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THEMclNTOSHMC2255SOLIDSTATESTEREOPOWERAMPLIFIE

eading Time: 31 Minutes Price

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V A R I O U S R E G U L A T O R Y A G E N C I E S R E Q U I R E T H A T W E B R I N G T H E F O L L O W I N G

IN F O R M A T IO N T O Y O U R A T T E N T IO N . P LE A S E R E A D IT C A R E F U L L Y .

WARNING: TO PREVENT FIRE OR SHOCKHAZARD, DO NOT EXPOSE THIS UNIT TO

RAIN OR MOISTURE.

The Mclntosh you have purchased is a Model

MC 2255. It has a serial number located on the rear panel

of the chassis. Record that serial number here:

Serial Number

The model, serial number and purchase date are im-

portant to you for any future service. Record the pur-

chase date here:

Purchase date

Upon application, Mclntosh Laboratory provides a

Three-Year Service Contract. Your Mclntosh authorized

Service Agency can expedite repairs when you provide

the Service Contract with the instrument for repair. To

assist, record your Service Contract number here:

Service Contract Number

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Your MC 2255 Stereo Power Amplifierwill give you many years of pleasantand satisfactory performance. If youhave any questions, please contact:

CUSTOMER SERVICE

Mclntosh Laboratory Inc.2 Chambers Street

Binghamton, New York 13903Phone: 60 7-723-3512

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. Mclntosh will provide all parts, materials

and labor needed to return the measuredperformance of the instrument to theoriginal performance limits. The SER-VICE CONTRACT does not cover anyshipping costs to and from the authoriz-ed service agency or the factory.

2. Any Mclntosh authorized service agencywill repair Mclntosh instruments at nor-mal service rates. To receive serviceunder the terms of the SERVICE CON-TRACT, the SERVICE CONTRACT CER-TIFICATE must be presented when theinstrument is taken to the service agency.

3. Al wa ys have s erv ice done by aMclntosh authorized service agency. If

the instrument is modified or damagedas a result of unauthorized repair, theSERVICE CONTRACT will be cancelled.Damage by improper use or mishandl-ing is not covered by the SERVICECONTRACT.

4. The SERVICE CONTRACT is issued toyou as the original purchaser. To pro-tect you from misrepresentation, this

contract cannot be transferred to a se-cond owner.

5. To receive the SERVICE CONTRACT,your purchase must be made from aMclntosh franchised dealer.

6. Your completely filled in application forthe SERVICE CONTRACT must be post-marked within 30 days of the date ofpurchase of the instrument.

7. To receive the SERVICE CONTRACT, allinformation on the application must befilled in. The SERVICE CONTRACT willbe issued when the completely filled inapplication is received by MclntoshLaboratory Incorporated in Binghamton,New York.

8. Units in operation outside the UnitedStates and Canada are not covered bythe Mclntosh Factory Service Contract,irrespective of the place of purchase.Nor are units acquired outside theU.S.A. and Canada, the purchasers ofwhich should consult with their dealerto ascertain what, if any, service con-tract or warranty may be available locally.

Copyright 1981 (c) by Mclntosh Laboratory Inc.

C o n t e n t s

SERVICE 1INSTALLATION 2

HOW TO CONNECT 4FRONT PANEL INFORMATION 9

REAR PANEL INFORMATION 11

PERFORMANCE LIMITS AND RATINGS 12PERFORMANCE CHARTS 13

TECHNICAL DESCRIPTION 15BLOCK DIAGRAM 17

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H o w t o C o n n e c t

INPUTSTEREO OPERATION

Use shielded cables to connect the signal fromthe preamplifier or signal source to the power

amplifier. To minimize the possibilty of hum theshielded cables should be run parallel to each otheror loosely twisted together. Locate the cables awayfrom speaker leads and AC power cords. All connec-tions are made on the back panel of the MC 2255.

For stereo operation, the left output of thepreamplifier should be plugged into the Left input

jack of the power ampl if ie r. The right output of thepreamplifier should be plugged into the Right(Mono) input jack of the power amplifier.

In stereo operation the MODE SWITCH must be in

the STEREO position.MONOPHONIC OR SINGLE CHANNEL OPERATION

A shielded cable from the signal source is plug-ged into the Right (Mono) input jack of the MC 2255only. The MODE SWITCH on the back panel of theamplifier must be placed in one of the MONO posi-tions. In the MONO positions the output of the rightchannel input amplifier is fed to both left and rightpower amplifiers. The Left INPUT is disconnected.Only the signal fed into the Right (MONO) input willbe amplified.

Be certain that the MC 2255 is never operated in

the stereo mode with the outputs connected formonophonic operation. Should the MODE SWITCHbe left in the STEREO position and the outputs re-main strapped for a mono parallel load, one channelwill at tempt to drive the other which causes high cir -culating currents and overheating.

OUTPUTBe certain the loudspeakers connected to the MC

2255 are capable of handling the power output of theamplifier.

Selection of the proper gauge wire to connect the

loudspeakers preserves the quality of sound repro-

duction for which the loudspeakers have been de-signed. If undersize wire is used, resistance is addedto the amplifier/loudspeaker combination whichadversely affects the performance. Added resis-

tance causes depreciation of damping characteris-tics, modification of frequency response and reduc-tion in power output.

Use lamp cord or wire with similar insulation toconnect the speakers to the amplifier. In all cases,the leads to and from the speaker should be twinconductor or twisted together. When using 8 ohmspeakers and for the normally short distances ofunder 30 feet between the amplifier and speaker, #18 wire or larger can be used. For distances over 30feet use larger diameter wire. Select the correct sizewire from the chart below. It is recommended that

the DC resistance of the speaker leads be less than5% of the speaker impedance. Resistance of theleads should be computed for the length of wire bothto and from the speaker or speakers.

For multiple speaker operation, run separateleads from the amplifier to the speakers.

MA XIMU M WIRE LENGTHSFor 4 Ohm Load For 8 Ohm Load

Feet15

25

4060

100

Meters4.57

7.62

12.1918.29

30.48

Feet

30

50

80120

200

Meters9.14

15.24

24.3836.5860.96

Wire

Gauge1816

14

12

10

Wire lengths above represent the wire resistanceequal to 5% of the speaker impedance.

STEREO OPERATION

Check the impedance of the speaker which isusually identified on the speaker itself or in theowner's manual. Connect a lead from the commonterminal of the left speaker to the amplifier LEFT

OUTPUT terminal strip COMmon screw. Connect

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another lead from the other terminal of theloudspeaker to the left output terminal marked forthe impedance of the speaker on the LEFT OUTPUTterminal strip. The right channel speaker is con-nected in the same manner to the RIGHT OUTPUTterminal strip.

When multiple speakers are to be connected toeither or both outputs, the combined load im-pedance must be caluclated. The load must be con-

nected to the appropriate impedance tap. Use thistable to aid in selecting the correct impedancematch:

Loadimpedancein ohms

0.8 to 1.61.6 to 3.23.2 to 6.46.4 and up

Connect for:

1 ohm output2 ohm output4 ohm output8 ohm output

If a load impedance is used that is lower than theoutput impedance tap, then reduced power and pos-sible distortion will result. If a load impedance is us-ed that is higher than the output impedance tap,then neither the signal nor the amplifier will beharmed but the power available is reduced.

FOR STEREO CONSTANT VOLTAGE OPERATION:

For outputvol tage of

25 volts

Connect for:

4 ohms

For monophonic operation using the MONPARALLEL mode, output impedances of , 1, 2, a4 ohms are accomodated. Connect as listed beloThe common output terminal is at ground potent

Load ImpedanceIn Ohms

1

2

4

Connect - SpeakerLead To:

Either Left or Right

Common TerminalEither Left or RightCommon Terminal

Either Left or RightCommon Terminal

Either Left or RightCommon Terminal

Connect + SpeakLead To:

Both Left and Rig

1 Ohm TerminaBoth Left and Rig2 Ohm Termina

Both Left and Rig4 Ohm Termina

Both Left and Rig8 Ohm Termina

FOR MONOPHONIC CONSTANT VOLTAGE LINEOPERATION

For output

voltage of

25 volts

Connected for:

2 ohm output (mono)

Be certain that the MC 2255 is never operatedthe stereo mode with the outputs connected monophonic operation. Should the MODE SWITbe left in the STEREO position and the outputs main strapped for a mono parallel load, one chanwill at tempt to drive the other which causes high culating currents and overheating.

AC POWERThe MC 2255 operates on 120 volts 50/60 Hz. T

auxiliary AC OUTLET on the MC 2255 is not fusedswitched.

MONOPHONIC OR SINGLE CHANNEL OPERATION

When the MC 2255 is used as a monophonic orsingle channel power amplifier the two channels arecombined to produce output up to 500 watts. Theoutputs must be connected as described below.

For monophonic operation using the MONO

BRIDGE mode, output impedances of 2, 4, 8, and 16ohms are accomodated. Output connections aremade by connecting to the output terminals aslisted below. Note that neither output terminal is atground potential.

Load ImpedanceIn Ohms

248

16

Connect - SpeakerLead To:

Left 1 Ohm TerminalLeft 2 Ohm TerminalLeft 4 Ohm TerminalLeft 8 Ohm Terminal

Connect + SpeakerLead To:

Right 1 Ohm TerminalRight 2 Ohm TerminalRight 4 Ohm TerminalRight 8 Ohm Terminal

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S te re o p h o n icC o n n e c t io n s

RIGHTSPEAKERMUST BE IN STEREO

POSITION FORSTEREO PROGRAMS

6

LEFTSPEAKER

PROG RAM SOURCE

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M o n o -B r id g eC o n n e c t io n s

(CONNECTIONS FOR8 OHM SPEAKER

ILLUSTRATED)

CONNECTIONS SHOWN FOR MONOWHEN MODE SWITCH

IS IN THE MONO-BRIDGE POSITION

SPEAKER

7

PROGRA M SOURCE

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M o n o -P a ra lle lC o n n e c t io n s

(CONNECTIONS FOR4 OHM SPEAKER

ILLUSTRATED)

SPEAKER

CONNECTIONS SHOWN FOR MONOWHEN MODE SWITCH

IS IN THE MONO-PARALLEL POSITION

8

PROGR AM SOURCE

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T h e F r o n t P a n e l C o n t r o l s a n dH o w t o U s e T h e mMETERS

Output power monitor meters indicate the output

power of each channel. Each meter has two primaryscales: WATTS and DECIBELS. When the METERRANGE switch is in one of the decibel (dB) posi-tions, peak signal readings are indicated on thelower DECIBEL scale. When the METER RANGEswitch is in the WATTS position, direct power inwatts is read from the upper watts scale. The metersare calibrated to read average watts. The in-termediate mark ings between the calibrations repre-sent, beginning with 250 watts, 100 watts, 50 watts,the indicated 25 watts, 10 watts, 5 watts, the in-dicated 2.5 watts, 1 watt, .5 watt, the indicated 0.25

watt, 0.1 watt, 0.05 watt, the indicated 0.025 watt,0.01 watt, 0.005 watt, the indicated 0.0025 watt, 0.001watt and 0.0005 watt. Although the meter calibra-tions are in average wat ts for a sine wave signal, themeters electrically respond to signal peaks. Themeters are voltage actuated and indicate power ac-curately when the amplifier is operated into ratedoutput load impedances.

The meters respond to the peak output of eachchannel. Ordinary meters lack the capability of in-dicating the short interval information in a soundwave. The mass of the meter movement is too greatto respond to the nearly instantaneous changes in

music program material. Short interval informatican have a duration as short as half a thousandth

a second. Ordinarily, a meter pointer moving over scale in such a short time would not be seeMclntosh has developed circuits that drive thmeters to respond to the short interval informatioin a sound wave to an accuracy of 90%. The eletrical pulse that drives the meter pointer is timstretched long enough so that the peak position the pointer can register in the persistence of visiocharacteristic of the retina of the human eye.

LEFT GAIN

The LEFT GAIN control adjusts the volume in thleft channel to the desired listening level. Turn t

control clockwise to increase the volume.

RIGHT/MONO GAIN

The RIGHT/MONO GAIN control adjusts thvolume in the right channel to the desired listeninlevel. Turn the control clockwise to increase thvolume. When the output of MC 2255 is connectefor monophonic operation and the rear panel MODSWITCH is in the MONO position the volume is cotrolled by the RIGHT/MONO GAIN control only.

METER RANGE

The METER RANGE switch has five positions.

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WATTS POWER

In the WATTS position the meter's primary cal ibra-tion is from .0025 watts (two and a half milliwatts),up to 250 watts, the rated power output of the MC2255. The meter is calibrated for 500 watts at theright hand end of the meter scale. While the MC 2255cannot reach this power level continuously, it ispossible for short interval peaks to exceed, con-siderably, the 250 watt continuous rating.

HOLD

In the HOLD position, the meter indicates WATTSand locks to the highest power peak in a sequenceof peaks. The meter will be driven to maximumpower and electronically held there until a higherpeak passes through the amplifier. If no furtherpeaks are reached the meter needle wil l very slow lyreturn to its rest position (decay rate: 6 dB perminute).

DECIBELS

In the other three positions of the METER RANGEswitch the meters will indicate the output of eachchannel in DECIBELS relative to 250 watts or anyother chosen reference.

0dB In this position of the swi tch , if theamplifier delivers 250 average watts, themeter indicates 0 dB; at 125 average wattsthe meter indicates - 3 dB. If the amplif ieris overdriven to +2 dB the indicated out-put would be 396 watts.

-10 dB In this position of the switch, if theamplifier delivers 25 average watts, themeter indicates 0 dB; at 12.5 average wattsthe meter indicates -3 dB.

-20dB In thi s posi tion of the switch, if theamplifier delivers 2.5 average watts, themeter indicates 0 dB; at 1.25 average wat tsthe meter indicates -3 dB.

HEADPHONES

The output of the front panel HEADPHONE jackhas been designed to feed low impedance dynamicstereo headphones.

The HEADPHONE output is not affected by theSPEAKER swit ch.

SPEAKERS

OFF: The loudspeakers are turned off when theSPEAKER switch is in the OFF position. You can lis-en to headphones in private.

THE SPEAKER SWITCH MUST BE IN THE "ON" PO-SITION TO HEAR MUSIC FROM THE LOUDSPEAK-

ERS.

ON: Music will be heard through the loudspeakers.

Use this as the normal listening position.

The power switch turns the MC 2255 ON or OFF.The switch does not control the power outlet on theback panel. If you wish to control the AC power froma preamplifier control center leave the switch in theON position. Be sure the AC cord of the MC 2255 isplugged into the controlled outlets on the rear of thepreamplifier control center.

OFF: In the OFF position the AC power to theamplifier is turned off.

POWER GUARD

POWER GUARD assures that the MC 2255 cannotbe overdriven, thus amplifier output clipping iseliminated. Clipping is caused when an amplifier isasked to produce more power output than it candeliver with low distortion. Amplifiers are capable ofdelivering large quantities of highly distorted powerwhen they are driven to clipping. The extra energycontent of the clipped signal will damage mostspeakers. Mclntosh's Power Guard circuit protects

your speakers from this kind of damage. The MC2255 has a built in "waveform comparator" thatcompares the wave shape of the output signal to theinput signal. If the disparity between the twosignals, due to overdrive, exceeds 0.5% (equivalentto 0.5% total harmonic distortion) a red LIMITindicator illuminates. With any further increase indistortion the Power Guard circuit operates tolimit the amplifier input dynamically so that theamplifier cannot be overdriven. Power Guardeliminates amplifier output clipping. As long as the

amplifier operates without

overload the NORMAL in-dicator illuminates.

NEW AUTOMATIC TESTSYSTEMThe MC 2255 contains anew automatic test circuit.When AC line power is ap-

plied, an LED digit illuminates to indicate which testis being performed. Starting with the numeral 7, itmakes the required measurement and verifies if it iswithin tolerance by lighting the "normal" PowerGuard green lights. It then counts down to six, per-

forms and veri fie s the next test, then five, four, three,etc. until it reaches "1" and then the speakers turnon. A "beep" tone is heard each time a test is per-formed. If a circuit should fail, the red "limit" lightswill come on and the sequence will stop at thatpoint. Speaker turn on will not occur until the faulthas been corrected; thereby protecting your systemfrom any further damage.

There are two user controls associated with theauto test circuit. They are located on the amplifiertop panel. Two switches control the speed of thecountdown (SLOW or FAST) and the presence of the

beep tone (ON-OFF).

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R e a r P a n e l i n f o r m a t i o nMODE SWITCH

The MC 2255 will operate in three modes, Stereo,Mono Bridge, and Mono Parallel. The Mono modesdi ff er in the loads they wil l drive and the connectionsequence to the speaker terminals.

LEFT and R IGHT OUTPUT TERMINA LS

For stereo operation, output impedances of 1, 2, 4and 8 ohms have been provided on a secure, screwtype barrier strip. For monophonic operation properin ter connec tio n provides 0.5, 1, 2, 4, 8, and 16 ohmsfrom the same barrier strips. See page 5 for connect-ing instructions.

INPUT JACKS

In the stereo mode of operation, both input jacksaccept signal. In the mono mode of operation onlythe Right (MONO) channel in put jack accepts signaland the Left channel input jack is disconnected.

INPUT LEVEL

The Input sens iti vi ty of the MC 2255 is 0.75 volts or2.5 V depending on the position of the INPUT LEVELswitch. All Mclntosh preamplifiers have been de-signed to deliver 2.5 volts output with rated input.For the best signal to noise ratio when using Mcln toshsource equipmen t, place the INPUT LEVEL switc h inthe 2.5 V position and the front panel LEFT and

RIGHT/MONO GAIN controls in the fully clockwiseposition. If more gain is desired the 0.75 V positiomay be used. For source equipment other tha

Mclntosh, set the switch in the position nearest tothe stated output rating of the source equipment.

AC POWER

The input to the MC 2255 is 120 volts 50/60 Hz aup to 12 amps. The primary circuit is protected by a15 Amp Fuse.

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P e r f o r m a n c e L i m i t sPERFORMANCE GUARANTEE

Performance Limits are the maximum deviationfrom perfection permitted for a Mclntosh instru-ment. We promise you the MC 2255 you buy must be

capable of performance at or exceeding these limitsor you get your money back. Mclntosh is the onlymanufacturer that makes this guarantee.

PERFORMANCEMclntosh audio power ratings are in accordance

with the Federal Trade Commission Regulation ofNovember 4, 1974 concerning power output claimsfor amplifiers used in home entertainment products.

POWER OUTPUT

STEREO250 watts minimum sine wave continuous average

power output, per channel, both channels operatinginto 1 ohm, 2 ohms, 4 ohms, or 8 ohms load im-pedance, which is:15.8 volts RMS across 1 ohm22.4 volts RMS across 2 ohms31.6 volts RMS across 4 ohms44.7 volts RMS across 8 ohms

MONO500 watts minimum sine wave continuous averagepower output in to 0.5 ohm, 1 ohm, 2 ohms, 4 ohms, 8ohms, or 16 ohms load impedance, which is:15.8 volts RMS across 0.5 ohm

22.4 volts RMS across 1 ohm2 ohms4 ohms8 ohms

16 ohms

31.6 volt s RMS across44.7 volts RMS across63.2 vo lts RMS across89.4 volt s RMS across

OUTPUT LOAD IMPEDANCESTEREO1 ohm, 2 ohms, 4 ohms, and 8 ohms; separate ter-minals are provided for each output.MONO-PARALLEL0.5 ohm, 1 ohm, 2 ohms, and 4 ohms; obtained byconnecting together the appropriate terminals of

both channels.MONO-BRIDGED2 ohms, 4 ohms, 8 ohms, or 16 ohms, balanced tocommon connections.

RATED POWER BAND20 Hz to 20,000 Hz

TOTAL HARM ONIC DISTORTIONSTEREO0.02% maximum harmonic distortion at any powerlevel from 250 milliwatts to 250 watts per channelfrom 20 Hz to 20,000 Hz, both channels operating.MONO

0.02% maximum harmonic distortion at any power

level from 250 milliwatts to 500 watts from 20 Hz t20,000 Hz.

INTERMOD ULATION DISTORTIONSTEREO

0.02% maximum if instantaneous peak power ouput is 500 watts or less per channel with both channels operating for any combination of frequencies20 Hz to 20,000 Hz.MONO0.02% maximum if instantaneous peak power ouput is 1000 watts or less for any combination of frequenices, 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 100,000 Hz + 0.25 -1 dB .

NOISE AND HUM

95 dB below rated output.

RATINGSDAMP ING FACTORGreater than 30

INPUT IMPEDANCE50,000 ohms.

INPUT SENSITIVITYSwitchable: 0.75 volt or 2.5 voltslevel control provided for higher input voltages.

POWER GUAR DClipping is prevented and THD does not exceed 2%

with up to 20 dB overdrive at 1 KHz.

GENERAL INFORMATIONPOWER REQUIREMENTS120 volts 50/60 Hz 0.7 to 12 amps.

SEMICONDUCTOR COMPLEMEN T85 silicon transistors47 silicon rectifiers and diodes14 integrated circuits

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P e r f o r m a n c e C h a r t s

PERFORMANCE CHARTS AREFOR ONE STEREO CHANNELWITH BOTH CHANNELS OPERATING.MONO OPERATION IS IDENTICALEXCEPT POWER IS DOUBLEDFOR THE SINGLE MONO OUTPUT.

13

FREQUENCY IN Hz

FREQUENCY IN Hz

FREQUENCY IN Hz

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POWER IN WATTS

FREQUENCY IN Hz

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T e c h n ic a l D e s c r ip t io nINPUT AMPLIFIER

Each channel input contains a complete seventransistor low power amplifier. A differential tran-sistor pair provides high input impedance and lownoise. The differential signals are combined in a cur-rent mirror circuit which drives a class A amplifierstage. The following output stage is driven by a com-plimentary pair of transistors with class AB biasing.The output signal drives the metering circuit, head-phone jack, and the high power output amplifier.This discrete transistor amplifier design wasselected for low noise, low distortion, adequatepower output capability to drive headphones, andfreedom from turn on and turn off transients.

The INPUT LEVEL selector and GAIN controls are

passive attenuators which precede the inputamplifier circuitry. Therefore, the input system to theamplifier cannot be overloaded when the controlsare correct ly set.

In the MONO mode of operation the input signalfeeds only the right input amplifier via theRIGHT/MONO GAIN control. The output of the rightinput amplifier feeds both output power amplifiersections. When MONO BRIDGE MODE is selectedthe left channel input amplifier is used as a phase in-verter before the left output amplifier. The outputchannels are therefore 180 out of phase with

respect to each other which is the correct arrange-ment for bridge output connections. When MONOPARALLEL MODE is selected the channels operatein phase. The channels, of course, also operate inphase for the STEREO MODE.

OUTPUT POWER AMPLIFIER

The power amplifier inputs are coupled to the in-put amplifier outputs through an electronic switch.The switch eliminates turn-on and turn-off transientsand is used for speaker switching. A junction FETand LED/light dependent resistor network make up

the switch. This combination allows the lowestpossible distortion when the amplifier is on and highisolation when the output power amplifier is off. Thecontrol signal to the switch is held off for the autotest countdown time. Therefore, transients that maycome into the amplifier from source equipment willnot be amplified or reach the loudspeaker. Since theHEADPHONE output and meters are powered by theinput amplifier, their operation is not affected by theSPEAKER switch or turn on delay system.

The first stage of the output power amplifier is adifferential transistor pair biased for best linearity.

The offset to the differential pair is adjustable. Cor-rect adjustment allows the lowest possible distor-

tion at low frequencies. A current mirror circuit com-bines the differential outputs into one signal whichis then amplified by a following class A voltageamplifier. Both the differential transitors and the

voltage amplifier are supplied by active currensources. The results are lower distortion and cleaneturn on characteristics.

The driver stage consisting of a complimentarypair of power transitors biased class AB follow thevoltage amplifier. Next, three complimentary pairsof rugged power transistors make up the power output stage. All power transistors are mounted on conservatively sized anodized aluminum heat sinksBecause of a unique connection of the bias networkthe output transistors operate class B and exhibit nocrossover distortion often associated with class B

operation. The heat sinks, therefore remain coowhen there is no output.

The turn-on and turn-off characteristics of thePower Amplifier are smooth and silent. No noisyrelay is required to disconnect the loudspeakers during turn-on time. Relays can cause noise and increased distortion if the contacts aren't perfectlyclean.

The amplifier output signal is fed to the output terminals through the output autotransformer. TheMclntosh designed interleaved multifilar wound

autotransformer is used to properly match theamplifier to stereo output load taps for 1, 2, 4 and 8ohms. The MC 2255 will deliver full power over theentire audio frequency range at any of these impedances. The autotransformer also protectsspeakers from damage in the event of amplifiefailure. Should a direct current component appear inthe output it is shunted by the autotransformer andDC cannot damage the speaker.

A Mclntosh patented Sentry Monitoring circuitconstantly monitors the output signal and instantlyreacts to prevent overload of the output transistors

At signal levels up to rated output this circuit hashigh impedance and has no effect upon the outputIf the power output exceeds design maximum, theSentry Monitoring circuit operates to limit the signato the output transistors. In the event of a short cir-cuit across the amplifier output or severe impedance mismatch the Sentry Monitoring circuit wilprotect the output transistors from failure. Bothpositive and negative halves of the output signal aremonitored and protected independently.

POWER GUARD PROTECTION CIRCUIT

The Mclntosh patented Power Guard circuiteliminates amplifier clipping due to overdrive. The

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circuit also illuminates red LIMIT indicator lampswhen the amplifier is driven beyond its maximumoutput capacity. Power Guard prevents loudspeakerdamage and eliminates harsh output distortioncaused by amplifier clipping.

The Power Guard circuit consists of a waveformcomparator which monitors the wave shape of theamplifier input and output signals. Normally there is

no disparity between these signals and the com-parator produces no output. When the amplifier isdriven beyond its maximum power capacity a dif-ference will develope. If the dispari ty exceeds 0.5%(equivalent to 0.5% total harmonic distortion) thecomparator output causes the red LIMIT indicatorsto light. If there is a further increase in the disparitythe comparator output controls an electronic at-tenuator at the amplifier input to reduce theamplifier gain, thus holding the amplifier output toits maximum undistorted value regardless of thedegree of overdrive to the amplifier. The amplifiermay be overdriven by 20 dB before the output distor-

tion exceeds 2%.The comparator is an especially compensated

operational amplifier integrated circuit. Its output isdetected by a full wave bridge that feeds signals tothe control circuitry for the LIMIT and NORMAL in-dicators and to the electronic attenuator at theamplifier input. The attenuator is a light emittingdiode/light dependent resistor network selectedespecially for its low distortion and time constantcharacteristics.

SYSTEM TEST CIRCUIT

When AC line power is applied to the MC 2255 theSystem Test circuit functions to verify operatingpotentials at 7 test locations within the amplifier.Upon applying power a digital display lights with thenumber 7. The digit is driven by a counter that stepsdown at a 1 second or .4 second rate. Each time thedigit changes electronic switches select the outputsof various voltage comparators connected to testpoints in the amplifier. If voltages are normal thegreen NORMAL Power Guard indicator lights, thereis an audible beep produced by an internal sounder,and the digit decrements to the next test number.When the count passes number 1 the loudspeakers

connect and the display blanks off. If a fault condi-tion exists the counter stops. The digit displayed in-dicates the circuit at fault. Servicing the amplifier isthereby simplified.

The test circuit uses logic counter, BCD decoder,lamp driver, and voltage comparator integrated cir-cuits.

METER CIRCUIT

The meter circuit has three basic sections: a loga-rithmic amplifier, a full wave rectifier, and a DC amp-lifier. In the WATTS ranges, the logarithmic amplifier

is used. In the DECIBEL ranges, the signal bypasses

this amplifier and goes directly to the full wave rectifiers through an attenuator which is controlled bthe METER RANGE switch.

The logarithmic amplifier consists of a high gaioperational amplifier with a biopolar connected silcon diode pair as feedback elements. These diodehave a uniform logarithmic characteristic over an 8dB range. Only 60 dB of this logarithmic range is us

ed in the MC 2255.The full wave rectifier circuit uses an operationa

amplifier with silicon diode feedback networks. Thamplified diode circuit has nearly perfect rectification characteristics. One rectifier detects only postive signals. The other responds only to negativsignals and produces a positive output. The outputof the rectifiers are combined at the operationaamplifier output, so the highest signal, eithepositive or negative, is the one that is indicated bthe meters. Gate diodes are used to charge a lowleakage capacitor which attains and holds a chargduring signal peaks. The operational amplifier provides a large amount of current so the capacitor cacharge suddenly. The charge on the peak holdincapacitor is amplified in a two transistor DCamplifier which is used to drive the meter. From thoutput of this amplifier there is a DC feedback network that connects back to the detector to assurexcellent overall linearity and frequency responseThe current drive to the meters has a peakincapacitor to accelerate the upscale response of thmeter needle. The meters also have a parallel shunresistor to correctly damp their action. In the WATTmode the discharge of the peak holding capacitor i

controlled by a resistor current source. In WATTHOLD, the resistor is disconnected so the peareading is retained. The rate of decay is about 6 dper minute.

POWER SUPPLY

The power supply is a conventional full wavbridge rectifier arrangement providing plus anminus 50 volts DC. Electronic regulators step dowand regulate plus and minus 15 volt sources for lowlevel circuits and plus 5 volts for the logic circuitsThermistors are used in the power transformeprimary circuit to limit the turn-on current.

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MclNTOSH LABORATORY INC.

2 CHAMBERS ST., BINGHAMTON, N.Y. 13903607-723-3512

The continuous improvement of its products is the policy ofMclntosh Laboratory Incorporated who reserve the right to

improve design without notice.

Printed in U.S.A.