McIntosh MC-75 User's Manual
Contents
1. At 38 watts output 0 1 db 10 cycles through 100 kilocycles Noise and Hum 90 db or more below rated output eo i Output Impedance 4 ohms 8 ohms 15 ohms 67 ohms 150 ohms and 600 ohms The 4 ohm 8 ohm and 16 ohm outputs are isolated from ground for convenience in connecting speakers for stereo The 600 ohm winding is balanced to ground Output Voltages 25 volts 70 7 volts 115 volts and 230 volts The 230 volt winding is center tapped Internal Impedance Less than 10 of rated load impedance Input Input impedance 250 000 ohms Input sensitivity 0 5 volts to 30 volts through gain control 2 0 volts with gain controlturned fully clockwise Power Requirements 117 or 125 volts AC 50 60 cps Uses 240 watis at 75 watts output 140 watts at zero signal output Tube Complement Voltage Amplifier 12AX7 Phase Inverter 12AU7 Drive Amplifier 12BH7 Cathode Follower Driver 12AT 12AZ7 Output 2 each KT 88 or 6550 PANEL FACILITIES KOTE USE CLASS 1 WIRE FOR 150 amp 600 9 OUTPUT USE CLASS 2 WIRE FOR OTHER OUTPUTS SPEAKER OUTPUT GAIN 2 0 SETTING FOR MclkTOSH PREAMP GND COM 4 8 l alas SOS 117V AG LINE VOLTAGE 300 WATT WW 125 pt Figure 3 End panel showing all input and output facilities 117 125 VOLTS 50 60 CPS 140 240 WATTS FUSE 3 24 0 5 VOLT SLO BLO INPUT INPUT The input sensitivity of the MC75 is 0 5 volt for 75 watts output The input impe2. Its resistance depends on its tem perature When the amplifier is 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 MC75 is first turned on but is not limited as the unit warms MECHANICAL SPECIFICATIONS Dimensions Chassis dimensions 1634 long x 814 wide x 814 high See INSTALLATION on Page 5 for minimum mounting space Weight Chassis only 44 pounne In shipping carton 49 pounds A Finish Chrome and Black Accessories Supplied Instruction Manual Mounting Template Mounting Flanges Octal output plug with cap ELECTRICAL SPECIFICATIONS Power Output 75 watts continuous Harmonic Distortion Less than 0 5 at 75 watts output or less 20 cycles through 20 000 cycles Typical per formance is 0 3 or less for 20 cycles and 20 000 cycles at full power Typical mid fre quency performance is less than O Ve at full power ntermodulation Distortion Less than 0 5 for any combination of fre quencies from 20 cycles through 20 000 cycles if instantaneous pos power is below 150 watts Phase Shift Less than 8 20 cycles through 20 000 cycles Frequency Range Ai 75 watts output 0 0 1 db 16 cycles through 40 000 cycles 0 0 5 db 16 cycles through 60 000 cycles
3. instrument Your good judgementin selecting McIntosh will be reinforced the more you use this equipment New safe ratings on all compo nents and tubes advanced engineering and cool operating design add to the long life built in every McIntosh product Reliability pro longs your investment without expensive maintenance costs The MC75 is a 75 watt power amplifier It is perfect for use in a monophonic home music system or as one channel in a stereo system In addition to home music system use the excellent performance of the MC75 is demanded by industrial users Some in dustrial applications are recording studios and broadcast stations high quality public address systems shake table driver and many instrumentation uses Just a few of the well known users of Mcintosh are U S Armed Forces National Bureau of Standards Boe ing Airplane Company Remington Rand Atomic Energy Commission Western Electric and many many more TECHNICAL DESCRIPTION The patented McIntosh 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 transforme
4. output transformer The new transformer design incorporates all of the benefits of the original McIntosh design and even further improves the power band width The MC75 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 Interieaving 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 MC75 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 MC75 limits current surges produced when the equipment is turned on The thermistor is a special type of resistor
5. volts 115 volts and 230 volts The 230 volt winding is center tapped The 4 ohm 8 ohm and 16 ohm output terminals are fed from taps on the secondary winding of the output transformer The 67 ohm 150 ohm and 600 ohm output ter minals are fed from taps on cathode winding in the primary of the output transformer The 25 volt output is fed from a tap on the secondary of the output transformer The 70 7 volt 120 volt and 240 volt output terminals are fed from taps on the cathode winding in the primary of the output trans former FUSE The MC75 uses a 3 2 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 equipment in the system The outlet will provide a maximum of 300 watts of power The AC outlet is not fused _ LINE VOLTAGE The MC75 operates on line voltages between 105 volts to 130 volts 50 cycles to 60 cycles The switch marked LINE VOLTAGE changes the tap on the primary of the power transformer The switch can be adjusted at any time INSTALLATION Adequate ventilation should always be provided in the installation of electronic in struments The trouble free life of these in struments can be extended by adequate ventilation lt 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 preve
6. Iii nio 5 h AMPLIFIER GENERAL DESCRIPTION _ _ TECHNICAL DESCRIPTION _ na Electrical Specifications Ss i Mechanical Specifications PANEL FACILITIES L SE a AS Input pean Baten Speaker Output Aaaa thts hilo Output i EENAA A AE Fuse 5 Keak es REA he eee OS Line Voltage es S ah Soot INSTALLATION ooh PEN CONNECTIONS amma AATE input ee we ee KNEE SASN Output prs AC Power aah BOT Set Pet OPERATION Bas ADJUSTMENT GUARANTEE i SENA FREE 3 YEAR FACTORY SERVICE CONTRACT CC Cr pocket TABLE OF CONTENTS Reading Time 13 Minutes ISSUE NO 1 ot AUUU U WWHONNND NNK H MAINTENANCE MANUAL cover pocket PARTS LIST COVer pocket MC7S POWER AMPLIFIER GENERAL DESCRIPTION The dramatic difference in the quality of music reproduction when you listen through McIntosh instruments is directly due to low distortion performance Careful devoted re search 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 recog nized the advanced technology of the Mcin tosh circuit by granting 6 patents Long life flexibility highest quality construction are characteristic designs in every Mcintosh
7. N THE APPLICATION NOW In Canada manufactured under license by McCurdy Radio Industries Ltd 22 Front Street West Toronto Canada Design subject to change without notice
8. and speaker use larger wire The loud speaker impedance is usually identified on the speaker itself Connect either one of the speaker leads to the screw on the barrier strip marked COM Connect the other speaker lead to the screw on the barrier strip marked with the number corre sponding to the speaker impedance The only adverse effect on the operation of a McIntosh amplifier when it is improperly matched is a reduction in the amount of dis tortion free power available to the loudspeak er Close impedance matching is desirable for maximum distortion free power Use this table to determine proper speak er connections If the speaker Connect the speaker impedance leads between is between COM and 3 2 to 6 5 ohms 4 ohms 6 5 to 13 ohms 8 ohms 13 to 26 ohms 16 ohms The octal socket marked OUTPUT has connections for all of the MC75 output im pedances and voltages Connections for the octal socket are listed in these tables If the impedance is Solder the leads between 4 ohms Pin 1 and 2 8 ohms Pin 1 and 3 16 ohms Pin 1 and 4 67 ohms Pin 5 and 6 150 ohms Pin 6 and 7 600 ohms Pin 7 and 8 The 600 ohm winding is balanced Pin 6 is the center tap of the 600 ohm winding and is ground For constant voltage applications use these connections Voltage required Connect to 25 volts Pin 1 and 3 70 7 volts Pin 5 and 6 120 volts Pin 6 and 7 240 volts Pin 7 and 8 For all applications using 150 ohms 600 ohms 120 v
9. dance is 250 000 ohms There is a GAIN control in the input circuit to permit the use of input voltages up to 30 volts In addition turning the GAIN control fully clockwise operates a switch that changes the input sensitivity to 2 0 volts for 75 watts output The 2 0 volt input position is for use with Mcintosh pre amplifiers The input jack marked 0 5 VOLT INPUT is located below the GAIN control SPEAKER OUTPUT Output connections are provided on the barrier strip marked SPEAKER OUTPUT for the normal speaker impedances of 4 ohms 8 ohms and 16 ohms They are also repeated at the octal socket marked OUTPUT A ground connection marked GND has been provided on the barrier strip The strap between the terminal marked GND and the terminal marked COM is provided to ground the secondary of the output transformer to the chassis for normal applications If it is desirable to have the secondary of the output transformer feed a balanced line the strap between the COM and GND terminals can be removed McIntosh Amplifiers are designed to operate with the secondary of the output transformer either grounded or not grounded The speaker output connections are fed from the secondary of the output transformer OUTPUT The octal socket marked OUTPUT has connections for 4 ohms 8 ohms 16 ohms 4 67 ohms 150 ohms or 600 ohms The 600 ohms is balanced to ground The octal socket also has connections for constant voltages of 25 volts 70 7
10. ge will line up over the three holes in the chassis Replace the three self tapping screws through the mounting flange and bottom cover Tne mounting flange is now firmly attached to the amplifier Repeat the same procedure to in stall the mounting flange on the other end of the amplifier Place the MC75 mounting template on the area where the amplifier is to be mounted Center punch the 4 slotted holes in the mounting flanges shown on the template Remove the template and instail 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 in the mounting flanges The 10 wood screws can then be tightened securely Figure 4 Bottom of MC75 with mounting flanges attached CONNECTING Plug the shielded cable from the program source into the jack marked 0 5 VOLT INPUT Speaker connections can be made either at the barrier strip or the octal socket The barrier strip is usually more convenient In compliance with the National Electrical Code Class II wiring can be used between the speaker and the amplifier at the 4 ohm 8 ohm 16 ohm or 67 ohm connection Class 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 used For distances over 100 feet be tween the amplifier
11. hen current in each tube is cut off to begin the idle period distortion 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 IL R E 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 Mcintosh 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 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 MC75 uses an advanced design of the McIntosh
12. n the control above the input jack marked GAIN totally clockwise to the right to the position marked 2 0 V SETTING FOR McINTOSH PREAMP When using a stereophonic McIntosh pre amplifier system turn the control above the input jack marked GAIN to the 12 o clock position on both amplifiers Adjust the GAIN control on one amplifier to one side or the other of the 12 o clock position until equal amplitude is obtained from the amplifiers If the MC75 is to be used with any program source other than a Mcintosh preamplifier the setting of this control should be adjusted to match the input voltage sensitivity of the MC75 to the output voltage of the program source For maximum flexibility set the amplifier gain control as follows turn the volume con trol on the program source equipment i e tape recorder tuner preamplifier other than McIntosh etc to the 12 o clock position Turn the gain control on the MC75 until the loudness 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 conven ient amount of loudness change in either direction from the 12 o clock position IMPORTANT The excellent performance that is inher ent in all McIntosh amplifiers does not de pend on the critical adjustment of controls in the output circuit The patented Mcintosh circuit delivers its advertised specifications without any need for these contr
13. nting insulation breakdown because of unnecessarily high operating temperatures The direct benefit of adequate ventilation is longer trouble free life The suggested minimum space for mount ing the MC75 is 1744 long x 10 wide x 10 high Always supply ventilation holes in the bottom of the amptifier mounting space and a means for the warm air to escape at the top The MC 5 can be mounted in any position except upside down If the 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 trans formers If the MC75 is to be permanently mounted use the two mounting flanges supplied with the amplifier The flanges are shipped sepa rately and must be attached to the amplifier Faas MOUNTING FLANGE ra CENTERPUNCH f 4 PLACES JZ CHASSIS OUTLINE k bottom at each end Turn the MC75 over with the transformers down and place it on a piece of cloth or card board to prevent scratches The amplifier is shipped with 4 plastic feet on the bottom cover Remove the 4 plastic feet Remove the three self tapping screws at one end of the amplifier that hold the bottom cover to the chrome 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 flan
14. ols and is not dependent on carefully balanced tubes for its performance With Mcintosh you can in stall the amplifier and forget it Your MC75 will give you many years of pleasant and satisfac tory performance If you have any questions concerning the operation ar 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 de fined in the 3 year factory service contract If the application is not mailed to McIntosh Laboratory 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 I
15. olts or 240 volts use Class wiring if the installation must meet the re quirements of the National Electrical Code AC POWER The MC75 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 FUSE The MC75 is fused with a slo blo type fuse rated at 3 2 amperes For maximum protec tion always replace with fuses of the same rating never larger The AC outlet on the MC75 is not fused OPERATING THE MC75 Before turning the MC75 on check all con nections 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 MC75 will operate with out 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 GAIN When using a monophonic McIntosh pre amplifier tur
16. r With the intro duction of the McIntosh amplifier new stand ards for distortion free performance were established PRIMARY NO 1 T PRIMARY NO 2 Figure 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 Figure 2 Representation of McIntosh output circuit IU S 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 AB or B 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 W
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