THE MclNTOSH MC 502 POWER AMPLIFIER
Reading Time: 28 Minutes
Price $2.00
VARIOUS REGULATORY AGENCIES REQUIRE THAT WE BRING THE
FOLLOWING INFORMATION TO YOUR ATTENTION. PLEASE READ IT
CAREFULLY.
WARNING: TO PREVENT FIRE OR SHOCK
HAZARD, DO NOT EXPOSE THIS UNIT TO
RAIN OR MOISTURE.
The Mclntosh you have purchased is a Model MC
502. It has a serial number located on the rear panel
of the chassis. Record that serial number here:
Serial Number
The model, serial number and purchase date are
important to you for any future service. Record the
purchase date here:
Purchase Date
Upon application, Mclntosh Laboratory provides a
Three-Year Service Contract. Your Mclntosh
authorized Service Agency can expedite repairs
when you provide the Service Contract with the instrument for repair. To assist, record your Service
Contract number here:
Service Contract Number
Your MC 502 Stereo Power Amplifier
will give you many years of pleasant and
satisfactory performance. If you
have any questions, please contact:
CUSTOMER SERVICE
Mclntosh Laboratory Inc.
2 Chambers Street
Binghamton, New York 13903
Phone: 607-723-3512
Take Advantage of 3 years
of Contract Service ...
Fill in the Application NOW.
Contents
HOW TO INSTALL... 2,3
HOW TO CONNECT... 4
THE FRONT PANEL CONTROLS
AND HOW TO USE THEM... 7
REAR PANEL INFORMATION... 8
PERFORMANCE LIMITS... 9
PERFORMANCE CHARTS ...10,11
TECHNICAL DESCRIPTION ...12,13
BLOCK DIAGRAM...14
MclNTOSH THREE YEAR SERVICE CONTRACT
An application for A THREE YEAR SERVICE CONTRACT is included with this manual.
The terms of the contract are:
1. Mclntosh will provide all parts, materials
and labor needed to return the measured
performance of the instrument to the
original performance limits. The SERVICE CONTRACT does not cover any
shipping costs to and from the authorized service agency or the factory.
2. Any Mclntosh authorized service agency
will repair Mclntosh instruments at normal service rates. To receive service
under the terms of the SERVICE CONTRACT, the SERVICE CONTRACT CERTIFICATE must be presented when the
instrument is taken to the service agency.
3. Always have service done by a
Mclntosh authorized service agency. If
the instrument is modified or damaged
as a result of unauthorized repair, the
SERVICE CONTRACT will be cancelled.
Damage by improper use or mishandling is not covered by the SERVICE
CONTRACT.
4. 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.
5. To receive the SERVICE CONTRACT,
your purchase must be made from a
Mclntosh franchised dealer.
6. Your completely filled in application for
the SERVICE CONTRACT must be post-
marked within 30 days of the date of
purchase of the instrument.
7. To receive the SERVICE CONTRACT, all
information on the application must be
filled in. The SERVICE CONTRACT will
be issued when the completely filled in
application is received by Mclntosh
Laboratory Incorporated in Binghamton,
New York.
8. Units in operation outside the United
States and Canada are not covered by
the Mclntosh Factory Service Contract,
irrespective of the place of purchase.
Nor are units acquired outside the
U.S.A. and Canada, the purchasers of
which should consult with their dealer
to ascertain what, if any, service contract or warranty may be available locally.
Copyright 1979 © by Mclntosh Laboratory Inc.
1
The PANLOC system of installing equipment conveniently and securely is a product of Mclntosh
research. By depressing the two PANLOC buttons
on the front panel, the instrument slide can be locked firmly in place or it can be unlocked so that the
chassis can slide forward, giving you easy access to
the top and rear panels.
The trouble-free life of an electronic instrument is
greatly extended by providing sufficient ventilation
to prevent the buildup of high internal temperatures
that cause deterioration. Allow enough clearance so
that cool air can enter at the bottom of the cabinet
and be vented from the top. With adequate ventilation the instrument can be mounted in any position.
The recommended minimum space for installation
is 15 inches (38.1 cm) deep, 17 inches (43.2 cm) wide,
and 4-1/2 inches (11.4 cm) high.
To install the instrument in a Mclntosh cabinet,
follow the instructions that are enclosed with the
cabinet. For any other type of installation follow
these instructions:
1. Open the carton and remove the PANLOC
brackets, hardware package, and mounting
template from the carton. Remove the MC 502 from
its plastic bag and place it upside down on the shipping pallet; unscrew the four plastic feet from the
bottom of the chassis.
2. Mark the cabinet panel.
Place the mounting template in the position on the
cabinet panel where the instrument is to be install-
ed, and tape it in place. The broken lines that repre-
sent the outline of the rectangular cutout also repre-
sent the outside dimensions of the chassis. Make
sure these lines clear shelves, partitions, or any
equipment. With the template in place, first mark the
six A and B holes and the four small holes that
locate the corners of the cutout. Then, join the four
corner markings with pencil lines, using the edge of
the template as a straightedge.
3. Drill Holes
Use a drill with a 3/16 inch bit held perpendicular to
the panel and drill the six A and B holes. Then, using
a drill bit slightly wider than the tip of your saw
blade, drill one hole at each of two diagonally opposite corners. The holes should barely touch the inside edge of the penciled outline. Before taking the
next step, make sure that the six A and B holes have
been drilled.
4. Saw the Panel Cutout
Saw carefully on the inside of the penciled lines.
First make the two long cuts and then the two short
cuts. After the rectangular opening has been cut out,
use a file to square the corners and smooth any irregularities in the cut edges.
2
5. Install the Mounting Strips
In the hardware package you will find two mounting
strips, and two sets of machine screws. For panels
that are less than 1/2 inch thick, use the 3/4 inch
screws; for panels that are more than 1/2 inch thick,
use the 1-1/4 inch screws.
Starting at the right-hand side of the panel, insert
a screw of the proper length into the center hole in
the panel, marked B on the templete. On the back of
the panel, align a mounting strip with the holes in
the panel and tighten the screw until the screwhead
is pulled into the wood.
Repeat this procedure to attach the mounting
strip to the left side of the panel.
6. Attach the PANLOC Brackets
Using two screws of the proper length in the A holes
on each side, attach the PANLOC brackets to the
cabinet panel; the short flange is mounted against
the front (face) of the cabinet panel. The screws
pass through the PANLOC bracket flange, the
cabinet panel, and then through the mounting strips
previously mounted.
7. Install the Instrument
Guide the AC power cord through the panel opening
to the back of the cabinet; then, slide the instrument
into the opening carefully so that the rails on the
bottom of each side of the chassis engage the
tracks on the mounting brackets. Continue to slide
the instrument into the cabinet until it is stopped by
the adjust position latches. Press the latches inward, this permits the instrument to slide into the
cabinet until its front panel is flush with the cabinet
panel. Depress the PANLOC buttons at the lower left
and right corners of the instrument panel to lock the
unit firmly in the cabinet. Depressing the PANLOC
buttons again will unlock the instrument so that it
can slide forward to the adjust position; if you press
inward on the adjust position latches then you can
remove the instrument from the cabinet.
3
How to Connect
INPUT
STEREO OR TWIN AMPLIFIER OPERATION
Use shielded cables to connect the signal from
the preamplifier or signal source to the power
amplifier input. To minimize the possibility of hum
the shielded cables should be run parallel to each
other or loosely twisted together. Locate the cables
away from speaker leads and AC power cords. All
connections are made on the back panel of the MC
502.
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.
MONOPHONIC OR SINGLE CHANNEL OPERATION
A shielded cable from the signal source is plugged into the Right (MONO) input jack of the MC 502
only. The MODE SWITCH on the back panel of the
amplifier must be placed in the MONO position. In
the MONO position the right channel input jack connects to both amplifiers. The Left INPUT is disconnected.
OUTPUT
Selection of the proper gauge wire to connect the
loudspeakers preserves the quality of sound repro-
duction for which the loudspeakers have been designed. If undersize wire is used, resistance is added
to the amplifier/loudspeaker combination which
adversely affects the performance. Added resistance causes reduction of damping characteristics,
modification of frequency response and reduction in
power output.
Use lamp cord, bell wire, or wire with similar type
of insulation to connect the speakers to the amplifier. In all cases, the leads to and from the speaker
should be twin conductor or twisted together. When
using 8 ohm speakers and for the normally short dis-
tances of under 30 feet between the amplifier and
speaker, No. 18 wire or larger can be used. For
distances over 30 feet between the amplifier and
speaker use larger diameter wire. Select the correct
size wire for the wire length from the chart. It is
recommended that the DC resistance of the speaker
leads be less than 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.
MAXIMUM WIRE LENGTHS
Wire
Gauge
22
20
18
16
14
12
10
Wire lengths above represent the wire resistance equal to 5%
of the speaker impedance.
For 4 Ohm Load
Feet
6
10
15
25
40
60
100
Meters
1.8
3.1
4.6
7.6
12.2
18.3
30.5
For 8 Ohm Load
Feet
12
20
30
50
80
120
200
Meters
3.7
6.1
9.1
15.2
24.4
36.6
66.0
For multiple speaker operation, run separate
leads from the amplifier to the speakers.
CONNECTING LOUDSPEAKERS FOR STEREO
Connect the leads from the left loudspeaker to the
Left and Common OUTPUT terminals on the MC 502.
Connect the leads from the right main loudspeaker
to the Right and Common terminals.
CONNECTING ONE LOUDSPEAKER FOR MONO
Connect leads from the loudspeaker to the Left
and Right OUTPUT terminals on the MC 502. Check
to be sure the MODE switch is in the MONO posi-
tion.
In MONO operation neither output terminal (Left
or Right) is at ground potential. The output is balanced to ground. Due precaution is required when connecting test instruments since neither terminal can
be grounded.
AC POWER
The amplifier AC power cord is plugged into a 120
volt 50/60 Hz wall outlet.
FUSE
A 4-amp fuse protects the MC 502 circuits. The
fuse does not protect additional equipment connected to the rear panel AC power outlets.
4
STEREO OPERATION
STEREO POSITION
PLACE SWITCH
IN
FOR
MODE
MONO
PROGRAM SOURCE
STEREO
Stereophonic
Connections
RIGHT
SPEAKER
LEFT
SPEAKER
POSITION
STEREO OPERATION
STEREO
PLACE SWITCH
IN
FOR
SOURCE
MODE
PROGRAM
MONO
STEREO
SPEAKER
Monophonic
Connections
Front Panel Controls and
How to Use Them
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. When the MC 502 is connected for mono-
phonic operation and the rear panel Mode Switch in
Mono the volume is controlled by the Right/Mono
Gain control only.
HEADPHONES
The output of the front panel HEADPHONE jack
has been designed to feed low impedance dynamic
stereo headphones. Plug dynamic headphones into
the front panel HEADPHONE jack. Adjust the front
panel LEFT GAIN and RIGHT/MONO GAIN control
for comfortable headphone listening. Electrostatic
headphones generally require higher power than
dynamic headphones. Connect them to the LEFT
and RIGHT OUTPUT contacts on the back of the MC
502.
SPEAKERS
OFF: The loudspeakers are turned off when the
SPEAKER switch is in the OFF position. You can
listen to headphones in private.
POWER
The power switch turns the MC 502 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 502 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.
POWER GUARD INDICATOR LAMPS
POWER GUARD assures that the power amplifier
section of the MC 502 can not be over driven, thus
amplifier output clipping is eliminated. Clipping is
caused when the amplifier is asked to produce more
power output than it can deliver with low distortion.
Amplifiers are capable of delivering large quantities
of power when they are driven to clipping and can
have more than 40% harmonic distortion. The extra
energy content of the clipped signal will damage
most speakers. A Mclntosh advancement helps to
protect your speaker from this kind of damage. The
MC 502 has a built in "waveform comparator" that
compares the wave shape of the output signal to the
input signal. If the non-linearity between the two
signals exceeds 0.5% the POWER GUARD circuit
operates. Operation is indicated when the red LIMIT
indicators turn on. As long as the amplifier operates
without overload the NORMAL indicator illuminates.
ON: Music will be heard through the
loudspeakers. Use this as the normal listening posi-
tion.
THE SPEAKER SWITCH MUST BE IN THE "ON"
POSITION TO HEAR MUSIC FROM THE
LOUDSPEAKERS.
7
Rear Panel Information
LEFT and RIGHT OUTPUT
For stereo operation, output connections for im-
pedances of 2.7 to 8 ohms are provided on a secure,
screw type barrier strip. For monophonic operation
proper interconnection provides 8 ohms from the
same barrier strip.
MODE SWITCH
The MC 502 can be used for stereo or mono (single
channel) operation. The MODE switch is used to
select the method of operation desired.
INPUT
In the stereo mode of operation both input jacks
accept signal. In the mono mode of operation only
the Right (MONO) channel input jack accepts signal
and the Left channel input jack is disconnected.
INPUT LEVEL
The input sensitivity of the MC 502 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 clockwise, the amplifier will deliver its rated
power. All Mclntosh preamplifiers have been designed to deliver 2.5 volts output with rated input. For the
best signal to noise ratio when using Mclntosh
source equipment, place 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 Mcln-
tosh set the switch in the position nearest to the
stated output rating of the source equipment.
AC POWER
The MC 502 is rated for 120 volts, 50/60 hertz. It
uses 0.2 amperes (20watts) when there is no signal
output and up to 4 amperes (400 watts) with both
channels delivering rated power. A 4 ampere fuse
protects the MC 502 electrically. The AC power
outlet provided for auxiliary equipment is neither
fused nor switched.
8
Performance Limits
We promise you that the MC 502 you buy must be
capable of performance at or exceeding these limits
at the time of purchase or you get your money back.
Mclntosh PERFORMANCE LIMITS are the maximum
departure from perfection permitted for a Mclntosh
instrument.
PERFORMANCE
Mclntosh 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:
75 watts into 2.7 to 4 ohm loads, 50 watts into 8
ohms loads, minimum sine wave continuous
average power output per channel from 20 Hz to
20,000 Hz, both channels operating.
Which is: 14.2 volts RMS across 2.7 ohms
17.3 volts RMS across 4 ohms
20.0 volts RMS across 8 ohms
MONO (Bridged):
150 watts into an 8 ohm load, minimum sine wave
continuous average power output from 20 Hz to
20,000 Hz.
Which is: 34.6 volts RMS across 8 ohms.
INTERMODULATION DISTORTION
STEREO:
.02% maximum per channel with both channels
operating for any combination of frequencies, 20 Hz
to 20,000 Hz.
MONO:
.02% maximum at any power level from 250
milliwatts to rated power 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
HUM AND NOISE
95 dB below rated output
RATINGS
OUTPUT VOLTAGES:
25 volts for distribution lines
DAMPING FACTOR
Greater than 50
INPUT IMPEDANCE
75,000 ohms
OUTPUT LOAD IMPEDANCE
Stereo: 2.7 ohms to 8 ohms.
Mono: 8 ohms obtained by connecting across the
output terminals of both channels.
RATED POWER BAND
20 Hz to 20 kHz
TOTAL HARMONIC DISTORTION
STEREO:
.02% maximum harmonic distortion at any power
level from 250 milliwatts to rated power per channel
from 20Hz to 20,000 Hz, both channels operating.
MONO:
.02% maximum harmonic distortion at any power
level from 250 milliwatts to rated power from 20 Hz
to 20,000 Hz.
INPUT SENSITIVITY
Switchable: 0.75 volt or 2.5 volts-Level control pro-
vided for higher input voltages.
POWER REQUIREMENT
120 Volts, 50/60 Hz, 0.2 to 4 amperes (20 to 400
watts).
MECHANICAL INFORMATION
SIZE: Front Panel measures 16 inches wide (40.6
cm) by 3 5/8 inches high (9.2 cm). Chassis measures
14 3/4 inches wide (37.5 cm) by 2 3/8 inches high (6.0
cm) by 14 1/2 inches deep (36.8 cm), including con-
nectors. Knob clearance required is 1 1/4 inches (3.2
cm) in front of the mounting panel.
FINISH: Front panel is anodized gold and black
with special gold/teal nomenclature illumination.
Chassis is black.
WEIGHT: 27 pounds (12.2 kg) net. 38 pounds (17.2
kg) in shipping carton
9
Performance Charts
240
220
200
180
160
140
WATTS
120
RMS
100
80
60
40
20
0
10
100
FREQUENCY IN Hz
1k
10k
100k
140
130
120
110
100
90
80
70
WATTS
60
RMS
50
40
30
20
10
0
10
- 1
0
-1
dB
-2
-3
100
FREQUENCY IN Hz
1k
10k
100k
10
100
1k
FREQUENCY IN Hz
10
10k
100k
.05
.04
.03
.02
DISTORTION
%
.01
0
20
100
FREQUENCY IN Hz
1k
10k 20k
.05
.04
.03
DISTORTION
.02
%
.01
0
1
10
RMS WATTS
100
1000
.05
.04
.03
DISTORTION
.02
%
.01
0
.1 1
.05
.04
.03
DISTORTION
.02
%
.01
0
.1
10
RMS WATTS
1 10
RMS WATTS
11
100
100
1000
1000
Technical Description
While the MC 502 performs a rather elementary
function as a basic stereo power amplifier, it does
contain an number of rather sophisticated circuits.
Each channel consists of an input preamplifier, a
power amplifier section, three separate protecting
control circuits, and a phase inverter which is integral with the left channel. The power supply includes positive and negative electronic regulators. A
description of each circuit follows:
Preamplifiers
Each channel's preamplifier is a high perfor-
mance low noise operational amplifier arranged for
a stage gain of 4.5 times. The input signals to the
MC 502 first pass through the input level switch attenuator, then to the left or right/mono gain controls,
next to electronic attenuators which are part of the
Power Guard circuit to be described later and then
to the preamplifiers. The left channel input path also
includes the mode switch which, in stereo, connects
to the normal stereo signal path or, if switched to
mono, directs the output from the right channel
preamplifier to the inverting input of the left channel
preamplifier. This mono arrangement drives the
following power amplifier stages in phase opposition so that mono output power is derived as the
sum of the left and right power channels.
Power amplifier sections
Each of the identical power amplifier sections
consist of 4 stages of amplification. At the input is a
differential transistor pair optimised for low noise.
Input and feedback signals are applied to this differential amplifier. The two outputs of the differential amplifier are combined in a current mirror and
the resulting signal drives the second stage, a class
A voltage amplifier. The second stage output feeds
complementary emitter followers which in turn drive
the output transistors which are also complementary emitter followers. Biasing for the driver and output transistors is accomplished in the base circuit of
the driver transistors using a temperature sensitive
transistor thermally coupled to the output transistor
heat sinks. Cool efficient operation is thereby ob-
tained.
Protection control circuits
Three control circuits are used. The first control
circuit is the Sentry Monitor which senses the cur-
rent flow in the output stage transistors. The sense
characteristic is shaped to confine current and
voltage conditions to the safe operating
characteristics of the output stage power tran-
sistors. This protection greatly contributes to the
reliable, long life, performance of the MC 502.
Power Guard is the second control circuit.
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
of the clipped signal will damage most loudspeakers, particularly delicate high frequency
tweeters. A new Mclntosh advancement helps protect your speakers from this kind of damage. The MC
502 has a built-in waveform comparator which compares the wave shape of the output signal to the input signal. If the disparity between the two signals
exceeds 0.5% (equivalent to 0.5% total harmonic
distortion) a red limit indicator illuminates. With any
further increase in distortion the Power Guard circuit will operate. This circuit limits the input
dynamically so that the amplifier cannot be overdriven. Power Guard eliminates amplifier output
clipping.
Power Guard does not limit the dynamic range or
the power output of the power amplifier. Clipping occurs when an amplifier is asked to exceed its design
limits and the capacity of the power supply. Since
Power Guard does not begin to work until this point
is reached, the power capability of the amplifier is
never affected.
The third control circuit is the speaker protection
and turn on delay arrangement. In most direct coupled circuits a failure of any transistor in the power
amplifier will cause a DC potential to appear in the
output. To assure that no damaging or interfering
DC appears across the output terminals, a special
very fast acting protector circuit constantly
monitors the output for DC. If, at any time, a constant DC level appears, the speakers are disconnected. The protective circuit reacts in milliseconds.
Speakers remain disconnected until the cause has
been fixed. Integral with this circuit is the transient
free turn on and turn off controller. The power
amplifier output signals pass through a heavy duty
relay before reaching the output terminals. The relay
is controlled by an electronic circuit which closes
the relay approximately two seconds after the power
switch is turned on and releases the relay almost instantly when the power is turned off.
The Sentry Monitor, Power Guard, and speaker
protector circuits were developed by Mclntosh
engineers and are each covered by U.S. patents
assigned to Mclntosh Laboratory.
12
Power Supply
Two high current power supplies - a positive 40
volt DC and a negative 40 volt DC - are used to drive
the output power amplifier. Two very large filter
capacitors, 10,000 microfarads each, are used to
store a large amount of energy to provide good filter-
ing and excellent voltage regulation. Good low frequency response and negligible low frequency
distortion in the power amplifier stages depends on
the regulation of the power supply. Two additional
electronically regulated power supplies are used in
the MC 502. A + 15 volt and a - 15 volt line supply
all operational amplifiers.
13
Diagram
STEREO POWER AMPLIFIER
MC 502
MclNTOSH
Block
14
MclNTOSH LABORATORY INC.
2 CHAMBERS ST., BINGHAMTON, N.Y. 13903-2699
607-723-3512
The continuous improvement of its products is the policy of
Mclntosh Laboratory Incorporated who reserve the right to
improve design without notice.
Printed in U.S.A.
039266
BE032003
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