Power Amplifier
MC303
Owner’s Manual
McIntosh Laboratory, Inc. 2 Chambers Street Binghamton, New York 13903-2699 Phone: 607-723-3512 www.mcintoshlabs.com
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Thank You
Table of Contents
Your decision to own this McIntosh MC303 Power Ampli-
fier ranks you at the very top among discriminating music
listeners. You now have “The Best.” The McIntosh dedica-
tion to “Quality,” is assurance that you will receive many
years of musical enjoyment from this unit.
Please take a short time to read the information in this
manual. We want you to be as familiar as possible with all
the features and functions of your new McIntosh.
Safety Instructions............................................................. 2
Thank You and Please Take a Moment.............................. 3
Technical Assistance and Customer Service..................... 3
Table of Contents ............................................................... 3
General Information .......................................................3-4
Connector Information ...................................................... 4
Introduction........................................................................ 4
Performance Features ........................................................ 4
Dimensions ........................................................................ 5
Installation ......................................................................... 6
Rear Panel Connections and Switch.................................. 7
How to Connect .............................................................. 8-9
Connection Diagrams (Separate Sheet) MC1A and MC1B
Front Panel Displays and Controls....................................10
How to Operate.................................................................11
Technical Description ..................................................12-15
Photos...........................................................................16-17
Specifications....................................................................18
Packing Instruction...........................................................19
Please Take A Moment
The serial number, purchase date and McIntosh Dealer
name are important to you for possible insurance claim or
future service. The spaces below have been provided for
you to record that information:
Serial Number: __________________________________
Purchase Date: __________________________________
Dealer Name: ___________________________________
General Information
Technical Assistance
Caution: The MC303 Amplifier weight is 150 pounds (70.3
kilograms). It requires two or more persons to safely
handle when moving the amplifier.
1. The following Connecting Cable is available from the McIn-
tosh Parts Department:
If at any time you have questions about your McIntosh
product, contact your McIntosh Dealer who is familiar
with your McIntosh equipment and any other brands that
may be part of your system. If you or your Dealer wish
additional help concerning a suspected problem, you can
receive technical assistance for all McIntosh products at:
Power Control Cable Part No. 170-202
Six foot, 2 conductor shielded, with two 1/8 inch stereo
mini phone plugs.
2. For additional connection information, refer to the owner’s
manual(s) for any component(s) connected to the MC303.
3. The MC303 mutes the speaker output for approximately two
seconds when first turned on.
4. In the event the MC303 overheats, due to improper ventilation
and/or high ambient temperature, the protection circuits will
activate. The Front Panel Power Guard LEDs will continu-
ously indicate ON and the audio will be muted. When the
MC303 has returned to a safe operating temperature, normal
operation will resume.
5. For the best performance it is important to minimize an im-
pedance mismatch. An impedance mismatch can occur when a
Loudspeaker of a given impedance rating is connected to the
MC303 Output Terminals with a different impedance rating.
For example, a Loudspeaker with an impedance rating of 2
Ohms is connected to the MC303 8 Ohm Output Terminals.
The impedance of a Loudspeaker actually varies as the
Loudspeaker reproduces different frequencies. As a result, the
nominal impedance rating of the Loudspeaker (usually mea-
sured at a midrange frequency) might not always agree with
the impedance of the Loudspeaker at low frequencies where
the greatest amount of power is required.
McIntosh Laboratory, Inc.
2 Chambers Street
Binghamton, New York 13903
Phone: 607-723-1545
Fax: 607-724-0549
Customer Service
If it is determined that your McIntosh product is in need of
repair, you can return it to your Dealer. You can also return
it to the McIntosh Laboratory Service Department. For
assistance on factory repair return procedure, contact the
McIntosh Service Department at:
McIntosh Laboratory, Inc.
2 Chambers Street
Binghamton, New York 13903
Phone: 607-723-3515
Fax: 607-723-1917
Copyright 2008 © by McIntosh Laboratory, Inc.
3
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Connector Information, Introduction and Performance Features
General Information, con’t
plifier circuitry uses Thermal Trak1 Output Transistors for
lower distortion and cool operation.
• Balanced and Unbalanced Inputs
Balanced connections guard against induced noise and al-
low long cable runs without compromising sound quality.
6. When discarding the unit, comply with local rules
or regulations. Batteries should never be thrown
away or incinerated but disposed of in accordance
with the local regulations concerning battery dis-
posal.
7. For additional information on the MC303 and other
McIntosh Products please vist the McIntosh Web
• Power Guard
The patented McIntosh Power Guard circuit prevents the
amplifier from being over driven into clipping, with its
harsh distorted sound that can also damage your valuable
loudspeakers.
8. When the MC303 is operated in the USA or Canada and the
main fuse, located on the rear panel, is in need of replace-
ment use only the following part:
Bussman ABC-15-R or Littlefuse 314-015P
Connector and Cable Information
• Sentry Monitor and Thermal Protection
McIntosh Sentry Monitor power output stage protection
circuits ensure the MC303 will have a long and trouble free
operating life. Built-in Thermal Protection Circuits guard
against overheating.
XLR Connectors
Below is the Pin configuration for the XLR Balanced Input
Connectors on the MC303. Refer to the diagram for con-
nection:
PIN 1: Shield/Ground
PIN 2: + Output
PIN 3: - Output
• Patented Autoformer
McIntosh designed and manufactured Output Autoformers
provide an ideal match between the amplifier output stages
and speaker loads of 2, 4 and 8 ohms. The Autoformers
also provide perfect DC protection for your valuable loud-
speakers.
PIN 2
PIN 1
PIN 3
Power Control Connector
The MC303’s Power Control Inputs receive On/Off signals
of +5 volts. An additional connection
Power
is for controlling the illumination of
Control
the MC303 Power Output Meters.
Meter
• Special Power Supply
The 1/8 inch stereo mini phone plug
connects to a McIntosh A/V Control
A regulated Power Supply, a very large Power Transformer
and Capacitors ensure stable noise free operation even
though the power line varies.
Illumination
Control
Ground
Center Power Control Output.
Note: The Power Control Connecting Cable is available from
the McIntosh Parts Department:
• Illuminated Power Meter
Data and Power Control Cable Part No. 170-202
Six foot, shielded 2 conductor, with 1/8 inch stereo mini
phone plugs on each end.
The Illuminated Power Output Watt Meter on the MC303
is peak responding, and indicates the power output of the
amplifier. The Front Panel Meter Illumination may be
switched Off at any time.
Introduction
Now you can take advantage of traditional McIntosh
standards of excellence in the MC303 Power Amplifier.
The 300 watt high current output per channel will drive
any high quality loudspeaker system to its ultimate per-
formance. The MC303 reproduction is sonically transpar-
ent and absolutely accurate. The McIntosh Sound is “The
Sound of the Music Itself.”
• Fiber Optic Solid State Front Panel Illumination
The even illumination of the Front Panel is accomplished
by the combination of custom designed Fiber Optic Light
Diffusers and extra long life Light Emitting Diodes
(LEDs).
• Glass Front Panel and Super Mirror Chassis Finish
The famous McIntosh Illuminated Glass Front Panel and
the Stainless Steel Chassis with Super Mirror Finish en-
sures the pristine beauty of the MC303 will be retained for
many years.
Performance Features
• Power Output
The MC303 is a Power Amplifier with a capability of 300
watts per channel (all channels driven) into 2, 4 or 8 ohm
speakers with less than 0.005% distortion. The Power Am-
1 ThermalTrak™ and ON Semiconductor are trademarks of Semicon-
ductor Components Industries, LLC
4
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Dimensions
Dimensions
The following dimensions can assist in determining the
best location for your MC303. There is additional informa-
tion on the next page pertaining to installing the MC303
into cabinets.
17-3/4"
45.09cm
10-7/8"
27.62cm
12-3/8"
31.43cm
Front View of the MC303
17-3/8"
44.15cm
Rear View of the MC303
F15AH 125V
12-1/4"
31.11cm
19-1/2"
49.50cm
18"
45.72cm
1-5/8"
4.12cm
10-1/2"
26.70cm
Side View of the MC303
3/16"
0.48cm
1-1/2"
3.81cm
14"
35.60cm
1-9/16"
3.96cm
5
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Installation
Installation
The MC303 can be placed upright on a table or shelf,
standing on its four feet. It also can be custom installed in
a piece of furniture or cabinet of your choice. The four feet
may be removed from the bottom of the MC303 when it is
custom installed as outlined below. The four feet together
with the mounting screws should be retained for possible
future use if the MC303 is
sure to cut out a ventilation hole in the mounting shelf ac-
cording to the dimensions in the drawing.
1 When the MC303 is installed together with other McIntosh
Components, check clearances on all components before
proceeding.
removed from the custom
installation and used free
17-9/16"
44.62cm
standing. The required
panel cutout, ventilation
cutout and unit dimen-
sions are shown.
Always provide adequate
ventilation for your
MC303. Cool operation
10 -13/16"
27.46cm
MC303 Front Panel
Custom Cabinet Cutout
ensures the longest pos-
sible operating life for any
electronic instrument. Do
not install the MC303 di-
rectly above a heat gener-
ating component such as a
high powered amplifier. If
all the components are in-
stalled in a single cabinet,
a quiet running ventila-
tion fan can be a definite
asset in maintaining all
the system components at
the coolest possible oper-
ating temperature.
6"
15.24cm
Cabinet
Front
Panel
Cutout Opening for Custom Mounting
Opening
for Ventilation
A custom cabinet instal-
lation should provide the
following minimum spac-
ing dimensions for cool
operation.
MC303 Side View
in Custom Cabinet
Chassis
Spacers
Cutout Opening for Ventilation
Allow at least 6 inches
(15.24cm) above the top,
2 inches (5.08cm) below
the bottom and 1 inch
(2.54cm) on each side of
the Power Amplifier, so
that airflow is not ob-
structed. Allow 19-1/2
inches (49.53cm) depth
behind the front panel.
Allow 1-5/8 inch (4.12cm)
in front of the mounting
panel1 for clearance. Be
Support
Shelf
1"
2.54cm
14-1/2"
36.83cm
21/32"
1.67cm
MC303 Bottom View
in Custom Cabinet
15-5/16"
38.91cm
14"
35.56cm
Cutout Opening
for Ventilation
Note: Center the cutout Horizontally on the unit.
For purposes of clarity, the above
illustration is not drawn to scale.
6
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Rear Panel Connections and Switch
UNBALANCED INPUT 3
for an audio cable from an
A/V Control Center or Pre-
amplifier audio output
POWER CONTROL IN
receives turn On/Off signal
from a McIntosh component
and the POWER CONTROL
OUT sends a turn On/Off
signal to the next McIntosh
Component
BALANCED INPUT 2
BALANCED INPUT 1
for an audio cable from
an A/V Control Center
or Preamplifier audio
output
for an audio cable from
an A/V Control Center
or Preamplifier audio
output
BALANCED INPUT 3
UNBALANCED INPUT 2
UNBALANCED INPUT 1
for an audio cable from
an A/V Control Center
or Preamplifier audio
output
for an audio cable from an
A/V Control Center or Pre-
amplifier audio output
for an audio cable from an
A/V Control Center or Pre-
amplifier audio output
F15AH 125V
OUTPUT 3 Connections
for 2 ohm, 4 ohm or 8 ohm
Loudspeaker
OUTPUT 2 Connections
for 2 ohm, 4 ohm or 8 ohm
Loudspeaker
OUTPUT 1 Connections
for 2 ohm, 4 ohm or 8 ohm
Loudspeaker
Connect the MC303
Main Fuse holder, refer to information on
power cord to a live AC
outlet. Refer to the rear
panel to determine the
correct voltage
the back panel of your MC303 to determine
the correct fuse size and rating. When used
in the USA and Canada refer to General
Information note number 8 on page 4.
7
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How to Connect for Center and Surround
Channels
the connection of a single Loudspeaker per amplifier
channel, with an impedance of 2 ohms, 4 ohms or 8
ohms.
Caution: The supplied AC Power Cord should not be
connected to the Rear Panel of the MC303
Amplifier until after the Loudspeaker Connec-
tions have been made. Failure to observe this
could result in Electric Shock. For additional
instruction on making Loudspeaker Connec-
tions contact your McIntosh Dealer or McIn-
tosh Technical Support.
Note: The remaining Amplifier Loudspeaker Terminals
should not be connected to another Loudspeaker.
6. Prepare the Loudspeaker Hookup Cables that attach to
the Amplifier by choosing one of the methods below:
Bare wire cable ends:
Carefully remove sufficient insulation from the cable
ends, refer to figures 1, 2 & 3. If the cable is strand-
ed, carefully twist
The connection instructions below, together with the
MC303 Connection Diagram located on the separate folded
sheet “Mc1A, is an example of a typical audio/video sys-
tem. Your system may vary from this, however the actual
components would be connected in a similar manner. For
additional information refer to “Connector and Cable In-
formation” on page 4.
1. For Remote Power Control, connect a power control
cable from the A/V Control Center or Preamplifier
Power Control Out to the Right Front Channel Power
Amplifier Power Control In. Connect a second power
control cable from the Right Front Channel Power
Amplifier Power Power Control Out to the Left Front
Channel Power Amplifier Power Control In.
2. Connect a power control cable from the Left Front
Channel Power Amplifier Power Power Control Out to
the MC303 POWER CONTROL IN.
3. Connect cables from the Balanced Outputs of the A/V
Control Center or Preamplifier (Left and Right Front
Channels) to the Left and Right Front Power Ampli-
fiers.
the strands together
as tightly as possible.
Note: If desired, the
Figure 2
Figure 3
Figure 1
twisted ends can
be tinned with solder to keep the strands together,
or attach spade lug and/or banana connector.
Spade lug or prepared wire connection:
Insert the spade lug connector or prepared section of
the cable end into the terminal side access hole, and
tighten the terminal
cap until the cable is
firmly clamped into
the terminal so the
Figure 4
Figure 5
Figure 6
wires cannot slip out. Refer to figures 4, 5 & 6.
Banana plug connection:
Insert the banana plug into the hole at the top of the
terminal. Refer to figures A and B.
Note: Banana
Plugs are
for use in
the United
States and
Canada
only.
Note: An optional hookup is to use the UNBALanced IN-
PUTS with unbalanced cables instead of balanced.
However, both type of connections for a given Input
should not be used at the same time.
4. Connect cables from the Balanced Outputs of the A/V
Control Center or Preamplifier (Center and Surround
Channels) to the MC303 BALANCED INPUTs (1-3).
When connecting Loudspeakers to the MC303 it is very
important to use cables of adequate size, so there is little to
no power loss in the cables. The size is specified in Gauge
Numbers or AWG (American Wire Gauge). The smaller
the Gauge number, the larger the wire size:
7. Connect the Loudspeaker hookup cables from a single
Loudspeaker to the output terminals that match the
impedance of OUPUT 1, being careful to observe the
correct polarities. Output impedance connections of 2
ohms, 4 ohms and 8 ohms are provided. If the Loud-
speaker’s impedance is in-between the available con-
nections, use the nearest lower impedance connection.
Refer to “General Information” Note 5 on page 3 for
additional information.
WARNING: Loudspeaker terminals are hazardous live
and present a risk of electric shock.
8. In a similar manner, connect a Loudspeaker to OUT-
PUT 2 and connect the remaining Loudspeaker to
OUTPUT 3.
Loudspeaker Cable Distance vs Wire Gauge Guide
25 feet
(7.62 meters)
or less
50 feet
(15.24 meters)
or less
100 feet
(30.48 meters)
or less
Loudspeaker
Impedance
2 Ohms
4 Ohms
8 Ohms
12AWG
14AWG
16AWG
10AWG
12AWG
14AWG
8AWG
10AWG
12AWG
9. Connect the MC303 Power Cord to a live AC outlet.
5. This McIntosh MC303 Power Amplifier is designed for
8
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How to Connect
How to Connect for Left, Center and Right
Channels
Caution: The supplied AC Power Cord should not be
connected to the Rear Panel of the MC303
Note: The remaining Amplifier Loudspeaker Terminals
should not be connected to another Loudspeaker.
6. Prepare the Loudspeaker Hookup Cables that attach to
the Amplifier by choosing one of the methods below:
Bare wire cable ends:
Amplifier until after the Loudspeaker Connec-
tions have been made. Failure to observe this
could result in Electric Shock. For additional
instruction on making Loudspeaker Connec-
tions contact your McIntosh Dealer or McIn-
tosh Technical Support.
Carefully remove sufficient insulation from the cable
ends, refer to figures 1, 2 & 3. If the cable is strand-
ed, carefully twist the strands together as tightly as
possible.
Note: If desired, the
The connection instructions below, together with the
MC303 Connection Diagram located on the separate folded
sheet “Mc1B, is an example of a typical audio/video sys-
tem. Your system may vary from this, however the actual
components would be connected in a similar manner. For
additional information refer to “Connector and Cable In-
formation” on page 4.
1. For Remote Power Control, connect a power control ca-
ble from the A/V Control Center or Preamplifier Power
Control Out to the MC303 POWER CONTROL IN.
2. Connect a second power control cable from the MC303
POWER CONTROL OUT to the Left and Right Sur-
round Channel Power Amplifier Power Control In.
3. Connect cables from the Balanced Outputs of the A/V
Control Center or Preamplifier (Left, Center and Right
Front Channels) to the MC303 BALANCED INPUTs
(1-3).
twisted ends can
be tinned with
Figure 2
Figure 3
Figure 1
solder to keep
the strands together, or attach spade lug and/or
banana connector.
Spade lug or prepared wire connection:
Insert the spade lug connector or prepared section
of the cable end into the terminal side access hole,
and tighten the terminal cap until the cable is firmly
clamped into the
terminal so the wires
cannot slip out. Refer
to figures 4, 5 & 6.
Banana plug connection:
Figure 4
Figure 5
Figure 6
Insert the banana plug into the hole at the top of the
terminal. Refer to figures A and B.
Note: Banana
Note: An optional hookup is to use the UNBALanced IN-
PUTS with unbalanced cables instead of balanced.
However, both type of connections for a given Input
should not be used at the same time.
Plugs are
for use in
the United
States and
4. Connect cables from the Balanced Outputs of the A/V
Control Center or Preamplifier (Surround Channels) to
the Left and Right Surround Channel Power Amplifier.
When connecting Loudspeakers to the MC303 it is very
important to use cables of adequate size, so there is little to
no power loss in the cables. The size is specified in Gauge
Numbers or AWG (American Wire Gauge). The smaller
the Gauge number, the larger the wire size:
Canada
only.
7. Connect the Loudspeaker hookup cables from a single
Loudspeaker to the output terminals that match the
impedance of OUPUT 1, being careful to observe the
correct polarities. Output impedance connections of 2
ohms, 4 ohms and 8 ohms are provided. If the Loud-
speaker’s impedance is in-between the available con-
nections, use the nearest lower impedance connection.
Refer to “General Information” Note 5 on page 3 for
additional information.
Loudspeaker Cable Distance vs Wire Gauge Guide
25 feet
(7.62 meters)
or less
50 feet
(15.24 meters)
or less
100 feet
(30.48 meters)
or less
Loudspeaker
Impedance
WARNING: Loudspeaker terminals are hazardous live
and present a risk of electric shock.
8. In a similar manner, connect a Loudspeaker to OUT-
PUT 2 and connect the remaining Loudspeaker to
OUTPUT 3.
2 Ohms
4 Ohms
8 Ohms
12AWG
14AWG
16AWG
10AWG
12AWG
14AWG
8AWG
10AWG
12AWG
9. Connect the MC303 Power Cord to a live AC outlet.
5. This McIntosh MC303 Power Amplifier is designed for
the connection of a single Loudspeaker per amplifier
channel, with an impedance of 2 ohms, 4 ohms or 8
ohms.
9
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Front Panel Displays and Switches
Meters indicate the Amplifier
Power Output of Channels 1-3
LEDs indicate when Amplifier Channels
1-3 POWER GUARD circuits activate
Standby Power On
Indicator
METER Switch selects
Meter Illumination Auto
(On) or Off
POWER Switch Turns
AC Power On/Off, or
On/Remote
10
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How to Operate
How to Operate
Power On
Power Guard
To have the MC303 automatically turn On or Off when an
A/V Control Center or Preamplifier turns on or off, rotate
the power switch to the REMOTE
position. For manual operation,
rotate the power switch to the ON
or OFF position as desired. Refer
to figure 8.
During normal operation, the Front Panel Power Guard
Indicators will momentarily illuminate during peaks in the
audio signals. Refer to figure 11. In
the event the MC303 overheats, due
to improper ventilation and/or high
ambient temperature, the internal
Figure 11
protection circuits will activate. The
Front Panel Power Guard Indicators
Note: There must be a power con-
trol connection between the
MC303 and the A/V Control
Center or Preamplifier, in
order for the remote power
will continuously illuminate and the audio will be muted.
When the MC303 has returned to a safe operating tem-
perature, normal operation will resume.
turn-on to function.
Figure 8
Meters
Rotate the METER LIGHTS Switch to select the Meter Il-
lumination Mode you desire. Refer to figure 9.
OFF - Meter Illumination
is switched OFF and
the meters will con-
tinue to indicate the
power output.
AUTO - Meter Illumination
is switched ON.
When Power Control
Input of the MC303
is connected to an
A/V Control Center
or Preamplifier with
Figure 9
Remote Meter Illumination Control, the Meter
Illumination will automatically be remotely
controlled (On/Off).
The MC303 Meters respond to all the musical informa-
tion being produced by the three Power Amplifier Circuits.
Refer to figure 10. They indicate to an accuracy of at least
95% of the power output with only a single cycle of a
2000Hz tone burst.
Figure 10
11
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Technical Description
McIntosh Laboratory, the company who introduced the
world’s first amplifier that could be called “High Fidelity”,
has done it again. The McIntosh engineering staff has cre-
20Hz to 20,000Hz. Typical performance at mid frequencies
is less than 0.0002%. The true distortion readings on the
MC303 are so low, it takes special measuring techniques
ated a power amplifier without compromise, using the most to make accurate readings. The MC303 can deliver the
advanced McIntosh circuit design concepts.
best possible performance from any type of high quality
loudspeaker system.
A continuous average power output rating of 300 watts
per channel and with an output current of greater than 60
amperes per channel, making this one of the most ad-
vanced and powerful amplifiers McIntosh has ever manu-
factured. The distortion limits for the MC303 are no more
than 0.005% at rated power output for all frequencies from
Creating an amplifier with this level of performance did
not come easily. Many months of design, testing and mea-
suring were required. Extensive controlled listening tests,
the ultimate form of measuring, were made before the final
design was accepted.
Inside Bottom View of the MC303
12
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Technical Description
POSITIVE
SENTRY
MONITOR
THERMAL
TRAK
DC BIAS
Power Amplifier
com
Block Diagram
(one channel shown)
NEGATIVE
SENTRY
MONITOR
BALANCED
AMP
Figure 12
Design Philosophy
range of music conditions with the benefits of lower distor-
tion and cooler operation.
The design philosophy incorporated in the MC303 in-
volved several different techniques, all based on sound
scientific logic. Refer to figure 12. Every stage of voltage
or current amplification must be as linear as possible prior
to the use of negative feedback. McIntosh engineers know
how to properly design negative feedback circuits so they
contribute to the extremely low distortion performance
expected from a McIntosh amplifier. The typical McIntosh
owner would never accept the approximately 100 times
higher distortion of many non-feedback designs.
The MC303 can provide greater than 60 amperes peak
output current to drive uneven speaker loads. Some poor
speaker designs have input impedance that dip to 1 or 2
ohms at various frequencies and the MC303 has the output
current reserve to drive them.
The high efficiency circuit design of the MC303 con-
tributes to low operating temperatures. More than 2,100
square inches of heat sink area keep the MC303 operating
safely with convection cooling. No fans are needed. Preci-
sion metal film resistors and low dielectric absorption film
All transistors are selected to have nearly constant cur-
rent gain over the entire current range they must cover. The capacitors are used in all critical circuit locations.
12 Power Transistors used in each channel of the MC303
Power Output
Autoformers
Circuitry, have
matched uniform
current gain, high
current band-
The output signal of the MC303 Power Amplifier Circuitry
is coupled to the Loudspeaker via the unique McIntosh
Output Autoformer. Refer to figure 15. The unequaled ex-
pertise of McIn-
width product and
a large active re-
gion safe operat-
ing area. Refer to
figure 13. These
Power Transistors
are the very latest
in semiconduc-
tor technology
tosh in the design
and manufactur-
ing of autoform-
ers is legendary
in the high fidel-
ity industry.
All solid state
power amplifier
output circuits
Figure 13
and incorporate
work best into
what is called
Figure 15
a new design known as ThermalTrak™. Refer
to figure 14. This allows for the instantaneous
and accurate monitoring of the Power Transis-
tor Temperature. The MC303 Power Output
Circuitry has a specially designed bias circuit
to take full advantage of the ThermalTrak™
Power Transistors and thus precisely controls
the power amplifier operation over a wide
an optimum load. This optimum load may vary consider-
ably from what a loudspeaker requires. In the case of more
than one Loudspeaker connected in parallel, the load to
the power amplifier may drop to two ohms or even less. A
power amplifier connected to a load that is lower than opti-
mum, causes more output current to flow, which results in
extra heat being generated in the power output stage. This
Figure 14
13
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Without Power Guard
increase in temperature will result in a reduced life
expectancy for the amplifier.
the shape of these waveforms.
If an amplifier channel is
The Autoformer creates an ideal match between the
Power Amplifier Output Circuitry and the Loudspeaker.
There is absolutely no performance limitation with an
Autoformer. Its frequency response from below 20Hz to
well beyond 20,000Hz exceeds the Power Output Circuitry
Frequency Response, and extends well beyond the audible
overdriven, there will be a dif-
ference between the two signal
waveforms. When the differ-
ence exceeds 0.3% the Power
Guard activates the PG light
and a dynamic electronic at-
range. Its distortion level is so low it is virtually impossible tenuator at the amplifier input
to measure. In the rare event of a power amplifier output
circuit failure, the McIntosh Autoformer provides absolute
protection from possible damage to your valuable Loud-
speakers.
reduces the input volume just
enough to prevent any further
increase in distortion. The
Power Guard circuit acts so
fast that there are absolutely
no audible side effects and
the sonic purity of the music
reproduction is perfectly
preserved. The MC303 Power
Amplifier with Power Guard
is not limited to just the rated
power output, but will actu-
ally produce distortion free
output well above its rated
power due to the McIntosh
philosophy of conservative
design.
Figure 18
With Power Guard
Protection Circuits
The different types of protection circuits incorporated in
the MC303 insure a long and safe operating life.
The MC303 incorporates the McIntosh Sentry Monitor
output transistor protection circuit. Refer to figure 16.
There is absolutely no
compromise in sonic
performance with this
circuit, and it ensures
safe operation of the
amplifier under even
Figure 19
the most extreme op-
erating conditions.
Normal
Operating
Area
The MC303 also
Power Supply Circuits
includes the unique
patented McIntosh
Power Guard Circuit.
Power Guard elimi-
nates the possibility
of ever overdriving
the amplifier into
To compliment the design of the MC303 Power Amplifier
Circuitry, there is a high current high voltage power supply
for the three channels. Refer to figure 20 on page 15. The
power amplifiers draw high current from the AC power
line. Therefore, it is important that they plug directly into
the wall outlet.
Sentry Monitor
Safety Area
Output Transistor Failure
Figure 16
The MC303 incorporates a very large high current
Main Power Transformer. Refer to figure 21. It is enclosed
clipping. Refer to figures 17, 18 and 19. An overdriven
amplifier can produce both audible and inaudible distortion in the legendary McIntosh Potted Enclosures and weighs
levels exceeding 40%. The audible distortion is unpleas-
ant to hear, but the inaudible ultrasonic distortion is also
38 lbs. The super size multiple large main filter capacitors
undesirable, since it can
Input Test Signal
damage valuable loudspeaker
system tweeters. You will
never experience the harsh
and damaging distortion due
to clipping.
The Power Guard circuit is a
waveform comparator, moni-
toring both the input and out-
put waveforms. Under normal
operating conditions, there
are no differences between
Figure 17
Figure 21
14
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Technical Description, con’t
Power Supply
Block Diagram
STANDBY
TRANSFORMER
STANDBY
SUPPLY
Figure 20
can store over 200 Joules of energy, for the three amplifier
channels, necessary for the wide dynamic range “Digital
Audio” demands. Refer to figure 22.
Figure 22
There is also a fully regulated low voltage power supply to
guarantee excellent signal to noise ratio. Refer to figure 22.
Also, most owners desire one power switch for the
whole audio system. The MC303 is equipped with a circuit
that provides remote Power Control from a McIntosh A/V
Control Center. When the A/V Control Center is switched
On, a (+5V) signal operates the power relay in the MC303.
The MC303 also has a remote Power Control Out Jack. The
Power Control signal from this jack is delayed by a frac-
tion of a second so that the turn on power surge of the next
power amplifier occurs at a later time. This helps prevent
power circuit overload that could trip circuit breakers or
blow fuses, a very important feature in a high power Home
Theater System employing multiple Power Amplifiers.
15
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16
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Photos
Angle View of the MC303
Rear View of the MC303
17
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Specifications
Specifications
Power Output
Power Requirements
Minimum sine wave continuous average power output per
channel, all channels operating:
300 watts into a 2 ohm, 4 ohm or 8 ohm load
100 Volts, 50/60Hz at 12 Amps
110 Volts, 50/60Hz at 11 Amps
120 Volts, 50/60Hz at 10 Amps
220 Volts, 50/60Hz at 6 Amps
230 Volts, 50/60Hz at 6 Amps
240 Volts, 50/60Hz at 6 Amps
Standby: Less than 1 watt
Output Load Impedance
2, 4 or 8 ohms
Rated Power Band
Note: Refer to the rear panel of the MC303 for the correct
20Hz to 20,000Hz
voltage.
Total Harmonic Distortion
Overall Dimensions
0.005% maximum harmonic distortion at any power level
from 250 milliwatts to rated power, 20Hz to 20,000Hz
Width is 17-3/4 inches (45.09cm)
Height is 12-3/8 inches (31.43cm) including feet
Depth is 22 inches (55.88cm) including the Front Panel and
Cables
Dynamic Headroom
1.8dB
Weight
Frequency Response
155 pounds (70.3 kg) net, 180 pounds (81.6 kg) in shipping
+0, -0.25dB from 20Hz to 20,000Hz
+0, -3dB from 10Hz to 100,000Hz
carton
Shipping Carton Dimensions
Width is 31 inches (78.74cm)
Depth is 28 inches (71.12cm)
Height is 17-1/4 inches (43.82cm)
Input Sensitivity (for rated output)
1.7 Volt Unbalanced
3.4 Volt Balanced
Signal To Noise Ratio (A-Weighted)
112dB below rated output
Intermodulation Distortion
0.005% maximum, if the instantaneous peak power output
does not exceed twice the rated power output for any com-
bination of frequencies from 20Hz to 20,000Hz.
Wide Band Damping Factor
Greater than 40
Input Impedance
10,000 ohms
Power Guard
Less than 2% THD with up to 14dB overdrive at 1,000Hz
Power Control Input
5VDC - 12VDC
Power Control Output
12VDC, 25mA maximum, delay 0.2 seconds
18
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Packing Instructions
Packing Instructions
In the event it is necessary to repack the equipment for
shipment, the equipment must be packed exactly as shown
below. It is very important that the four feet are attached to
the bottom of the equipment. This will ensure the proper
equipment location on the bottom foam pad. Failure to do
this will result in shipping damage.
Quantity Part Number Description
1
1
2
3
034105
034104
034439
034441
Shipping carton top
Shipping carton bottom
Foam Pad (top and bottom)
Foam Ring
Use the original shipping carton and interior parts only
if they are all in good serviceable condition. If a shipping
carton or any of the interior part(s) are needed, please call
or write Customer Service Department of McIntosh Labo-
ratory. Please see the Part List for the correct part numbers.
4
018445
Feet
19
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McIntosh Laboratory, Inc.
2 Chambers Street
Binghamton, NY 13903
The continuous improvement of its products is the
policy of McIntosh Laboratory Incorporated who
reserve the right to improve design without notice.
Printed in the U.S.A.
McIntosh Part No. 04109800
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