOR MONO BRIDGE.

Operated with the MODE switch in the MONO-BRIDGEposition, the circuit subtracts the left channel from theright channel L-R). The remaining difference signal is allthat is then fed to the speaker.

HOW TO CONNECT 7

The MONO-BRIDGE operation of the MC 7270 amplifieralways requires the speaker load to be connected be-tween two opposite channel impedance terminals. Thecommon terminals are not used. Do not connect leads in-dependently between left and right channel impedancetaps as this will cause high circulating currents andoverheating. Connect the speaker leads to the output ter-minals as listed in this chart:

Load Impedance Connect the Connect theIn Ohms -Speaker Cables to: +Speaker Cables to:

2 Left 1 Ohm Terminal Right 1 Ohm Terminal4 Left 2 Ohm Terminal Right 2 Ohm Terminal8 Left 4 Ohm Terminal Right 4 Ohm Terminal16 Left 8 Ohm Terminal Right 8 Ohm 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 ohmMONO-BRIDGE output.

8 HOW TO CONNECT

STEREOPHONICCONNECTIONS

MUST BE INSTEREO POSITIONFOR S T E R E O PROGRAMS

RIGHTS P E A K E R

MONO-BRIDGEDCONNECTIONS

(CONNECTIONS FOR8 OHM S P E A K E R ILLUSTRATED)

CONNECTIONS SHOWN FOR MONOWHEN MODE SWITCHIS IN MONO-BRIDGE POSITION

CONNECTIONS 9

PROGRAM SOURCE

PROGRAM SOURCE

SPEAKER

LEFTSPEAKER

METERS

Output power monitor meters calibrated in both wattsand decibels, indicate the output power of each channel.The upper scale on the meter has been calibrated toshow average watts and the lower scale, decibels.Reading from right to left, the marks between thenumbers indicating watts are (from the indicated 270watts): first mark, 108 watts. Second 54 watts, the in-dicated 27 watts, 10.8 watts, 5.4 watts, the indicated 2.7watts, 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 shortinterval information in a sound wave. The mass of themeter movement is too great to respond to the nearly in-stantaneous changes in music program material. Short in-terval information can have a duration as short as half athousandth of a second. Ordinarily, a meter pointer mov-ing over its scale in such a short time could not be seen.Mclntosh has developed circuits that drive the meters torespond to the short interval information in a sound waveto an accuracy of 90%. The electrical pulse that drives themeter pointer is time stretched so that the peak positionof the pointer can register in the persistence of visioncharacteristic of the retina of the human eye. The pointercan be locked at the peak indication by switching to theHOLD position of the METER range switch.

LEFT CAIN

Use the LEFT GAIN control to adjust the output in theleft channel to the desired listening level. Turn the controlclockwise to increase the output.

RIGHT/MONO GAINUse the RIGHT/MONO GAIN control to adjust the out-

put in the right channel to the desired listening level. Turnthe control clockwise to increase the output.

When connected for monophonic operation use theRIGHT/MONO GAIN to adjust the total output to thedesired listening level. The rear panel MODE SWITCHmust be in the MONO-BRIDGE position.

10 FRONT PANEL CONTROLS

METER RANGE

The METER RANGE switch has two positions: WATTSand HOLD.WATTS

In the WATTS position, the meter needle indicates thevariations in program loudness. Although the primary out-put calibration of the meters is from 0.0027 watts (2.7milliwatts) up to 270 watts, the rated power output of theMC 7270, the additional indication to the right of the 270watts mark, is 540 watts. While the MC 7270 cannotreach this power level continuously it is possible for shortinterval peaks to considerably exceed the 270 watts con-tinuous rating.

HOLDIn the HOLD position, the meter needle locks to the

highest power peak in a sequence of peaks. The meter isdriven to maximum power, electronically held there untila higher peak passes through the amplifier, which movesthe meter needle to a new indication. If no further peaksare reached the meter needle will very slowly return toits rest position (decay rate: 6 dB per minute).

POWERThe power switch turns the MC 7270 ON or OFF. The

switch does not control the power outlet on the backpanel. If you wish to control the AC power from apreamplifier control center, leave the switch in the ONposition. Be sure the AC cord of the MC 7270 is pluggedinto the controlled outlets on the rear of the preamplifiercontrol center.

OFF: In the OFF position the AC power to the amplifieris turned off.

FRONT PANEL CONTROLS 11

THE MclNTOSH (EXCLUSIVE) POWER GUARDDIGITAL DYNAMICS PROTECTION CIRCUIT

Improved recordings and recording techniques have im-posed higher power demands on today's amplifiers. Poor-ly designed amplifiers can present music listeners with aform of harsh unpleasant distortion due to amplifieroverload (hard clipping). Clipping, which looks and actslike non musical square waves, is caused when theamplifier is asked to produce more power output withlow 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 decreasesthe pleasure and enjoyment you get from listening. Thisform of distortion (clipped signal) also produces extra heatenergy which will damage most speakers. Mclntoshleadership in engineering has developed the Power Guardcircuit which - (1) dynamically prevents power amplifiersfrom being overdriven into hard clipping - (2) assures thatthe amplifier will produce its maximum output without in-creased distortion - (3) protects your speaker from ex-cessive heating. Power Guard is a patented Mclntoshdesign (U.S. patent #4048573).

The MC 7270 has a circuit that compares the waveshape of the output signal to the input signal. If thedisparity between the two signals, due to overdrive, ex-ceeds an average of 0.3% (equivalent to 0.3% total har-monic distortion) an amber POWER GUARD indicator il-luminates. With any further increase in distortion thePOWER GUARD circuit operates to limit the amplifier in-put dynamically so that the amplifier cannot be over-driven. POWER GUARD eliminates amplifier output clip-ping. POWER GUARD only operates when the amplifier isasked to deliver more power than that for which it wasdesigned. While the power output remains within theselimits the POWER GUARD indicators do not illuminate.

12 FRONT PANEL CONTROLS

MODE SWITCHThe MC 7270 will operate in two modes, Stereo and

Mono Bridge.

LEFT AND RIGHT OUTPUT TERMINALSFor stereo operation, output impedances of 1, 2, 4 and

8 ohms have been provided on a secure, screw type bar-rier strip. For rnonophonic operation proper interconnec-tion provides 2, 4, 8, and 16 ohms at the same barrierstrips. See page 7 for connecting instructions.

INPUT JACKSIn the stereo mode of operation, both Left and Right in-

put jacks function. In the mono mode of operation onlythe Right (MONO) channel input jack accepts signal. Inmono, the Left channel input jack is disconnected.

INPUT LEVELThe input sensitivity of the MC 7270 is 0.75 volts or

2.5V depending on the position of the INPUT LEVELswitch. For the best signal to noise ratio when using Mcln-tosh source equipment, place the INPUT LEVEL switch inthe 2.5V position and the front panel LEFT andRIGHT/MONO GAIN controls in the fully clockwise posi-tion. If more gain is desired the 0.75V position may beused. All Mclntosh preamplifiers have been designed todeliver 2.5 volts output with rated input. For sourceequipment other than Mclntosh, set the switch in theposition nearest to the stated output rating of the sourceequipment.

AC POWERThe input to the MC 7270 is 120 volts 50/60 Hz at up to

12 amps. The primary circuit is protected by a 15 AmpFuse.

REAR PANEL INFORMATION 13

PERFORMANCE GUARANTEEPerformance Limits are the maximum deviation from

perfection permitted for a Mclntosh instrument. We pro-mise you that when you purchase a new MC 7270 from aMclntosh franchised dealer, it will be capable of or can bemade capable of performance at or exceeding these limitsor you can return the unit and get your money back.Mclntosh is the only manufacturer that makes this state-ment.

PERFORMANCEMclntosh audio power ratings are in accordance with

the Federal Trade Commission Regulation of November 4,1974 concerning power output claims for amplifiers usedin home entertainment products.

POWER OUTPUTSTEREO270 watts minimum sine wave continuous average poweroutput, 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 ohm23.2 volts RMS across 2 ohms32.9 volts RMS across 4 ohms46.5 volts RMS across 8 ohms

MONO-BRIDGE540 watts minimum sine wave continuous average

power output into 2 ohms, 4 ohms, 8 ohms, or 16 ohmsload impedance, which is:

32.9 volts RMS across 2 ohms46.5 volts RMS across 4 ohms65.7 volts RMS across 8 ohms93.0 volts RMS across 16 ohms

OUTPUT LOAD IMPEDANCESTEREO

1 ohm, 2 ohms, 4 ohms, and 8 ohms; separate terminalsare provided for each output.MONO-BRIDGED

2 ohms, 4 ohms, 8 ohms, or 16 ohms, balanced toground.

RATED POWER BAND20 Hz to 20,000 Hz

TOTAL HARMONIC DISTORTIONSTEREO

0.02% maximum harmonic distortion at any power levelfrom 250 milliwatts to 270 watts per channel from 20 Hzto 20,000 Hz, both channels operating.

MONO0.02% maximum harmonic distortion at any power level

from 250 milliwatts to 540 watts from 20 Hz to 20,000Hz.

INTERMODULATION DISTORTIONSTEREO

0.02% maximum if instantaneous peak poweroutput is 540 watts or less per channel with bothchannels operating for any combination of fre-quencies, 20 Hz to 20,000 Hz,

MONO0.02% maximum if instantaneous peak power

output is 1080 watts or less for any combination offrequencies, 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 HUM100 dB below rated output.

RATINGSDAMPING FACTOR

Greater than 30

INPUT IMPEDANCE20,000 ohms.

INPUT SENSITIVITYSwitchable: 0.75 volt or 2.5 volts-level control provided

for higher input voltages.

POWER GUARDClipping is prevented and THD does not exceed 2% with

up to 20 dB overdrive a 1 kHz.

GENERAL INFORMATIONPOWER REQUIREMENTS

120 volts 50/60 Hz; 13 amps.

SEMICONDUCTOR COMPLEMENT79 silicon transistors31 silicon rectifiers and diodes7 integrated circuits

MECHANICAL INFORMATIONSIZE

16 3/16 inches wide (41.1 cm) by 7 1/8 inches high (18.1cm) by 14 1/2 inches deep (36.8 cm), including connec-tors. Knob clearance required is 1 1/4 inches (3.2 cm) infront of mounting panel.

FINISHThe front panel is a combination of glass and black

anodized aluminum. The chassis is black.

WEIGHT82 pounds (37.2 kg) net, 96 pounds (43.5 kg) in shipping

carton.

14 PERFORMANCE LIMITS

PERFORMANCE CHARTS 15

20

FREQUENCY IN Hz

100 1k 10k 20k

0

-10

-20

-30

-40

-50

-60

-70

-80

-90

FREQUENCY IN Hz

100 1k 10k

FREQUENCY IN Hz

100 1k

MC 7270HARMONIC DISTORTION VS FREQUENCYAT RATED POWER

MC 7270FREQUENCY RESPONSE AT 1 WATT OUTPUT

MC 7270CHANNEL SEPARATION VS FREQUENCY

OUTPUT IMPEDANCE: 8INPUT IMPEDANCE: 1k

0.05

0.04

0.03

0.02

0.01

0

HA

RM

ON

IC D

IST

OR

TIO

N I

N %

20 10k 20k

0

-1

-2

-3

-4

-5

-6

-7

-8

-9

-10

10

OU

TP

UT

IN

dB

100k

OU

TP

UT

IN

dB

0.1

POWER OUTPUT IN WATTS

1000

0-1

POWER OUTPUT IN WATTS

1000

16 PERFORMANCE CHARTS

1.0 10 100

0,06

0.05

0.04

0.03

0.02

0.01

0

IM D

IST

OR

TIO

N I

N %

MC 7270

INTERMODULATION DISTORTION VS POWER OUTPUT

BOTH CHANNELS DRIVEN

FREQUENCY IN Hz

100 1k 10k

MC 7270 POWER OUTPUT VS FREQUENCY0.02% HARMONIC DISTORTION

BOTH CHANNELS OPERATING INTO 8 OHMS

1 10 100

0.05

0.04

0.03

0.02

0.01

0

HA

RM

ON

IC D

IST

OR

TIO

N I

N %

MC 7270HARMONIC DISTORTION VS POWER OUTPUT

BOTH CHANNELS OPERATING INTO 8 OHMS

20kHz1kHz

20Hz

300

400

300

200

100

010

PO

WE

R O

UT

PU

T IN

W

AT

TS

100k

INPUT AMPLIFIER

Each channel input contains a complete seven transistorlow power amplifier. A differential transistor pair provideshigh input impedance with low noise. The differentialsignals are combined in a current mirror circuit whichdrives a class A amplifier stage. The following outputstage is a complimentary pair of transistors with class ABbiasing. The output signal drives the metering circuit andthe high power output amplifier. This discrete transistorinput amplifier design has low noise, low distortion andfreedom from turn-on/turn-off transients.

The INPUT LEVEL selector and GAIN controls (as passiveattenuators) which precede the input amplifier, prevent in-put overloaded when the controls are correctly set.

In the MONO BRIDGE mode of operation the inputsignal feeds only the right input amplifier using theRIGHT/MONO GAIN control. The output of the right inputamplifier feeds both output power amplifier sections withthe left input amplifier as a phase inverter to the left out-put amplifier. The phase inversion puts the outputamplifiers 180° out of phase with respect to each otherto 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 poweramplifier input through a junction FET electronic switch.The control signal to the switch is held for about one se-cond to eliminate turn-on/turn-off transients and preventtransients that may originate in the source equipmentfrom being amplified and fed to the loudspeakers.

The first stage of the output power amplifier is a dif-ferential transistor pair biased for best linearity. The offsetto the differential pair is adjustable. Correct adjustmentassures the lowest possible distortion at low frequencies.In each channel, current mirror circuit combines the dif-ferential outputs into one signal which is amplified by aclass A voltage amplifier. Both the differential transitorsand the voltage amplifier are supplied by active currentsources. The results are lower distortion and cleaner turn-on characteristics.

The driver stage consists of a complimentary pair ofpower transitors biased class AB. Three complimentarypairs of rugged power transistors mounted on over-sizedanodized aluminum heat sinks, make up the power out-put stage. A unique design of the bias network permitsthe output transistors to operate class B but prevents thecrossover distortion associated with class B operation. Theheat sinks remain cool when there is no output.

The amplifier output signal is fed to the output terminalsthrough the output autotransformer. The Mclntosh design-ed interleaved multifi lar wound autotransformer is used to

properly match the amplifier to stereo output load tapsfor 1, 2, 4 and 8 ohms. The MC 7270 will deliver fullpower over the entire audio frequency range at any ofthese impedances. The autotransformer also protectsspeakers from damage in the event of amplifier failure.Should a direct current component appear in the output itis shunted to ground by the autotransformer. DC cannotdamage the speaker.

A Mclntosh patented Sentry Monitoring circuit constantlymonitors the output signal and instantly reacts to preventoverload of the output transistors. At signal levels up todesign maximum the circuit has high impedance and hasno effect upon the output. Should the power output ex-ceed design maximum, the Sentry Monitoring circuitoperates to limit the signal to the output transistors. Inthe event of a short circuit across the amplifier output orsevere impedance mismatch the Sentry Monitoring circuitwill protect the output transistors from failure. Positiveand negative halves of the output signal are independent-ly monitored and protected.

POWER GUARD PROTECTION CIRCUIT

The Mclntosh patented Power Guard circuit eliminatesamplifier clipping due to overdrive. The circuit illuminatesamber POWER GUARD indicator lamps when theamplifier drive trys to exceed the maximum output capaci-ty. Power Guard prevents loudspeaker damage andeliminates harsh output distortion caused by amplifier clip-ping.

In the Power Guard circuit, the output waveform is com-pared to the input waveform. As long as there isn't anydisparity between these signals, the circuit hasn't any in-fluence on the performance of the amplifier. Should theamplifier drive try to exceed the amplifier's maximumpower capacity, a difference will develop. If the disparityexceeds 0.03% on the average, (equivalent to 0.3% totalharmonic distortion) the difference causes the amberPOWER GUARD indicators to light. A further increase inthe disparity, controls an electronic attenuator at theamplifier input to reduce the amplifier gain, thus holdingthe amplifier output to its maximum undistorted valueregardless of the degree of overdrive to the amplifier.

The difference signal is fed to a specially compensatedoperational amplifier integrated circuit. Its output isdetected by a full wave bridge that feeds signals to thecontrol circuitry for the POWER GUARD indicators and tothe electronic attenuator at the amplifier input. The at-tenuator is a light emitting diode/light dependent resistornetwork selected specially for its low distortion and timeconstant characteristics.

TECHNICAL DESCRIPTION 17

METER CIRCUITThe 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 opera-

tional amplifier with a bipolar connected silicon diodepair as feedback elements. These diodes have a uniformlogarithmic characteristic over an 80 dB range. Only 60 dBof this logarithmic range is used in the MC 7270.

The full wave rectifier circuit uses an operational ampli-fier with silicon diode feedback networks. This amplifieddiode circuit has nearly perfect rectification characteristics.One rectifier detects only positive signals. The otherresponds only to negative signals and produces a positiveoutput. The outputs of the rectifiers are combined at theoperational amplifier output, so the highest signal, eitherpositive or negative, is the one that is indicated by themeters. Gate diodes are used to charge a low leakagecapacitor which attains and holds a charge during signalpeaks. The operational amplifier provides a large amountof current so the peak holding capacitor can charge sud-denly. The charge on the peak holding capacitor isamplified in a two transistor DC amplifier which is used todrive the meter. The output of the amplifier has a DCfeedback network connected to the detector to assure ex-cellent overall linearity and frequency response. The cur-rent drive to the meters has a peaking capacitor to ac-celerate the upscale response of the meter needle. Themeters also have a parallel shunt resistor to correctlydamp their action. In the WATTS mode the discharge ofthe peak holding capacitor is controlled by a resistor cur-rent source. In WATTS HOLD, the resistor is disconnectedso the peak reading is retained. The rate of decay inWATTS HOLD is about 6 dB per minute.

POWER SUPPLYThe power supply is a full wave bridge rectifier arrange-

ment providing plus and minus 52 volts DC. Electronicregulators step down and regulate plus and minus 15 voltsources for low level circuits and plus 5 volts for the logiccircuits. Thermistors are used in the power transformerprimary circuit to limit the turn-on current.

18 TECHNICAL DESCRIPTION

BLOCK DIAGRAM 19

MclN

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SH

MC

7270

ST

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PO

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MP

LIF

IER

"...Proves outwhat my grandfathertold me, years ago:

'BUY THE BEST...QUALITY LASTS"

Mclntosh ownerJ. C. Mc. Orchard Lake, Ml

XRT 18Isoplanar Loudspeaker Systemwith optional base stand

MR 7082AM/FM Tuner

C 31VHome Entertainment Control Center

Handcrafted with pride in the United States by dedicated, highly trained craftspeople

MC 7270Power Amplifier

20

Music reproducing instrumentsthat carry the Mclntosh namehave always been designed tomaintain the Mclntosh reputationfor best sound, for durability andfor long life. Mclntosh has, since1949, lead the industry in tech-nological advancement. Mclntoshhas always earned the foremostreputation for quality perform-ance, Mclntosh has provided useroriented facilities and appearanceand Mclntosh design always pro-vides ease of maintenance orrepair. Regardless of theMclntosh combination youchoose you are always assuredyou have chosen the best, withthe latest technology that leads tosuperior sound reproduction...technology whose integrityhas been proven by time.