McIntosh MC-2125 User's Manual
Contents
1. DS m SL p CANNA 7 N Ve gt NS Dx VARG HH e HLHR ve m t 77 1277 MY A Sy ee ate i f od 2 26 e 4 W HH N 7 N 7 NN Ff NN 44 4 HN ENS v II 45929 a n 2909885 dM Nn 4 2 ANS se Q a P 4 4 11 12 14 16 Naar N AN o Y 7 Wii N 7 g 2 2 Ph q 77 NN 74 WM gt N S 2 777722 How to Connect INPUT STEREO OR TWIN AMPLIFIER OPERATION Use shielded cables to connect the signal from the pre amplifier or signal source to the power amplifier To mini mize the possibility of hum the shielded cables should be run parallel to each other or loosely twisted together Lo cate the cables away from speaker leads and AC power cords All connections are made on the back panel of the MC 2125 For stereo operation the left output of the preamplifier should be plugged into the Left input jack of the power amplifier The right output of the preamplifier should be plugged into the Right MONO input jack of the power amplifier In stereo or twin amplifier operation the MODE SWITCH must be in the STEREO position For twin amplifier opera tion a separate signal source can be connected to each input MONOPHONIC OR SINGLE CHANNEL OPERATION A shielded cable fr2. 2125 FSA THE McINTOSH MC 2125 SOLID STATE STEREO POWER AMPLIFIER Reading Time 31 Minutes Price 1 25 Your MC 2125 Stereo Power Amplifier will give you many years of pleasant and satisfactory performance If you have any questions please contact CUSTOMER SERVICE Mcintosh Laboratory Inc 2 Chambers Street Binghamton New York 13903 Phone 607 723 3512 WARNING TO PREVENT FIRE OR SHOCK HAZARD DO NOT EXPOSE THIS UNIT TO RAIN OR MOISTURE Take Advantage of 3 years of FREE Service Fillinthe Application NOW gt e q T NN IAA HAAS YA No GEO N ZA SAN WILLE Ses 77777 wu Contents SERVICE INSTALLATION HOW TO CONNECT FRONT PANEL INFORMATION REAR PANEL INFORMATION PERFORMANCE LIMITS AND RATINGS PERFORMANCE CHARTS TECHNICAL DESCRIPTION BLOCK DIAGRAM gt NN M AMAN 7 SWAY NUN y Ny ye THREE YEAR SERVICE CONTRACT An application for a FREE THREE YEAR SERVICE CONTRACT is included with this manual The terms of the contract are 1 will provide all parts materials and labor needed to return the measured performance of the In strument to the original performance limits free of any charge The SERVICE CONTRACT does not cover any shipping costs to and from the authorized service agency or the factory Any Mcintosh authorized service agency will repair
3. HOLIMS INVIAS 3NOHJQY3H 43MOd 440 NO v 3NOH4QY3H 33804 HSNANI wo o 104100 ez o 801231044 YOLVIN93Y yayvaas vy lo 104170 INJISNVAL aste 143 ve lo 440 NO vo lo E n ABINAS 831411038 nni 31 25 3340 43A1307334 O IndNI 1431 BILIWI INJA SOLINOW AYINIS 31N 0419313 NIY9 1431 33 N30035 5dWYT dv 300W indino 39 15 104100 20 16 McINTOSH LABORATORY INC 2 CHAMBERS ST BINGHAMTON N Y 13903 607 723 3512 Design subject to change without notice Printed in U S A 038 885
4. less of temperature The predriver has a generous amount of emitter degeneration for low distortion Another complimentary pair of power transistors biased for class AB operation make up the output driver stages Their bias is developed across a thermistor which holds the bias current stable regardless of temperature The output stage consists of 4 rugged complimentary power transistors connected in single ended push pull parallel This stage is connected as a balanced emitter fol lower which allows great stability and low distortion Due to a unique arrangement of the output bias network the output transistors are operated class B free of crossover distortion Heat is not produced by these transistors when there is no output The bias network for the output stage is temperature compensated assuring class B operation at any temperature or power level The output transistors and drivers are mounted on a generous sized black anodized aluminum heat sinks The free flow of room temperature air that passes through the oversized heat sinks provides the cooling necessary for the long life of components The MC 2125 has transient free turn on and turn off characteristics The output of the amplifier is switched by a heavy duty relay to the output autotransformer The relay is driven by a transistor switch The control signal to this 14 Technical Description transistor is derived from a long time constant capacitor charging network The s
5. SH MEH Il 114110 T LV ISNOdSIN ADNINOIYA NI 011231430 3313W 13 NI ISNOdSIA INPUT AMPLIFIER Separate input amplifiers are used for the right and left 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 for driving the power amplifier sections In addition impe dance matching is provided for the POWER GUARD system input attenuator When the MODE selector is switched to MONO only the right channel input GAIN control POWER GUARD attenua tor and input amplifier is used to drive both power ampli fiers POWER AMPLIFIER There are two identical power amplifier sections At the input to each power amplifier is a monolithic differential transistor selected for low noise The monolithic design ylelds closely matched transistors for low distortion The input 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 are fully com plimentary and have high efficiency and negligible distor tion The predriver is a complimentary transistor pair biased to nearly class A operation for low crossover distortion Bias is supplied by a transistor which serves as a temperature tracking circuit so the predriver is properly biased regard
6. all McIntosh instruments at normal service rates To re ceive the free service under the terms of the SERVICE CONTRACT the SERVICE CONTRACT CERTIFICATE must accompany the instrument when taken to the service agency Always have service done by a Mclntosh authorized service agency If the instrument is modified or dam aged as a result of unauthorized repair the SERVICE CONTRACT will be cancelled Damage by improper use BAIS TIPS in same 4 J Copyright 1976 by McIntosh Laboratory Inc ARA p 45442477 or mishandling is not covered by the SERVICE CON TRACT The SERVICE CONTRACT is issued to you as the original purchaser To protect you from misrepresentation this contract cannot be transferred to a second owner For your protection Mclntosh selects only dealers who have technical competence to guide purchasers fairly and provide service when necessary To receive the SERVICE CONTRACT your purchase must be made from a Mcintosh franchised dealer Your completely filled in application for a SERVICE CONTRACT must be postmarked within 30 days of the date of purchase of the instrument To receive the SERVICE CONTRACT all information on the application must be filled in The SERVICE CON TRACT will be issued when the completely filled in application is received at Mclntosh Laboratory In corporated in Binghamton New York LA SSS SN
7. on the capacitor from exceeding that which is required for a full scale meter reading This prevents meter damage that might result from slamming the meter end stop The charge on the peak holding capacitor is amplified in a two transistor DC feedback amplifier which is used to drive the meter From the output of this amplifier there is a DC feedback network that feeds the detectors to assure excel lent overall linearity and frequency response The current drive to the meters has a peaking capacitor to accelerate the upscale response of the meter needle The meters also have a parallel shunt resistor to correctly damp their action In the WATTS mode the discharge of the peak holding capacitor is controlled by a resistor current source In WATTS HOLD the resistor is disconnected so the peak reading is retained The rate of decay is 10 dB per minute 15 380 Sd4WV1 NY 33 N3n035 1N3330 les YOLYNNILLY pond 104 ion 3IN0813313 DU Dios ONOW QNY 117 340 384 314WyS 831411038 L 3AVM 831IWI1 nna 1N3380 JOLINOW IDIIMS 181835 quvno Y3MOd AYOMLIN 1080334 H21IMS qavno 2 3OLVNNILLW 1031N02 YaZAIVNY 011301514 AJOMLIN 19480333 YHOMLIN YSOMI3N 39015 11000 10V80333 y0Y80334 wo o 104110 vz 0 801231034 349345 vs lo barres LNAISNYAL HOR da To 440 NO va lo HILIMS 114100 te n Y3WAOISNYAL
8. LY WHEN THE MODE MODE SWITCH IS IN MONOPOSITION SWITCH MONO LEFT GAIN METER RANGE SPEAKERS HOLD 1048 ER SS METERS Output power monitor meters indicate the output power of each channel Each meter has two primary scales WATTS and DECIBELS With the METER RANGE switch in one of the decibel dB positions peak signal readings are indicated on the lower DECIBEL scale The meters show peak output of the monitored channel Ordinary meters lack the capability of indicating the short interval informa tion in a sound wave The mass of the meter movement is too great to respond to the nearly instantaneous changes in music program material The short interval information can have a duration as short as half a thousandth of a sec ond Ordinarily a meter pointer moving over its scale in such a short time would not be seen Mcintosh has de veloped circuits that drive the meters to respond to the short interval information in a sound wave to an accuracy of 98 The electrical pulse that drives the meter pointer is time stretched long enough so that the peak position of the pointer can register in the persistence of vision char acteristic of the retina of the human eye With the METER RANGE switch in the WATTS positions direct power in watts is read from the upper watts scale The meters are calibrated in average watts for a sine wave signal but respond to signal peaks So a 120 a
9. O JINOWAYH NI NOILYOLSIG 9INONNVH 1 A9N3n0333 00002 000701 001 02 IWIL ee TEMO TUBE FA EIER Hub E IRE E gu Um Ae SE HEP OL AO FANGE AVE AE Zeta 539181 Wv49011I9 0 108 804 AP OZ A8 NIAIYOYIAO SI INdNI u3i3l1dWV 9 144112 1 915 104100 31VNIWIT3 01 GYVND Duneuadg 5 9UUBUI 4108 JJUUBUS Jad 43404 40 NOILOV 9NIMOHS WYOJ3AVM TUNDIS 114100 9 JBU 3JULWJOJJAd 1014100 031 9 IV 011801510 JINOWAYH 0 yO U 1014100 YIMOd SA NOILYOLSIC NOLLVINGOWYILNI 3aniridWv 1VN9IS 114100 so 90 Lo 80 NI 011801510 NOILVINGOWSIINI 12 SGNODISITIIW NI 0001 006 008 00 009 006 0 1 L 00b 006 002 001 1504 8h QNV 5 011504 010 S 1S808 INOL QNO23SITIIW 006 319NIS V 01 35 04538 3313W Z183H NI A9N300333 000 001 00001 000 T 001 01 ULL TL mul rr mti 100000 bL _ AA OBL 804 E Hl T Mi bir i E EE RR a ee QHL 0 LY HLGIMGNYS Y3MOd 02 Ov 09 08 001 021 091 08 001 SLIVM 39V33AV NI 114100 83 04 NI 011231330 4313W SQNO23SITIIW NI 0001 006 008 004 009 006 00 00 000 001 0 0c 0 09 08 001 15808 INOL QNO23S 08218 005 319NIS V 01 ISNOASIY YILIM Z143H NI 128300383 000 001 00001 00071 001 01 ML I Nur Ju F 11181008 FA LIS ma UN mg i
10. OLTAGES 25 volts for distribution lines DAMPING FACTOR STEREO 16 at 2 ohms output 50 at 4 ohms output 20 at 8 ohms output 14 at 16 ohms output MONO 16 at 1 ohm 50 at 2 ohms 20 at 4 ohms and 14 at 8 ohms output INPUTIMPEDANCE 100 000 ohms INPUT SENSITIVITY Switchable 0 75 volt or 2 5 volts Level control provided for higher input voltages GENERAL INFORMATION POWER REQUIREMENTS 120 volts 50 60 Hz 50 watts at zero signal output 460 watts at rated output SEMICONDUCTOR COMPLEMENT 45 silicon transistors 45 silicon rectifiers and diodes 8 integrated circuits ae MECHANICAL INFORMATION Front panel measures 16 inches wide 40 64 cm by 5 7 16 inches high 13 81 cm Chassis measures 15 inches wide 38 1 cm by 5 inches high 12 7 cm by 13 inches deep 33 02 cm including connectors Knob clearance required is 11 inches 3 81 cm in front of mounting panel FINISH Front panel is anodized gold and black with special gold teal nomenclature illumination Chassis is chrome and black WEIGHT 65 pounds 29 5 kg net 77 pounds 35 kg in shipping carton SLIVM 39V33AV NI 114100 YIMOd SLIVM 39V33AV NI 114110 YIMOd 06 6 q 006 HHAH fs T DD TIEF RR TE ze CRE tit e ET E ELIT A Tr RR DICE dp MEET d RET ED Eo ra TREE Ff E dl v e HILL HI NOIINOISI
11. c amplifier is used In the DECIBEL ranges the signal goes directly to the full wave rectifiers through an attenuator which is selected by the METER RANGE switch The logarithmic amplifier consists of a high gain opera tional amplifier with a bipolar connected silicon diode pair as feedback elements These diodes have a uniform loga rithmic characteristic over an 80 dB range Only 60 dB of the logarithmic portion of this range has been used in the MC 2125 A second diode pair follows the operational amplifier for compensation of diode contact potential oince the compensation circuit has a temperature coeffi cient thermistors are used to stabilize the logarithmic output with temperature changes The full wave rectifier circuit uses a pair of operational amplifiers with silicon diode feedback networks These amplified diode circuits have nearly perfect rectification characteristics One rectifier detects only positive signals The other responds only to negative signals and produces a positive output The outputs of the detectors are combined in a diode gate so the highest signal either positive or nega tive is the one that is indicated by the meters These gate diodes charge a low leakage capacitor which attains and holds a charge during signal peaks The operational am plifier detectors provide a large amount of current so the capacitor will charge suddenly Connected to the peak holding capacitor is a diode clamp to prevent the charge
12. cates 0 dB at 6 average watt the meter indicates 3 dB OUTPUTMODE The NORMAL green and LIMIT red indicators are a new Mcintosh development As long as the amplifier operates without overload the NORMAL indicator illu minates A waveform comparator in the MC 2125 con stantly compares the amplifier input and output wave form Waveform difference of the output wave is con verted to a voltage which is used as a control signal to turn off the NORMAL indicator and to turn on the LIMIT indicator The LIMIT indicator is on whenever the waveform difference exceeds 0 5 Generally these waveform differences are the result of applying voltages at the input in excess of its rating Because the limit in dicator circuit can show an overload condition as short as one cycle of a 20 000 Hz signal a holding circuit keeps the red indicator on long enough for the eye to see If the am plifier output is mismatched or shorted the LIMIT indica tor will illuminate when the meters indicate less than rated output HEADPHONES The output of the front panel HEADPHONE jack has been designed to feed low impedance dynamic stereo head phones Electrostatic headphones generally require higher power than dynamic headphones Connect them to the LEFT and RIGHT MAIN SPEAKER contacts on the back of the MC 2125 Plug dynamic headphones into the front panel HEADPHONE jack Adjust the front panel LEFT GAIN and RIGHT MONO GAIN control for comfortable headphone
13. f the clipped signal will dam age most speakers A new Mclntosh advancement helps to protect your speaker from this kind of damage The MC 2125 has a built in waveform comparator that com pares the wave shape of the input signal with the output signal If the non linearity between the two signals exceeds 0 5 a front panel signal illuminates in red LIMIT With the 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 so that the amplifier cannot be overdriven which eliminates amplifier output clipping MODE SWITCH The MC 2125 can be used in these different manners stereo or as a twin amplifier and mono Because of the excellent channel separation of the MC 2125 it can fulfill all of these functions When the outputs are properly con 10 nected the MODE switch is used to select the method of operation desired INPUT In the stereo or twin amplifier mode of operation both input jacks accept signal In the mono mode of operation the Left channel input jack is disconnected and only the Right MONO channel input jack accepts signal INPUT LEVEL The input sensitivity of the MC 2125 is 0 75 volts or 2 5 V depending on the position of the INPUT LEVEL switch With indicated voltage applied the amplifier will deliver its rated power All Mclntosh preamplifiers have been designed to deliver 2 5 volts output with rated input For
14. ip on the rear panel The meter is calibrated for 300 watts at the right hand end of the meter scale While the MC 2125 cannot reach this power level continuously it is possible for short interval peaks to exceed considerably the 120 watt continuous rating HOLD In the HOLD position the meter indicates WATTS and locks to the highest power peak in a sequence of peaks The meter will be driven to maximum power and elec tronically held there until a higher peak passes through the amplifier If no further peaks are reached the meter needle will very slowly return to its rest position decay rate 10 dB per minute Both WATTS and HOLD will be illuminated in the METER rectangle when the METER RANGE switch is in the HOLD position DECIBELS In the other three positions of the METER RANGE switch the meters will indicate the output of each chan nel in DECIBELS relative to 120 watts or any other appro priate arbitrarily chosen reference 0 dB In this position of the switch if the amplifier delivers 120 average watts or 240 peak watts the meter indicates 0 dB at 60 average watts the meter indicates 3 dB If the amplifier is overdriven to 1 dB the indicated output would be 151 watts 10 dB In this position of the switch if the amplifier delivers 12 average watts the meter indicates O dB at 6 average watts the meter indicates 3 dB 20 dB In this position of the switch if the amplifier delivers 1 2 average watts the meter indi
15. listening The HEADPHONE output is not affected by the SPEAKER switch SPEAKERS OFF The loudspeakers are turned off when the SPEAKER switch is in the OFF position You can listen to headphones in private THE SPEAKER SWITCH MUST BE IN THE ON POSITION TO HEAR MUSIC FROM THE LOUDSPEAKERS ON Music will be heard through the loudspeakers Use this as the normal listening position POWER The power switch turns the MC 2125 ON or OFF The switch does not control the power outlet on the back panel If you wish to control the AC power from a preamplifier control center leave the switch in the ON position Be sure the AC cord of the MC 2125 is plugged into the controlled outlets on the rear of the preamplifier control center OFF In the OFF position the AC power to the amplifier is turned off 52 2 2 Ja R K OK A ied A Rear Panel Information LEFT and RIGHT OUTPUT For stereo operation output impedances of 2 4 8 and 16 ohms have been provided on a secure screw type bar rier strip For monophonic operation proper interconnection provides 1 2 4 and 8 ohms from the same barrier strips POWER GUARD Amplifiers are capable of delivering large quantities of power when they are driven to clipping Clipping is caused when the amplifier is asked to produce more power output than it can deliver with low distortion A clipped amplifier can have more than 40 harmonic distortion The extra energy content o
16. m NORMAL green to LIMIT red at a preset distortion level Also built into the trigger circuit is a timing capacitor which holds the LIMIT indicator on long enough to be visible even when the duration of the overload is much shorter The LIMIT indica tor will be illuminated for an overload condition as short as one cycle of a signal at 20 000 Hz If the amplifier output is mismatched or shorted the LIMIT indicator will be turned on when the meters indicate less than 120 watts The same control voltage used to activate the LIMIT indicator also actuates the POWER GUARD system If the control voltage is greater than is needed to illuminate the LIMIT indication then the POWER GUARD begins op erating The control voltage is used to operate a light emitting diode light dependent resistor LDR 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 of excessive input signal The POWER GUARD circuit is activated when the rear panel switch is in the NORMAL position POWER GUARD protection is disabled in the OUT position The LIMIT indi cator operates regardless of the position of the POWER GUARD switch METER CIRCUIT The meter circuit has three basic sections the logarith mic amplifier full wave rectifier and DC amplifier In the WATTS ranges the logarithmi
17. om the signal source is plugged into the Right MONO input Jack of the MC 2125 only The MODE SWITCH on the back panel of the amplifier must be placed in the MONO position In the MONO position the output of the right channel input amplifier is fed to both left and right power amplifiers The Left INPUT is discon nected Only the signal fed into the Right MONO input will be amplified Should the MODE SWITCH be left in the STEREO position and the output transformers remain strapped for a monophonic load one channel will attempt to drive the other which causes high circulating currents and overheating Be certain that the MC 2125 is never operated in the stereo mode with the outputs connected for monophonic operation OUTPUT For multiple speaker operation run separate leads from the amplifier to the speakers Because of the high power available from the MC 2125 be sure to use large diameter speaker leads In all cases the leads to and from the speaker should be twin conductor or twisted together Use lamp cord bell wire or wire with similar type of insulation to connect the speakers to the amplifier For the normally short distances of under 20 feet between the amplifier and speaker 18 wire or larger can be used For distances over 20 feet between the amplifier and speaker use larger diameter wire Select the correct size wire for the wire dis tance from the chart lt is recommended that the DC re sistance of the speaker leads no
18. ped to a monophonic load one channel will attempt to drive the other and cause circulating currents and Ifthe speaker The hot side overheating Be certain that the MC 2125 is never op or load of the line is Connect a jumper erated in the stereo mode with the outputs connected for impedance is connected to wire between monophonic operations 1ohm Left 2 Left 2 and Right 2 2 ohms Left 4 Left 4 and Right 4 AC POWER 4 ohms Left 8 Left 8 and Right 8 The MC 2125 operates on 120 volts 50 60 Hz The aux 8 ohms Left 16 Left 16 and Right 16 iliary AC OUTLET on the MC 2125 is not fused or switched MAXIMUM WIRE LENGTHS Wire For 4 Ohm Load For 8OhmLoad For 16 Ohm Load Gauge Feet Meters Feet Meters Feet Meters 22 6 1 83 12 3 66 24 7 32 20 10 3 05 20 6 10 40 12 20 18 15 4 57 30 9 14 60 18 30 16 25 7 62 50 15 24 100 30 50 14 40 12 19 80 24 38 160 48 80 12 60 18 29 120 36 58 240 73 20 10 100 30 48 200 60 96 400 122 00 These wire lengths represent the wire resistance equal to 5 of the speakers im pedance Use of smaller wire sizes will produce more resistance which causes more power loss and less loudspeaker damping Stereophonic Connections PROGRAM SOURCE RIGHT SPEAKER STEREO MUST IN STEREO PA POSITION FOR STEREO IA SPEAKER PROGRAMS MODE SWITCH MONO Monophonic Connections PROGRAM SOURCE STEREO CONNECT FOR MONO USE ON
19. s 21 9 volts RMS across 4 ohms 31 0 volts RMS across 8 ohms 43 8 volts RMS across 16 ohms MONO 240 watts minimum sine wave continuous average power output into 1 ohm 2 ohms 4 ohms or 8 ohms load impedance which is 15 5 volts RMS across 1 ohm 21 9 volts RMS across 2 ohms 31 0 volts RMS across 4 ohms 43 8 volts RMS across 8 ohms OUTPUT LOAD IMPEDANCE STEREO 2 ohms 4 ohms 8 ohms and 16 ohms separate terminals are provided for each output MONO 1 ohm 2 ohms 4 ohms and 8 ohms ob tained by connecting together the appro priate terminals of both channels RATED POWER BAND 20 Hz to 20 000 Hz TOTAL HARMONIC DISTORTION STEREO 0 1 maximum harmonic distortion at any power level from 250 milliwatts to 120 Performance Limits and Ratings watts per channel from 20 Hz to 20 000 Hz both channel operating MONO 0 1 maximum harmonic distortion at any power level from 250 milliwatts to 240 watts from 20 Hz to 20 000 Hz INTERMODULATION DISTORTION STEREO 0 1 maximum if instantaneous peak power output is 240 watts or less per channel with both channels operating for any combination of frequencies 20 Hz to 20 000 Hz MONO 0 1 maximum if instantaneous peak power output is 480 watts or less for any combination of frequencies 20 Hz to 20 000 Hz FREQUENCY RESPONSE at one watt output 20 Hz to 20 000 Hz 0 0 25 dB 10 Hz to 100 000 Hz 0 3 0 dB NOISE AND HUM 95 dB below rated output RATINGS OUTPUT V
20. t If a load impedance is used that is lower than the output impedance tap then reduced power and possible distortion will result If a load impedance is used that is higher than the output impedance tap then neither the signal nor the amplifier will be harmed but the power available is limited FOR STEREO OR TWIN AMPLIFIER CONSTANT VOLTAGE OPERATION For output voltage of Connect for 25 volts 8 ohms MONOPHONIC OR SINGLE CHANNEL OPERATION When the MC 2125 is used as a monophonic or single channel power amplifier the two channels are combined to produce output up to 240 watts The outputs must be tied together at the appropriate load impedance tap In connect ing a load to the MC 2125 for single channel operation connect the common side of the load to the LEFT CHANNEL OUTPUT terminal marked COM the other lead is con nected 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 Load Load impedance impedance in ohms Connect for in ohms Connect for 0 8 to 1 6 1 ohm output 3 2 to 6 4 4ohm output 1 6 to 3 2 2 ohm output 6 4 and up 8 ohm output FOR MONOPHONIC CONSTANT VOLTAGE LINE OPERATION For output voltage of Connected for 25 volts 4 ohm output mono For multiple speaker operation run separate leads from the amplifier to the speakers Should the MODE SWITCH be left in the STEREO position and the output transformers be strap
21. t be over 5 of the speaker impedance Up to 10 can be tolerated Resistance of the leads should be computed for the length of wire both to and from the speaker or speakers Be certain the loudspeakers connected to the MC 2125 are capable of handling the power output of the am plifier STEREO OR TWIN AMPLIFIER OPERATION Check the impedance of the speaker which is usually identified on the speaker itself or in the speaker owner s manual Connect a lead from the common terminal of the left speaker to the amplifier LEFT OUTPUT terminal strip COMmon 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 OUTPUT terminal strip The right channel speaker is connected in the same manner on the RIGHT OUTPUT terminal strip For stereo or twin channel operation it is not necessary to use the same impedance loudspeaker on each output Connect each channel for the impedance desired Full power will be delivered to each properly connected speaker When multiple speakers are to be connected to either or both 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 correct impedance match Load Load impedance impedance in ohms Connect for in ohms Connectfor 1 6 to 3 2 2 ohm output 6 4 to 13 8 ohm output 3 2 to 6 4 4 ohm output 13 and up 16 ohm outpu
22. t transistors In the event of a short circuit across the amplifier output or severe impedance mismatch the Sentry Monitoring circuit will protect the output transistors from failure Both positive and negative halves of the output signal are monitored independently LIMIT INDICATOR AND POWER GUARD The front panel NORMAL green and LIMIT red indica tors in the OUTPUT MODE rectangle are activated by a newly designed Mcintosh circuit The MC 2125 has a built in waveform comparator The waveform comparator electrically compares the amplifier s output waveform with the input waveform Should the waveform differ ences reach 0 596 the red LIMIT indicator on the front panel is turned on and the NORMAL indicator is turned off If the differences increase to 1 the POWER GUARD circuit is activated POWER GUARD provides an unusual margin of safety for loudspeakers by the prevention of amplifier clipping yet permits the amplifier to deliver designed maximum power The input waveform and the output waveform are compared in an integrated circuit differential amplifier Any differences due to distortion of the output waveform are converted to a control voltage This control voltage is applied differentially to a high gain operational amplifier where it is amplified about 100 times The amplified control voltage is detected by a full wave bridge rectifier then enters a Schmidt trigger which is programmed to seguence the front panel indicators fro
23. the best signal to noise ratio when using Mclntosh source equipment always have the INPUT LEVEL switch in the 2 5 V position and the front panel LEFT and RIGHT MONO GAIN controls in the fully clockwise position If more gain is desired the 0 75 V position may be used For source equipment other than Mcintosh set the switch in the position nearest to the stated output rating of the source equipment AC POWER The MC 2125 is rated for 120 volts 50 60 hertz It uses 50 watts when there is no signal output and 460 watts with both channels delivering rated power An 8 ampere fuse protects the MC 2125 electrically An auxiliary AC power outlet has been provided The auxiliary outlet is neither fused nor switched PERFORMANCE GUARANTEE Performance Limits are the maximum deviation from per fection permitted for a Mclntosh instrument We promise you that the MC 2125 you buy must be capable of per formance at or exceeding these limits or you get your money back Mcintosh is the only manufacturer that makes this guarantee PERFORMANCE McIntosh audio power ratings are in accordance with the Federal Trade Commission Regulation of November 4 1974 concerning power output claims for amplifiers used in home entertainment products POWER OUTPUT STEREO 120 watts minimum sine wave continuous average power output per channel both channels operating into 2 ohms 4 ohms 8 ohms or 16 ohms load impedance which is 15 5 volts RMS across 2 ohm
24. verage watt indication also means 240 instantaneous peak watts The meters are voltage actuated and indicate power accurately when the amplifier is operated into Mentosk MC 2125 POWER AMPLIFIER OUTPUT MODE LIMIT NORMAL RIGHT MONO GAIN y HEADPHONES o PAMOC Front Panel Information rated output load impedances The illuminated word be tween the two meters indicates which scale is in use LEFT GAIN Use the LEFT GAIN control to adjust the volume in the left channel to the desired listening level Turn the control clockwise to increase the volume RIGHT MONO GAIN Use the RIGHT MONO GAIN control to adjust the volume in the right channel to the desired listening level Turn the control clockwise to increase the volume MONO With the output of MC 2125 connected for monophonic operation and the rear panel MODE SWITCH in MONO the volume is controlled by the RIGHT MONO GAIN control only METER RANGE The METER RANGE switch has five positions WATTS In the WATTS position the meter s primary calibration is from 0012 watts 1 2 milliwatts up to 120 watts the advertised maximum low distortion power output of the MC 2125 The meter s indicated power output is accurate if the load impedance is connected to the matching output transformer tap at the LEFT and RIGHT OUTPUT barrier Meintosh MC2125 POWER AMPLIFIER OUTPUT MODE LIMIT NORMAL MONO GAIN HEADPHONES e r 22 PANLOC str
25. witch turns on the relay approxi mately two seconds after the MC 2125 is turned on The same circuit has a short turn off time constant which turns off the relay before the amplifier s main power supply has had a chance to drop The relay is also controlled by the SPEAKER switch The output signal is fed through a match ing network to the front panel HEADPHONE jack The HEADPHONE output is designed to feed low impedance dynamic stereo headphones The signal is always available at the HEADPHONE jacks The HEADPHONE output is not switched The amplifier output signal is fed to the output terminals through the autotransformer The Mclntosh designed in terleaved multifilar wound autotransformer is used to properly match the amplifier to any output impedance tap The MC 2125 will deliver full power over the entire audio frequency range at any of these impedances The auto transformer also protects speakers from damage in the event of amplifier failure Should a direct current compo nent appear in the output it is shunted by the autotrans former and cannot damage the speaker The Mcintosh patented Sentry Monitoring circuit con stantly monitors the output signal and instantly reacts to prevent overload of the output transistors At signal levels up to rated output this circuit has high impedance and has no effect upon the output If the power output exceeds design maximum the Sentry Monitoring circuit oper ates to limit the signal to the outpu
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