McIntosh MC-275 User's Manual
Contents
1. Unfosh a TABLE OF CONTENTS GENERAL DESCRIPTION 1 Speakers Stereo or Twin Amp 5 TECHNICAL DESCRIPTION 1 a no Line Output Stereo or Twin Amp 6 Electrical Specifications 3 as een 3 Line Output Mono 6 echanical Specifications AC Power gt PANEL FACILITIES 4 OPERATION 7 Input 4 Output 4 ADJuSsTMENTS Line Output 4 Line Voltage 7 Fuse 4 Input Switch 7 AC Outlet 4 GUARANTEE 8 Line Voltage 5 FREE 3 YEAR FACTORY SERVICE INSTALLATIONS CONTRACT APPLICATION cover pocket CONNECTIONS OS SCHEMATIC DIAGRAM cover pocket Input Stereo 5 Input Twin Amp 5 PARTS LIST cover pocket Input Mono 5 RESISTANCE AND VOLTAGE CHART cover pocket MC275 STEREO POWER AMPLIFIER GENERAL DESCRIPTION Over one million watts of amplifier output power capacity have been manufactured by McIntosh since 1947 In this 1 000 000 watts of audio power there are less than 10 watts of distortion capacity and less than 1 100 watt of noise capacity The dramatic difference in the quality of music reproduction when you listen through McIntosh instruments is directly due to low distortion performance Careful devoted research is a way of life at Mcintosh The world s finest amplifier is the creation of per severing resourceful Mcintosh engineers McIntosh Laboratory is the only manufac turer in the entire industry to guarantee the lowest distortion at all audio frequencies at full power The U S Patent Office has rec ognized the advance2. Industries Ltd 22 Front Street West Toronto Canada Design subject to change without notice Meontost 2 CHAMBERS STREET BINGHAMTON N Y Made In U S A Phone Area Code 607 723 5491 Printed in U S A
3. is produced at the instant of cut off and again at the instant when current flows This form of distortion is known as Notch Distortion and was well illustrated by Mr Pen Tung Sah in the Proceedings of the Volume 24 pp 1522 1541 in 1936 Imperfect coupling between the primary windings found in all conventional output transformers produces the condition which permits notch distortion Trying to improve coupling in a conventional transformer de creases the power response at both low and high frequencies heating the output tubes and lowering the available power output The Melntosh Unity Coupled output cir cuit and bifilar transformer is the first com mercial breakthrough that eliminates notch distortion by coupling both output tubes almost to perfection In the McIntosh trans former the extremely close coupling of the bifilar windings removes the condition which permits notch distortion Furthermore the the two output tubes are arranged as partial cathode followers Half of the output circuit is in the cathode and half in the plate of each tube The output tubes now are operating in a local feedback loop which reduces their distortion reduces their internal generator resistance and reduces their balance requirements The Mcintosh circuit in reality perfects push pull high efficiency output circuits The MC275 uses an advanced design of the Mcintosh output transformer The new transformer design incorporates all of th
4. of 2 0 volts This position has been provided to be used with McIntosh preampli fiers or any other high output preamplifiers The BALANCE control assists in balancing the overall stereo system for equal loudness Equal loudness is affected by room acous tics furniture placement speakers and many other things With a monophonic signal adjust this control until both loudspeakers are equal in sound output TWIN AMP With the input switch in the center posi tion marked TWIN AMP the input of the MC275 is connected to the two jacks marked TWIN INPUT located below the two center control knobs marked GAIN The input sensi tivity in this position is 0 5 volt This position has been provided for use with a low output stereo preamplifier an electronic crossover network or as two completely separated monophonic 75 watt amplifiers The two center control knobs marked GAIN should be adjusted to match the input sensitivity of the MC275 to the output voltage of the program source equipment To balance a stereo system using the TWIN AMP input position use a monophonic signal from the program source A mono phonic signal supplies the same voltage to both channels Turn the volume control on the source equipment to the 12 o clock posi tion half rotation Turn up the GAIN con trol on the LEFT input until the loudness in the room is not quite as loud as you like to listen Then turn the GAIN control on the RIGHT input until it is equally as l
5. used For distances over 100 feet between the amplifier and speaker use larger wire The loudspeaker impedance is usually identified on the loudspeaker itself Connect one of the leads from the left loudspeaker to the screw marked COM on the left barrier strip Connect the other lead from the left loudspeaker to the screw marked with the number corresponding to the speaker im pedance on the LEFT barrier strip Connect one left speaker Connect one right speaker If the speaker lead to screw LEFT COM lead to the screw marked impedance is and the other to RIGHT COM andthe otherto 4 ohms LEFT 4 RIGHT 4 8 ohms LEFT 8 RIGHT 8 16 ohms LEFT 16 RIGHT 16 The only adverse effect on the operation of a McIntosh amplifier when it is improperly matched is a reduction in the amount of distortion free power available to the loud speaker Close impedance matching is de sirable for maximum distortion free power Use this table to determine proper speaker connections Connect the Ifthe speaker impedance speaker leads is between between COM and 3 2 to 6 5 ohms 4 ohms 6 5 to 13 ohms 8 ohms 13 to 26 ohms 16 ohms OUTPUT MONOPHONIC When the MC275 is to operate as a 150 watt monophonic amplifier the outputs of the two channels combine to produce a single 150 watt output This chart lists the proper connections and interconnections for monophonic operation 6 If the Connect one speaker speaker lead
6. 5 INPUT STEREO The shielded cable from the left output of the Mcintosh preamplifier is plugged into the jack marked STEREO INPUT L The shielded cable from the right output of the McIntosh preamplifier is plugged into the jackmarked STEREOINPUT R INPUT TWIN AMP The shielded cable from the left program source is plugged into the jack marked TWIN INPUT L The shielded cable from the right program source is plugged into the jack marked TWIN INPUT R INPUT MONO The shielded cable from the program source is plugged into the jack marked MONO INPUT OUTPUT STEREO OR TWIN AMPLIFIERS For stereo or twin channel operation it is not necessary to use the same impedance load on each output Simply connect each output for the impedance desired Warning Do not parallel the amplifier out puts when using the amplifier as a stereo amplifier or twin channel amplifier Damage to the output tubes may result if parallel operation is attempted when the output volt ages are not the same phase and amplitude Speakers are connected at the barrier strips marked OUTPUT on the left end of the panel In compliance with the National Electrical Code Class II wiring can be used between the speaker and the amplifier atthe 4 ohm 8 ohm or 16 ohm connection Class II wiring is lamp cord bell wire or other wire with this type of insulation For the normally short distances of under 100 feet between the amplifier and speaker 18 wire or larger can be
7. CHANICAL Dimensions Chassis dimensions 17 long x 1214 wide x 8 high See INSTALLATION on Page 5 for minimum mounting space Weight Chassis only 67 2 pounds In shipping carton 6 pounds SPECIFICATIONS Center tap internally grounded 300 ohms One side internally grounded 34 ohms 75 ohms Output Voltage Stereo or Twin Amp Isolated from ground 25 volts Center tap internally grounded 230 volts One side internally grounded 70 7 volts 115 volts Mono Isolated from ground 25 volts 70 7 volts Center tap internally grounded 230 volts One side internally grounded 70 7 volts 115 volts Internallmpedance Less than 10 of rated load impedance Input Input impedance 250 000 ohms Input sensitivity 0 5 volts to 30 volts through gain control with input switch in MONO or TWIN AMP position With the switch in the STEREO position sensi tivity is 2 0 volts through a balance control Power Requirements 105 volts to 130 volts at 50 60 cycles 240 watts at zero signal output 400 watts at rated output The power trans former is tapped for 117 volts and 125 volts Tube Complement Voltage Amplifier One 12AX7 drives both amps Phase Inverter Two 12AU7 Drive Amplifier Two 12BH7 Cathode Follower Drive Two 12AT7 or 12AZ7 Output Four KT88 or 6550 SPECIFICATIONS Finish Chromium and Black Accessories Supplied OwnersManual Mounting Template Mounting Flanges Octal output plug
8. d technology of the McIntosh circuit by granting 6 patents Long life flexibility highest quality con struction are characteristic designs in every McIntosh instrument Wide electrical and thermal margins of safety for all components and tubes advanced engineering and cool operating design add to the long life built into every McIntosh product Reliability pro longs your investment without expensive maintenancecosts The MC275 has on one chassis two 75 watt power amplifiers In addition to use as a stereo amplifier the flexibility of the MC275 permits it to be used as a monophonic amplifier that delivers 150 watts or as two separate 75 watt amplifiers with each chan nel amplifying completely separate pro grams or as two amplifiers for use with an electronic crossover network Such flexi bility permits maximum use for greatest return from your investment TECHNICAL DESCRIPTION The patented Mclntosh Unity Coupled circuit and output transformer have estab lished Mcintosh amplifiers as the unchal lenged leaders in the audio field Before 1947 low distortion at high power and high efficiency was impossible A com pletely new engineering approach resulted in an amplifier that for the first time permitted high power with distortion below 1 That new engineering produced the Mcintosh Unity coupled circuit and the Mcintosh bifilar wound output transformer With the intro duction of the McIntosh amplifier new stand ards
9. e benefits of the original McIntosh design and even further improves the power band width The MC275 output transformer has three primary windings wound trifilarly The new third winding is connected between the plates of the cathode follower driver Leakage inductance lack of coupling between the primary and secondary wind ings of the output transformer limits the high frequency response of an amplifier The primary and secondary windings of the McIntosh output transformer are interleaved five times to improve coupling The inter leaving is accomplished by winding groups of primary layers then secondary layers alter nately until the total transformer is wound Interleaving helps to extend the Mcintosh power band width to over 100 000 cycles Since 1947 this new technology has been built into every Mcintosh power amplifier Good voltage regulation in the power supply permits overloads without overshoot or blocking good transient response and complete stability To improve regulation a silicon rectifier power supply is used in the MC275 In addition to better voltage regula tion the silicon rectifier allows even higher operating efficiency cooler operation and longer amplifier life To greatly extend tube and component life a thermistor in the MC275 limits current Surges produced when the equipment is turned on The thermistor is a special type of resistor Its resistance depends on its tem perature When the amplifier is
10. erminal strips marked OUTPUT provide stereo connections for the normal speaker impedances of 4 ohms 8 ohms and 16 ohms For monophonic oper 4 ation connections for 2 ohms 4 ohms 8 ohms 16 ohms and 32 ohms are provided The terminal strips may also be connected for a constant voltage output of 25 volts in either stereo or mono or for 70 7 volts in mono The secondary of the Mcintosh output transformer may be operated either grounded or not grounded As supplied the secondary of the MC275 is not grounded If grounding is desirable a wire may be connected be tween chassis and common of either or both output terminal strips The barrier strip output connections are fed from the secondary of the output trans former LINE OUTPUT See also pages 5 6 and 7 The octal socket marked LINE OUTPUT has connections for 70 7 volts 115 volts 230 volts or 600 ohms for stereo operation For monophonic operation the octal socket provides connections for 70 7 volts 115 volts 230 volts or 300 ohms The 600 ohm winding and the 300 ohm winding are balanced to ground Pin 1 of the octal socket is the center tap for these wind ings and is grounded The connections at the octal socket are fed from taps on the cathode winding in the primary of the output transformer FUSE The MC275 uses a 5 0 ampere slo blo type fuse The auxiliary AC socket is not fused AC OUTLET The auxiliary AC outlet can be used to supply power to other equ
11. f the 600 winding Pin 1 is grounded For all applications using 600 ohms use Class wiring if the installation must meet the requirements of the National Electrical Code LINE OUTPUT MONOPHONIC The octal socket marked LINE OUTPUT has connections for operating the MC275 monophonically at 70 7 volts 115 volts or 300 ohms For operating at 25 volts connec tions are made at the barrier strips To feed a 25 volt line connect one side to the screw marked COM on the LEFT barrier strip Connect the other side to the screw marked 8 on the LEFT barrier strip Then connect with a wire the screw marked COM on the LEFT barrier strip to the screw marked COM on the RIGHT barrier strip Connect with another wire the screw marked 8 on the LEFT barrier strip to the screw marked 8 on the RIGHT barrier strip The 34 ohm and the 70 7 volt connec tions are the same To connect for 34 ohms or 70 7 volts solder one of the leads to pin 1 in the octal socket The other lead is soldered to pin 7 A wire is connected be tween pin 7 and pin 8 For 150 watt monophonic operation 70 7 volts is available from the secondary of the output transformers isolated from ground Connect one lead to the terminal strip screw marked LEFT COM and the other lead to the screw marked RIGHT 16 Connect a wire between RIGHT COM and LEFT 16 To connect for 300 ohms one lead is soldered to pin 2 The other lead is sol dered to pin 3 Connect pin 2 to pin 4 w
12. flanges are shipped sepa rately and must be attached to the amplifier bottom at each end Turn the MC275 over with the transform ers down and place on a piece of cloth or cardboard to prevent scratches The ampli fier is shipped with 4 plastic feet on the bot tom cover Remove the 4 plastic feet Re move the three self tapping screws at one end of the amplifier that hold the bottom cover to the chromium chassis Place the square edge of the mounting flange with the three round holes positioned over the three holes in the bottom cover The three holes in the mounting flange will line up over the three holes in the chassis Replace the three self tapping screws through the mounting flange and bottom cover The mounting flange is now firmly attached to the ampli fier The mounting flange on the other end of the amplifier is installed in the same way HTING FLANGE Fig 4 Bottom of MC275 with mounting flanges attached Place the MC275 mounting template on the area where the amplifier is to be mounted Center punch the mounting shelf through the 4 slotted holes in the mounting flanges shown on the template Remove the template and install four 10 round head screws in the center punch marks Do not tighten these screws Place the amplifier over the screw heads and slide the amplifier to either side in the slotted holes of the mounting flanges The 10 wood screws can then be tightened securely CONNECTING THE MC27
13. for distortion free performance were established PRIMARY NO PRIMARY NO 2 Fig 1 Representation of bifilar winding The Mcintosh output transformer is unique It has two primary windings which are wound bifilarly In the bifilar technique both primary wires are wound side by side Each turn of primary number one is next to the same turn of primary number two There is almost complete magnetic coupling between the two wires The magnetic coupling is rein forced by the capacitance between the two wires OUTPUT Fig 2 Representation of McIntosh output circuit 10 5 Patent No s 2 477 074 2 545 788 2 646 467 2 654 058 2 860 192 2 929 028 In the McIntosh Unity Coupled circuit one of the bifilar primary windings is connected through the power supply to the plate and cathode of one of the output tubes The other bifilar primary winding is similarly connected to the other tube All low distortion high power amplifiers use push pull output circuits known as Class AB or Two tubes are arranged in a balanced circuit This permits each tube to operate alternately somewhat over half the time Compared to full time operation of the tubes the push pull method reduces heating and permits more power from a given type of tube Despite this advantage of the conven tional push pull circuit one problem in par ticular remained to be solved When current in each tube is cut off to begin the idle period distortion
14. ipment in the system The outlet will provide a maximum of 300 watts of power The AC outlet is notfused LINE VOLTAGE TheMC275operatesonanylinevoltage be tween 105 volts and 130 volts 50 cycles to 60 cycles The switch marked LINE VOLTAGE changes the tap on the primary ofthe power transformer The switch can be adjusted at any time INSTALLATION Adequate ventilation extends the trouble free life of electronic instruments It is generally found that each 10 centigrade 18 F rise in temperature reduces the life of electrical insulation by one half Adequate ventilation is an inexpensive and effective means of preventing insulation breakdown that results from unnecessarily high operat ing temperatures The direct benefit of ade quate ventilation is longer trouble free life The suggested minimum space for mount ing the MC275 is 20 long x 14 wide x 10 high Always allow for air flow either by ventilation holes or space next to the bottom of the amplifier and a means for the warm air to escape at the top The MC275 can be mounted in any posi tion except upside down Ifthe amplifier is to be installed on a vertical surface it is recom mended that the transformers be on the down side The advantage of this position is that the flow of heat from the tubes rises vertically and does not tend to heat the transformers Ifthe MC275 is to be permanently mounted use the two mounting flanges supplied with the amplifier The
15. ith a wire With another wire connect pin 3 to pin 5 Pin 1 is the center tap of the 300 ohm winding The 300 ohm winding is balanced to ground AC POWER The MC275 operates on 105 volt to 130 volt 50 to 60 cycle power The amplifier will be turned on and off if its power cord is plugged in one of the auxiliary AC outlets on the program source OPERATING THE MC275 Before turning the MC275 on check all connections and plugs to see that they are firmly and correctly connected Check to make sure that the tubes are firmly seated in the proper sockets After the following adjustments have been completed the MC 275 will operate without any further attention ADJUSTMENTS LINE VOLTAGE The LINE VOLTAGE switch should be set to the position that more closely matches the average line voltage in your area The local electric power company can usually tell you the average line voltage in your area If the line voltage is between 105 volts and 120 volts put the LINE VOLTAGE switch in the 117 volt position If the line voltage is between 120 volts and 130 volts put the switch in the 125 volt position If the line voltage is un known it is safer to operate with the switch in the 125 volt position INPUT SWITCH STEREO With the input switch in the left position marked STEREO the input of the MC275 is connected to the two jacks marked STEREO INPUT located below the BALANCE control The amplifier has an input sensitivity in this position
16. off the therm istor has a high resistance value about 79 ohms Just after the amplifier is turned on the current which flows through the therm istor heats it and causes its resistance to decrease to a low value less than 7 ohms Current is thus limited when the MC275 is first turned on but is not limited as the unit warms ELECTRICAL Power Output Stereo or twin amplifier 75 watts con tinuous per channel Monophonic 150 watts continuous Harmonic Distortion Less than 0 5 at rated output or less 20 cycles through 20 000 cycles Typical performance is 0 3 or less at 20 cycles and 20 000 cycles at full power Typical performance in the mid frequencies is 0 1 or less at full power Intermodulation Distortion Less than 0 5 for any combination of frequencies from 20 cycles through 20 000 cycles if instantaneous peak power is below twice the rated power Phase Shift Less than 8 20 cycles through 20 000 cycles Frequency Range At rated output 0 0 1 db 16 cycles through 40 000 cycles 0 0 5 do 16 cycles through 60 000 cycles At one half rated output 0 1 db 10 cycles through 100 000 cycles Noise and Hum 90 db or more below rated output Output Impedance Stereo or Twin Amp Isolated from ground 4 ohms 8 ohms 16 ohms Center tap internally grounded 600 ohms One side internally grounded 67 ohms 150 ohms Mono Isolated from ground 2 ohms 4 ohms 8 ohms 16 ohms 32 ohms ME
17. oud as the left channel The system is now balanced for loudness and provides the most convenient amount of loudness change when using the volume control on the source equipment MONO With the switch in the right position marked MONO the inputs of the MC275 are internally connected in parallel to the jack 7 marked MONO INPUT This jack is located below the right control knob marked GAIN The input sensitivity in this position is 0 5 volt The GAIN control should be adjusted to match the input voltage sensitivity of the MC275 to the output voltage of the program source For maximum flexibility set the GAIN con trol as follows turn the volume control on the program source equipment i e tape recorder tuner preamplifier etc to the 12 o clock half rotation position Turn the GAIN control on the MC275 until the loud ness of the sound from the speaker is just a little louder than you normally like to listen The volume control on the program source equipment now has the most convenient amount of loudness change in either direc tion from the 12 o clock position IMPORTANT The excellent performance that is inher ent in all Mclntosh amplifiers does not depend on the critical adjustment of bias or balance controls in the output circuit The patented Melntosh circuit delivers its adver tised specifications without any need for these controls and is not dependent on care fully balanced tubes for its performance With McIntosh
18. to the screw impedance markedLEFT COM Connect a Connect another is and the other to wire between wire between 2 ohms LEFT 4 LEFT COM and LEFT 4 and RIGHT COM RIGHT 4 4 ohms LEFT 8 LEFT COM and LEFT 8 and RIGHT COM RIGHT 8 8 ohms LEFT 16 LEFT COM and LEFT 16 and RIGHT COM RIGHT 16 16 ohms RIGHT 8 RIGHT COM and LEFT 8 32 ohms RIGHT 16 RIGHT COM and LEFT 16 When connected as outlined the MC275 operates as a 150 watt monophonic amplifier LINE OUTPUT STEREO or TWIN AMPLIFIERS The octal socket marked LINE OUTPUT has connections for 70 7 volts 115 volts and 600 ohms The 25 volt line is fed from the barrier strips For 25 volt line operation connect one of the left leads to the screw marked COM on the LEFT barrier strip The other left lead is connected to the screw marked 8 on the LEFT barrier strip Connect the right leads in the same manner on the RIGHT barrier strip The 67 ohm and 70 7 volt connections are the same To connect for 67 ohm or 70 7 volt operation the octal plug supplied in the hardware bag is used Solder one of the left leads to pin 7 and the other to pin 1 Solder one of the right leads to pin 8 and the other to pin 1 Pin 1 is the ground side The 600 ohm connections for the left side are made between pin 2 and pin 3 The 600 ohm connections for the right side are made between pin 4 and pin 5 The 600 ohm winding is balanced to ground Pin 1 on the octal socket is the center tap o
19. with cap PANEL INE VOLTAGE LIN II 129 FACILITIES Fig 3 The end panel of the MC275 showing all input and output facilities INPUT The input of the MC275 has a three posi tion lever switch to permit the amplifier to be used in any one ofthree ways 1 As a monophonic 150 watt amplifier 2 As twin 75 watt amplifiers used with an electronic crossover network or two completely separate amplifiers or in stereo applications requiring 0 5 volt sensitivity in the power amplifiers 3 As a stereo amplifier used with a McIntosh preamplifier or other high Output stereo program sources The input marked STEREO has a sensi tivity of 2 0 volts controlled by a BALANCE control The input impedance is 250 000 ohms The input marked TWIN AMP has a sensi tivity of 0 5 volt Each TWIN AMP input is controlled by a GAIN control The GAIN con trol allows signal sources with output from 0 5 volt up to 30 volts to be connected with out overloading the input to the amplifier The input impedance is 250 000 ohms The input marked MONO has a sensitivity of 0 5 volt The MONO position of the input switch parallels the inputs to both ampli fiers When the outputs are properly con nected the MC275 becomes a 150 watt monophonic amplifier The MONO input has a GAIN control that permits connecting signal sources up to 30 volts without overloading the amplifier s input The input impedance is 250 000 ohms OUTPUT The two barrier t
20. you can install the amplifier and forget it The MC275 you have purchased will give you years of pleasant and satisfactory per formance If you have any questions con cerning the operation or maintenance of this amplifier please contact Customer Service McIntosh Laboratory Inc 2 Chambers Street Binghamton New York Our telephone number is 723 5491 The direct dial area code is 607 GUARANTEE McIntosh Laboratory Incorporated guar antees this equipment to perform as adver tised We also guarantee the mechanical and electrical workmanship and components of this equipment to be free of defects for a period of 90 days from date of purchase This guarantee does not extend to compo nents damaged by improper use nor does it extend to transportation to and from the factory 3 YEAR FACTORY SERVICE CONTRACT An application for a FREE 3 YEAR FAC TORY SERVICE CONTRACT is included in the pocket in the back cover of this manual The FREE 3 YEAR FACTORY SERVICE CONTRACT will be issued by Mcintosh Laboratory upon receipt of the completely filled out applica tion form The term of this contract is defined in the 3 year factory service contract If the application is not mailed to Mcintosh Labo ratory only the services offered under the standard 90 day guarantee will apply on this equipment TAKE ADVANTAGE OF 3 YEARS OF FREE FACTORY SERVICE BY FILLING IN THE APPLICATION NOW In Canada Manufactured under license by McCurdy Radio
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