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THE MclNTOSH MC 2200 SOLID STATE STEREO POWER AMPLIFIER

Your MC 2200 Stereo Power Amplifierwill give you many years of pleasantand satisfactory performance. If youhave any questions, please contact:

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

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

Take Advantage of 3 years

of FREE Service . ..

Fill in the Application NOW.

ContentsSERVICE

INSTALLATION

HOW TO CONNECT

FRONT PANEL INFORMATION

REAR PANEL INFORMATION

PERFORMANCE LIMITS AND RATINGS

PERFORMANCE CHARTS

TECHNICAL DESCRIPTION

BLOCK DIAGRAM

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2

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THREE YEAR SERVICE CONTRACTAn application for a FREE THREE YEAR SERVICE CONTRACT is included with this manual.

The terms of the contract are:

1. Mclntosh will provide all parts, materials andlabor needed to return the measured perform-ance of the instrument to the original per-formance limits free of any charge. TheSERVICE CONTRACT does not cover any ship-ping costs to and from the authorized serviceagency or the factory.

2. Any Mclntosh authorized service agency willrepair all Mclntosh instruments at normalservice rates. To receive the free service underthe terms of the SERVICE CONTRACT, theSERVICE CONTRACT CERTIFICATE must ac-company the instrument when taken to theservice agency.

3. Always have service done by a Mclntoshauthorized service agency. If the instrumentis modified or damaged, as a result of un-authorized repair the SERVICE CONTRACTwill be cancelled. Damage by improper use

or mishandling is not covered by the SERV-ICE CONTRACT.

4. The SERVICE CONTRACT is issued to you asthe original purchaser. To protect you frommisrepresentation this contract cannot betransferred to a second owner.

5. For your protection Mclntosh selects onlydealers who have technical competence toguide purchasers fairly, and provide servicewhen necessary. To receive the SERVICECONTRACT your purchase must be madefrom a Mclntosh franchised dealer.

6. Your completely filled in application for aSERVICE CONTRACT must be postmarkedwithin 30 days of the date of purchase ofthe instrument.

7. To receive the SERVICE CONTRACT all in-formation on the application must be filledin. The SERVICE CONTRACT will be issuedwhen the completely filled in application isreceived at Mclntosh Laboratory Incorporatedin Binghamton, New York.

Copyright © 1976 by Mclntosh Laboratory Inc.1

The MC 2200 can be housed in the Mclntosh L22 cabinet,installed in a custom cabinet or, with the addition of RACKMOUNTING ADAPTERS (RMA 7), mounted in a standard 19inch rack panel. The MC 2200 weighs 85 pounds (38.6 kg).In any installation it is necessary to provide proper supportfor the weight. The mounting methods described and theamplifier have been designed to accommodate the weight

so be sure that the panel and its mounting can also handlethe weight.

INSTALLATION IN MclNTOSH CABINETRemove the instrument and hardware package from the

carton. Remove the MC 2200 from its plastic bag and placeit upside down on the shipping pallet; unscrew the fourplastic feet from the bottom of the chassis. Use the screwsfrom the plastic feet to secure the instrument to the cabi-net. Do not substitute longer screws. Longer screws maypenetrate the electronic components in the chassis whichwould cause severe damage. Do not overtighten the screws.

INSTALLATION IN A CUSTOM CABINETThe trouble-free life of an electronic instrument is

greatly extended by providing sufficient ventilation to pre-vent the buildup of high internal temperatures that causedeterioration. Allow enough clearance so that cool air canenter at the bottom of the cabinet and be vented from thetop. With adequate ventilation the instrument can bemounted in any position. The recommended minimumspace for installation is 15 inches (38.1 cm) deep, 17 inches(43.2 cm) wide, and 8 inches (30.3 cm) high.

Vertical mounting the MC 2200 requires careful con-sideration. The total weight is transmitted to the panel andshelf on which the amplifier is to be mounted. Thoughtshould be given to providing a means for raising the MC2200 out of the panel. More consideration should be givento providing adequate ventilation at the panel in verticalmounting. The heat sinks are at the rear of the chassispermitting the heat to pass over the entire chassis. It is thentrapped behind the panel causing the overall temperatureto rise. In any case, air flow for cooling can be improved bythe use of a properly installed low noise fan. Adequatecooling will extend the life of the components.

Fig. 1. Top view of the panel cutout and mounting shelf.

2

1/4" DIA. (2 HOLES)

CUTOUT

FRONT PANELMTG. SURFACE

Fig. 2. Views showing mounting shelf in position.

INSTALLATION PROCEDURE

1. Mark for PositionMark cabinet panel where the instrument is to be

installed. The solid lines in Fig. 2 represent the outline ofthe rectangular cutout and also represent the outsidedimensions of the chassis. Make sure these lines clearshelves, partitions, or any equipment. The broken linerepresents the outside dimension of the panel.

2. Drill HolesUsing a drill bit slightly wider than the tip of your saw

blade, drill one hole at each of two diagonally oppositecorners. The holes should barely touch the inside edge ofthe outline.

3. Saw Panel CutoutSawing carefully on the inside of the lines, first make

the two long cuts and then the two short. After therectangular opening has been cut out, use a file tosquare the corners and smooth any irregularities in thecut edges.

4. Shelf InstallationInstall the shelf on which the MC 2200 will sit. It

must be capable of supporting the total weight of theinstrument. Drill the two ¼" holes shown in Fig. 1.

5. Install InstrumentGuide the AC power cord through the panel opening

to the back of the cabinet; then, slide the instrument intothe opening until its front panel is flush with the cabinetpanel. Secure the MC 2200 to the shelf with #10 woodscrews of the proper length.

½" thick shelf—use 1" screws5/8

" thick shelf—use 11/8" screws

¾" thick shelf—use 1¼" screwsDo not use screws longer than this recommended

length. They may penetrate electronic componentsinside the chassis and cause severe damage. In anyevent, the screw length must not exceed the shelfthickness by more than ½".

Insert two proper length screws through the mount-ing shelf and into the holes in the bottom of the ampli-fier. Do not overtighten the screws.

INSTALLATION IN A 19 INCH RACK• The RACK MOUNTING ADAPTERS install on each end of

the front panel. On each side of the amplifier, three screwssecure the front panel bracket to the chassis. These threescrews are removed, the RACK MOUNTING ADAPTERplaced over the screw holes and the screws replaced. Theentire weight is transmitted through the RACK MOUNTINGADAPTERS to the rack. No weight is carried by the frontpanel. Additional support for the weight can be providedat the rear of the chassis as desired.

Fig. 3. Rack Mounting Adapters installed.

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6 9/16"

DEPTH REQUIRED15".

TOP OF SHELF & BOTTOMOF CUTOUT SHOULD COINCIDE.

SHELF

MOUNTING SCREWSDIFFERENCE BETWEEN SCREWLENGTH & TOTAL SHELF THICKNESSMUST NOT EXCEED 1/2"

3"

INPUTSTEREO OR TWIN AMPLIFIER OPERATION

Use shielded cables to connect the signal from the pre-amplifier or signal source to the power amplifier input. Tominimize the possibility of hum the shielded cables shouldbe run parallel to each other or loosely twisted together.Locate the cables away from speaker leads and AC powercords. All connections are made on the back panel of theMC 2200.

For stereo operation, the left output of the preamplifiershould be plugged into the Left input jack of the poweramplifier. The right output of the preamplifier should beplugged into the Right (MONO) input jack of the poweramplifier.

For twin amplifier operation a separate signal source canbe connected to each input. In stereo or twin amplifieroperation the MODE SWITCH must be in the STEREO posi-tion.

MONOPHONIC OR SINGLE CHANNEL OPERATIONA shielded cable from the signal source is plugged into

the Right (MONO) input jack of the MC 2200 only. TheMODE SWITCH on the back panel of the amplifier must beplaced in the MONO position. In the MONO position theright channel input jack connects to both left and rightpower amplifiers. Mono output is obtained by strappingboth outputs in parallel. The Left INPUT is disconnected.Only the signal fed into the Right (MONO) input will beamplified.

Should the MODE SWITCH be left in the STEREO positionand the output transformers remain strapped for a mono-phonic load, one channel will attempt to drive the otherwhich causes high circulating currents and overheating. Becertain that the MC 2200 is never operated in the stereomode with the outputs connected for monophonic opera-tion.

OUTPUTBe certain the loudspeakers connected to the MC 2200

are capable of handling the power output of the ampli-fier.

For multiple speaker operation, run separate leads fromeach speaker to the amplifier. Because of the high power

available from the MC 2200, large diameter speaker leadsmust be used. Use lamp cord, bell wire, or wire with similartype of insulation to connect the speakers to the amplifier.In all cases, the leads to and from the speaker should betwin conductor or twisted together. When using 8 ohmspeakers and for the normally short distances of under 30feet between the amplifier and speaker, #18 wire orlarger can be used. For distances over 30 feet between theamplifier and speaker use larger diameter wire. Select thecorrect size wire for the wire distance from the chart. It isrecommended that the DC resistance of the speaker leadsbe less than 5% of the speaker impedance. Up to 10% canbe tolerated. Resistance of the leads should be computed forthe length of wire both to and from the speaker or speakers.

Wire

Gauge22201816141210

MAXIMUM WIRE

For 4 Ohm Load

Feet6

1015254060

100

Meters1.833.054.577.62

12.1918.2930.48

LENGTHS

For 8

Feet1220305080

120200

Ohm Load

Meters3.666.109.14

15.2424.3836.5860.96

Wire lengths above represent the wire resistance equal to5% of the speakers impedance. Use of smaller wire sizeswill produce more resistance which causes more powerloss and less loudspeaker damping.

STEREO OR TWIN AMPLIFIER OPERATIONCheck the impedance of the speaker which is usually

identified on the speaker itself or in the speaker owner'smanual. Connect a lead from the common terminal of theleft speaker to the amplifier LEFT OUTPUT terminal stripCOMmon screw. Connect another lead from the other termi-nal of the left speaker to the screw with the number cor-responding to the speaker impedance on the LEFT OUTPUTterminal strip. The right channel speaker is connected inthe same manner on the RIGHT OUTPUT terminal strip. Forstereo or twin channel operation it is not necessary to usethe same impedance loudspeaker on each output. Connect

4

each channel for the impedance desired. Full power willbe delivered to each properly connected speaker.

When multiple speakers are to be connected to either orboth outputs, the combined load impedance must be calcu-lated. The load must be connected to the appropriate im-pedance tap. Use this table to aid in selecting the correctimpedance match:

Loadimpedancein ohms0.8 to 1.61.6 to 3.2

Connect for1 ohm output2 ohm output

Loadimpedancein ohms3.2 to 6.46.4 and up

Connect for4 ohm output8 ohm output

If a load impedance is used that is lower than the outputimpedance tap, then reduced power and possible distortionwill result. If a load impedance is used that is higher thanthe output impedance tap, then neither the signal nor theamplifier will be harmed but the power available is limited.

FOR STEREO OR TWIN AMPLIFIER CONSTANT VOLTAGEOPERATION:For outputvoltage of Connect for25 volts 4 ohms

MONOPHONIC OR SINGLE CHANNEL OPERATIONWhen the MC 2200 is used as a monophonic or single

channel power amplifier the two channels are combined toproduce output up to 400 watts. The outputs must be tiedtogether at the appropriate load impedance tap. In connect-ing a load to the MC 2200 for single channel operationconnect the common side of the load to the LEFT CHANNELOUTPUT terminal marked COM., the other lead is con-nected:

FOR MONOPHONIC CONSTANT VOLTAGE LINEOPERATIONFor outputvoltage of Connected for25 volts 2 ohm output (mono)

For multiple speaker operation, run separate leads fromeach speaker to the amplifier.

Should the MODE SWITCH be left in the STEREO positionand the output transformers be strapped for a monophonicload, one channel will attempt to drive the other and causecirculating currents and overheating. Be certain that theMC 2200 is never operated in the stereo mode with theoutputs connected for monophonic operations.

AC POWERThe MC 2200 operates on 120 volts 50/60 Hz. The aux-

iliary AC OUTLET on the MC 2200 is not fused or switched.

If the speakeror loadimpedance is

0.5 ohm1 ohm2 ohms4 ohms

The hot sideof the line isconnected toLeft 1Left 2Left 4Left 8

Connect a jumperwire betweenLeft 1 andLeft 2 andLeft 4 andLeft 8 and

Right 1Right 2Right 4Right 8

Do not connect unequal impedance taps together

If the load impedance is between any of the above fig-ures, select the best impedance match from:

Loadimpedancein ohms0.4 to 0.80.8 to 1.6

Connect for0.5 ohm output1 ohm output

Loadimpedancein ohms1.6 to 3.23.2 and up

Connect for2 ohm output4 ohm output

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Front PanelInformation

LEFT GAINUse the LEFT GAIN control to adjust the volume in the

left channel to the desired listening level. Turn the controlclockwise to increase the volume.

RIGHT/MONO GAINUse the RIGHT/MONO GAIN control to adjust the volume

in the right channel to the desired listening level. Turn thecontrol clockwise to increase the volume.

MONOWith the output of MC 2200 connected for monophonic

operation and the rear panel MODE SWITCH in MONO thevolume is controlled by the RIGHT/MONO GAIN controlonly.

LIMIT/NORMALThe NORMAL (green) and LIMIT (red) indicators are a

new Mclntosh development. As long as the amplifieroperates without overload the NORMAL indicator illu-

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minates. A waveform comparator in the MC 2200 con-stantly compares the amplifier input and output waveform.Waveform difference of the output wave is converted toa voltage which is used as a control signal to "turn off" theNORMAL indicator and to "turn on" the LIMIT indicator.The LIMIT indicator turns on whenever the waveform dif-ference exceeds 0.5%. Generally, these waveform differ-ences are the result of applying voltages at the input inexcess of its rating. Because the limit indicator circuit canshow an overload condition as short as one cycle of a20,000 Hz signal a holding circuit keeps the red indi-cator on long enough for the eye to see.

HEADPHONESThe output of the front panel HEADPHONE jack has been

designed to feed low impedance dynamic stereo head-phones. Electrostatic headphones generally require higherpower than dynamic headphones. Connect them to theLEFT and RIGHT OUTPUT contacts on the back of the

MC 2200. Plug dynamic headphones into the front panelHEADPHONE jack. Adjust the front panel LEFT GAIN andRIGHT/MONO GAIN control for comfortable headphonelistening.

POWERThe power switch turns the MC 2200 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 preamplifiercontrol center leave the switch in the ON position. Be surethe AC cord of the MC 2200 is plugged into the controlledoutlets on the rear of the preamplifier control center.

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

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Rear Panel InformationLEFT and RIGHT OUTPUT

For stereo operation, output impedances of 1, 2, 4 and8 ohms have been provided on a secure, screw type barrierstrip. For monophonic operation proper interconnectionprovides 0.5,1, 2 and 4 ohms from the same barrier strips.

POWER GUARDAmplifiers are capable of delivering large quantities of

power when they are driven to clipping. Clipping iscaused when the amplifier is asked to produce more poweroutput than it can deliver with low distortion. A clippedamplifier can have more than 40% harmonic distortion.The extra energy content of the clipped signal will dam-age most speakers. A new Mclntosh advancement helpsto protect your speaker from this kind of damage. TheMC 2200 has a built in "waveform comparator" that com-pares the wave shape of the input signal with the outputsignal. If the non-linearity between the two signals exceeds0.5% a front panel signal illuminates in red (LIMIT}. Withthe POWER GUARD switch in the NORMAL position, 1.0%distortion will cause the POWER GUARD circuit to operate.The POWER GUARD circuit limits the input dynamically sothat the amplifier cannot be overdriven. This eliminatesamplifier output clipping. Recommended operation is withthe POWER GUARD switch in the NORMAL position at alltimes.

MODE SWITCHThe MC 2200 can be used in these different manners:

stereo (or as a twin amplifier) and mono. Because of the

excellent channel separation of the MC 2200, it can fulfillall of these functions. When the instrument is properly con-nected the MODE switch is used to select the method ofoperation desired.

INPUTIn the stereo or twin amplifier mode of operation both

input jacks accept signal. In the mono mode of operationthe Left channel input jack is disconnected and only theRight (MONO) channel input jack accepts signal.

INPUT LEVELThe input sensitivity of the MC 2200 is 0.75 volts or

2.5 V depending on the position of the INPUT LEVEL switch.With indicated voltage applied and the GAIN control clock-wise, the amplifier will deliver its rated power. All Mclntoshpreamplifiers have been designed to deliver 2.5 volts out-put with rated input. For the best signal to noise ratio whenusing Mclntosh source equipment, always have the INPUTLEVEL switch in the 2.5 V position and the front panel LEFTand RIGHT/MONO GAIN controls in the fully clockwiseposition. If more gain is desired the 0.75 V position may beused. For source equipment other than Mclntosh set theswitch in the position nearest to the stated output rating ofthe source equipment.

AC POWERThe MC 2200 is rated for 120 volts, 50/60 hertz. A

10 ampere fuse protects the MC 2200 electrically. Anauxiliary AC power outlet has been provided. The auxiliaryoutlet is neither fused nor switched.

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Performance Limits and RatingsPERFORMANCE GUARANTEE

Performance Limits are the maximum deviation from per-fection permitted for a Mclntosh instrument We promiseyou that the MC 2200 you buy must be capable of per-formance at or exceeding these limits or you get yourmoney back, Mclntosh is the only manufacturer that makesthis guarantee.

PERFORMANCEMclntosh audio power ratings are in accordance with theFederal Trade Commission Regulation of November 4,1974concerning power output claims for amplifiers used in homeentertainment products.

POWER OUTPUTSTEREO200 watts minimum sine wave continuousaverage power output, per channel, bothchannels operating into 1 ohm, 2 ohms, 4ohms, or 8 ohms load impedance, which is:

14.1 volts RMS across 1 ohm20.0 volts RMS across 2 ohms28.3 volts RMS across 4 ohms40.0 volts RMS across 8 ohms

MONO400 watts minimum sine wave continuousaverage power output into 0.5 ohm, 1ohm, 2 ohms, or 4 ohms load impedance,which is:

14.1 volts RMS across 0.5 ohm20.0 volts RMS across 1 ohm28.3 volts RMS across 2 ohms40.0 volts RMS across 4 ohms

OUTPUT LOAD IMPEDANCESTEREO1 ohm, 2 ohms, 4 ohms, and 8 ohms; sepa-rate terminals are provided for each outputMONO0.5 ohm, 1 ohm, 2 ohms, and 4 ohms; ob-tained by connecting together the appro-priate terminals of both channelsRATED POWER BAND20 Hz to 20,000 HzTOTAL HARMONIC DISTORTIONSTEREO0.1% maximum harmonic distortion at anypower level from 250 milliwatts to 200

watts per channel from 20 Hz to 20,000Hz, both channels operating

MONO0.1% maximum harmonic distortion at anypower level from 250 milliwatts to 400watts from 20 Hz to 20,000 HzINTERMODULATION DISTORTIONSTEREO0.1% maximum if instantaneous peak power output is 400watts or less per channel with both channels operating forany combination of frequencies, 20 Hz to 20,000 HzMONO0.1% maximum if instantaneous peak power output is 800watts or less for any combination of frequencies, 20 Hz to20,000 HzFREQUENCY RESPONSE (at one watt output)20 Hz to 20,000 Hz, +0 -0.25 dB10 Hz to 100,000 Hz +0 -3.0 dBNOISE AND HUM95 dB below rated output

RATINGSOUTPUT VOLTAGES25 volts for distribution linesDAMPING FACTORSTEREO16 at 1 ohm output, 50 at 2 ohms output, 30 at 4 ohms out-put, 16 at 8 ohms outputMONO16 at 0.5 ohms, 50 at 1 ohm, 30 at 2 ohms, and 16 at 4ohms outputINPUT IMPEDANCE100,000 ohmsINPUT SENSITIVITYSwitchable: 0.75 volt or 2.5 volts—Level control providedfor higher input voltages

GENERAL INFORMATIONPOWER REQUIREMENTS120 volts 50/60 Hz, 70 to 550 wattsSEMICONDUCTOR COMPLEMENT43 silicon transistors24 silicon rectifiers and diodes

2 integrated circuitsMECHANICAL INFORMATION

SIZEFront panel measures 16 inches wide (40.7 cm) by 6 31/32inches high (17.7 cm). Chassis measures 14 5/8 incheswide (37.2 cm) by 6 9/16 inches high (16.7 cm) by 14 1/2inches deep {36.8 cm), including connectors. Knob clear-ance required is 1 1/2 inches (3.8 cm) in front of mountingpanelFINISHFront panel is anodized gold. Chassis is chrome and blackWeight73 pounds (38.6 kg) net, 87 pounds (44.0 kg) in shippingcarton

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TechnicalDescriptionINPUT AMPLIFIER

Separate input amplifiers are used for the right andleft channels. Each input amplifier is a two transistor feed-back amplifier which has unity gain. They are used to pro-vide high input impedance and low output impedance fordriving the power amplifier sections. In addition, impe-dance matching is provided for the POWER GUARD systeminput attenuator.

When the MODE selector is switched to MONO, only theright channel input, GAIN control, POWER GUARD attenua-tor, and input amplifier is used to drive both power am-plifiers.

POWER AMPLIFIERThere are two identical power amplifier sections. At the

input to each power amplifier is a monolithic differentialtransistor selected for low noise. The monolithic designyields closely matched transistors for low distortion. Theinput and the feedback signals are applied to this differen-tial amplifier. The second stage is a class A voltage ampli-fier using a single transistor with feedback.

The predriver, driver, and output sections follow and arefully complimentary with high efficiency and negligible dis-tortion. The predriver is a complimentary transistor pairbiased to nearly class A operation for low crossover dis-tortion. Bias is supplied by a transistor which serves asa temperature tracking circuit so the predriver is properlybiased regardless of temperature. The predriver has agenerous amount of emitter degeneration for low distortion.

Another complimentary pair of power transistors biasedfor class AB operation make up the output driver stages.Their bias is developed across a termistor which holds thebias current stable regardless of temperature.

The output stage consists of 6 rugged complimentarypower transistors connected in single ended push-pullparallel. This stage is connected as a balanced emitter fol-lower which allows great stability and low distortion. Dueto a unique arrangement of the output bias network, theoutput transistors are operated class B free of crossoverdistortion. Heat is not produced by these transistors whenthere is no output. The bias network for the output stageis temperature compensated assuring class B operationat any temperature or power level.

The output transistors and drivers are mounted on gener-ous sized black anodized aluminum heat sinks. The freeflow of room temperature air that passes through the over-sized heat sinks provides the cooling necessary for the longlife of components.

The MC 2200 has transient free turn on and turn offcharacteristics. This is because the output of the amplifier isswitched by a heavy duty relay to the output auto trans-former. The relay is driven by a transistor switch. The con-trol to this transistor is derived from a long time constantcapacitor charging network. The switch turns on the relayapproximately two seconds after the MC 2200 is turned on.The same circuit has a short turn-off time constant whichturns off the relay before the amplifier's main power supplyhas had a chance to drop. The output signal is fed through amatching network to the front panel HEADPHONE jack. TheHEADPHONE output is designed to feed low impedancedynamic stereo headphones.

The amplifier output signal is fed to the output terminalsthrough the autotransformer. The Mclntosh designed inter-leaved multifilar wound autotransformer is used to properlymatch the amplifier to any output impedance tap. The MC2200 will deliver full power over the entire audio frequencyrange at any of these impedances. The MC 2200 will notoverheat when driving low impedance loads {1 ohm, 2ohms or 4 ohms). The autotransformer also protects speak-ers from damage in the event of amplifier failure. Should adirect current component appear in the output it is shuntedby the autotransformer and cannot damage the speaker.

The Mclntosh patented Sentry Monitoring circuit con-stantly monitors the output signal and instantly reacts toprevent overload of the output transistors. At signal levelsup to rated output this circuit has high impedance and hasno effect upon the output. If the power output exceedsdesign maximum, the Sentry monitoring circuit oper-ates to limit the signal to the output transistors. In theevent of a short circuit across the amplifier output orsevere impedance mismatch the Sentry Monitoringcircuit will protect the output transistors from failure.The positive and negative halves of the output signal aremonitored independently.

LIMIT INDICATOR AND POWER GUARDThe front panel NORMAL (green) and LIMIT (red) indica-

tors are activated by a newly designed Mclntosh circuit. TheMC 2200 has a built in "waveform comparator." The wave-form comparator electrically compares the amplifier's out-put waveform with the input waveform. Should the wave-form differences reach 0.5%, the red LIMIT indicator on thefront panel is turned on and the NORMAL indicator isturned off. If the differences increase to 1% the POWERGUARD circuit is activated. POWER GUARD provides anunusual margin of safety for loudspeakers by the preven-tion of amplifier clipping yet permits the amplifier to de-liver designed maximum power.

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The input waveform and the output waveform arecompared in an integrated circuit differential amplifier.Any differences, due to distortion of the output waveform,are converted to a control voltage. This control voltage isapplied differentially to a high gain operational amplifierwhere it is amplified about 100 times. The amplifiedcontrol voltage is detected by a full wave bridge rectifierthen enters a Schmidt trigger which is programmed tosequence the front panel indicators from NORMAL (green)to LIMIT (red} at a preset distortion level. Also built intothe trigger circuit is a timing capacitor which holds theLIMIT indicator on long enough to be visible even when theduration of the overload is much shorter. The LIMIT indica-tor will be illuminated for an overload condition as shortas one cycle of a signal at 20,000 Hz.

The same control voltage used to activate the LIMITindicator also actuates the POWER GUARD system. If thecontrol voltage is greater than is needed to illuminatethe LIMIT indication then the POWER GUARD begins op-erating. The control voltage is used to operate a lightemitting diode/light dependent resistor (LOR) network.The LDR is part of an attenuator between the volume con-trol and the input amplifier. The input to the power am-plifier can be attenuated to as little as one tenth, if neces-sary, in order to maintain low distortion in the event ofexcessive input signal.

The POWER GUARD circuit is activated when the rearpanel switch is in the NORMAL position. POWER GUARDprotection is disabled in the OUT position. The LIMIT indi-cator operates regardless of the position of the POWERGUARD switch.

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MclNTOSH LABORATORY INC.

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

Design subject to change without notice.Printed in U.S.A.

038-910BE022004