Mcintosh MC255 User Manual

McIntosh Laboratory, Inc. 2 Chambers Street Binghamton, New York 13903-2699 Phone: [phone] www.mcintoshlabs.com

MC255

Power Amplifier

Owner’s Manual

Important Safety Information is supplied in a separate document “Important Additional Operation Information Guide”

Thank You

Your decision to own this McIntosh MC255 Five
Channel Power Amplifier ranks you at the very top
among discriminating music listeners. You now have
“The Best.” The McIntosh dedication 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.

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: ________________________________

Technical Assistance

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:

McIntosh Laboratory, Inc.
2 Chambers Street
Binghamton, New York 13903
Phone: [phone]
Fax: [phone]

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: [phone]
Fax: [phone]

Table of Contents

Safety Instructions ..................................................... 2

(Separate Sheet) ................... Important Additional

Operation Information Guide

Thank You and Please Take a Moment ....................... 2

Technical Assistance and Customer Service .............. 2

Table of Contents ........................................................ 2

General Information ................................................... 2

Connector and Cable Information ..............................3

Introduction ................................................................. 3

Performance Features .................................................3

Dimensions .................................................................4

Installation ..................................................................5

Rear Panel Connections and Switch ........................... 6

Output Terminals and How to Connect ................... 7-8

Connection Diagram (Separate Sheets) ........Mc1A-2A

Front Panel Displays and Controls ............................ 10

How to Operate ......................................................... 11

Technical Description .......................................... 12-15

Photos ................................................................... 16 -17

Specifications ............................................................ 18

Packing Instruction ................................................... 19

General Information

1. For additional connection information, refer to the
owner’s manual(s) for any component(s) connected
to the MC255.

2. The MC255 mutes the speaker output for approxi­mately two seconds when first turned on.

3. For the best performance and safety it is important
to always match the impedance of the Loudspeaker
to the Power Amplifier connections. Refer to “How
to Connect” pages 7 and 8.

Note: The impedance of a Loudspeaker actually var-

ies as the Loudspeaker reproduces different
frequencies. As a result, the nominal impedance
rating of the Loudspeaker (usually measured 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. Contact the Loudspeaker Manufac­turer for additional information about the actual
impedance of the Loudspeaker before connecting

it to the McIntosh MC255.

4. In the event the MC255 over heats, due to improper
ventilation and/or high ambient temperature, the
protection circuits will activate. The Front Panel
Power Guard LED will continuously indicate ON
and the audio will be muted. When the MC255 has
returned to a safe operating temperature, normal
operation will resume.

5. 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 disposal.

6. For additional information on the MC255 and
other McIntosh Products please visit the McIntosh
Website at www.mcintoshlabs.com.

Copyright 2019 © by McIntosh Laboratory, Inc.

2

Cable Information, Introduction and Performance Features

Connector and Cable Information

XLR Connectors

Below is the Pin configuration for the XLR Balanced
Input, Input/Output Connectors on the MC255. Refer
to the diagram for connection:

IN

PIN 1: Shield/Ground
PIN 2: + Input/Output
PIN 3: - Input/Output

PIN 2 PIN 1

PIN 3

Power Control Connector

The MC255 Power Control Input receives an On/Off
signal from +5 to +12 volts. The Power Control Output
will in turn provide a +12 volt
Output Signal with a total cur­rent up to 50mA. An additional
connection is for controlling
the illumination of the MC255

Trigger
Control

Meter
Illumination
Control

Ground

Power Output Meters. The 1/8
inch stereo mini phone plug connects to a McIntosh
Preamplifier or A/V Control Center Power Control
Output.

Output Terminal Connector

When cables with spade lugs are used
for Loudspeaker Connection, the spade
lugs need an opening of at least 3/10 inch
(7. 6m m).

3/10 of an inch
(7.6millimeters)

Introduction

Now you can take advantage of traditional McIntosh
standards of excellence in the MC255 Power Ampli­fier. This Five Channel Power Amplifier with high
current output will drive any pair of high quality
Loudspeakers. The MC255 reproduction is sonically
transparent and absolutely accurate. The McIntosh
Sound is “The Sound of the Music Itself.”

Performance Features

• Power Output

The MC255 is a Five Channel Home Theater Power
Amplifier with a capability of 250 watts from the
Front Three Channels (Left, Center and Right) and
200 watts from the Surround Channels (Left and
Right) into 4 or 8 ohm speakers with less than 0.005%
distortion. The Power Amplifier Circuitry uses Ther­malTrak
cool operation.

• Balanced and Unbalanced Inputs

Balanced connections guard against induced noise and
allow long cable runs without compromising sound
qual it y.

• 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.

• Zone B Operation

The Zone B on the MC255 provides Two Channel
Music Playback in a second room with 200 watts
per channel, while at the same time Zone A provides
Three Channel Home Theater Playback.

• Sentry Monitor and Thermal Protection

McIntosh Sentry Monitor power output stage protec­tion circuits ensure the MC255 will have a long and
trouble free operating life. Built-in Thermal Protection
Circuits guard against overheating.

1

ThermalTrak™ and ON Semiconductor are trademarks of Semi-

1

Output Transistors for lower distortion and

conductor Components Industries, LLC

• Special Power Supply

A very large Power Transformer and Large Capaci­tors ensure stable noise free operation even though the
power line varies.

• Illuminated Power Meters

The Illuminated Power Output Watt Meters on the
MC255 are peak responding, and indicate the true
power output of the amplifier. The Front Panel Meter
Illumination may be switched Off at any time.

• McIntosh Custom Binding Posts

McIntosh patent pending gold plated output terminals
deliver high current output. They accept large diam­eter wire and spade lugs. Banana plugs may also be
used only in the United States and Canada.

• Glass Front Panel and Super Mirror Chassis
Finish

The famous McIntosh Illuminated Glass Front Panel
uses long life Light Emitting Diodes (LEDs) and
the Stainless Steel Chassis with Super Mirror Fin­ish ensures the pristine beauty of the MC255 will be
retained for many years.

3

Dimensions

Front View of the MC255

Rear View of the MC255

Side View of the MC255

17-

1/2

"

44.5cm

8-

7/8

"

22.5cm

9-

7/16

"

24.0cm

11-

3/4

"

29.9cm

16

-13/16

"

43.0cm

18-

1/4"

46.4cm

3/16

"

0.5cm

16-

5/8

"

42.2cm

13/16

"

2.1cm

12"

30.5cm

8-

5/16

"

21.1cm

2"

5.1cm

3"

7.62cm

5/8

"

1.6cm

The following dimensions can assist in determining
the best location for your MC255.

Dimensions

4

Installation

Installation

The MC255 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 MC255 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
MC255 is removed from the custom installation and
used free standing. The required panel cutout, ventila­tion cutout and unit dimensions are shown.
Always provide adequate ventilation for your MC255.
Cool operation ensures the longest possible operating
life for any electronic instrument. Do not install the
MC255 directly above a heat generating component
such as a high powered amplifier. If all the compo­nents are installed in a single cabinet, a quiet running
ventilation fan can be a definite asset in maintaining
all the system components at the coolest possible oper­ating temperature.
A custom cabinet installation should provide the fol­lowing minimum spacing dimensions for cool opera­tion.
Allow at least 6 inches (15.24cm) above the top, 2
inches (5.08cm) below the bottom, 3 inches (7.62cm)
behind the rear panel and 2 inches (5.08cm) on each
side of the Power Amplifier, so that airflow is not
obstructed. Allow 7/8 inch (2.22cm) in front of the
mounting

ventilation hole in the mounting shelf according to
the dimensions in the drawing.

1

When the MC255 is installed together with other McIn-

tosh Components, check clearances on all components
before proceeding.

1

panel for clearance. Be sure to cut out a

MC255 Front Panel
Custom Cabinet Cutout

Cabinet
Front
Panel

Opening
for Ventilation

MC255 Side View
in Custom Cabinet

MC255 Bottom View
in Custom Cabinet

Note: Center the cutout Horizontally
on the unit. For purposes of
clarity, the above illustration
is not drawn to scale.

Cabinet Front Panel

Support
Shelf

17"

43.18cm

Opening for Ventilation

Cutout Opening for Custom Mounting

6"

15.24cm

Cutout Opening for Ventilation

3"

7.62cm

10-1/2"

26.7cm

16"

40.64cm

Cutout Opening
for Ventilation

14-11/16"

37.30cm

Chassis
Spacers

5

Rear Panel Connections and Switch

OUPUTS 2 Connec­tions (ZONE B) for Left
Surround Channel and
for an 8 ohm or 4 ohm
Loudspeaker

OUPUTS 4 Connec­tions (ZON E B) for
Right Channel Sur­round for an 8 ohm or
4 ohm Loudspeaker

Fuse holder, refer to
the nomenclature next
to the fuse holder to
determine the correct
fuse size and rating

Connect the MC255
power cord to a live
AC outlet. Refer to
information on the
back panel of your
MC255 to determine
the correct voltage
for your unit

OUPUTS 5 Connec­tions for Right Front
Channel and for an 8
ohm or 4 ohm Loud­speaker

POWER CONTROL IN ZA (Zone
A) receives a turn On/Off signal
from a McIntosh component for all
five channels

POWER CONTROL OUT
ZA (Zone A) sends a turn
On/Off signal to the next
McIntosh Component

OUPUTS 1 Connec­tions for Left Front
Channel and for an 8
ohm or 4 ohm Loud­speaker

OUPUTS 3 Con­nections for Center
Channel and for
an 8 ohm or 4 ohm
Loudspeaker

Sets the impedance
of the Connected
Loudspeakers

6

The Power
Auto Off
Feature is
either ENable
or DISable

POWER CONTROL IN ZB (Zone B) re­ceives a turn On/Off signal from a McIntosh
component for the LS and RS two channels

POWER CONTROL
OUT ZB (Zone B)
sends a turn On/Off
signal to the next Mc­Intosh Component

INPUT 5 Balanced
Connection for the
Right Front Chan­nel

INPUT 5 Unbalanced
Connection for the
Right Front Channel

INPUTS 2 & 4
Unbalanced Con­nection for the Left
and Right Surround
Channels

INPUT 4 Balanced Con­nection for the Right Sur­round Channel

INPUT 3
Balanced
Connection
for the Cen­ter Channel

INPUT 2 Balanced
Connection for the Left
Surround Channel

INPUT 1 Balanced
Connection for the
Left Front Channel

INPUTS 1 & 3 Unbalanced
Connection for the Left Front
and Center Channels

Output Terminals

Figure H

When connecting the Loudspeaker Hookup Cables to
the MC255 Power Amplifier Output Terminals please
follow the steps below:

1. Rotate the top of the Output Terminal Post coun­terclockwise until an opening

appears. Refer to gures A and

B.

2. Insert the Loudspeaker hookup
cable into the Output Terminal
Post opening or the cable spade
lug around the center post of

Figu re A

Opening

Figu re B

the Output Terminal. Refer to

gure C.

3. Rotate the top of the Output
Terminal Post clockwise until it

is nger tight. Refer to gure D.

4. Place the supplied McIntosh
Wrench over the top of the Out-

Figu re C Figu re D

put Terminal and rotate it one
quarter of a turn (90° ) to secure
the Loudspeaker Cable Connec­tion. Do not over tighten. Refer

to gure E.

Figu re E

How to Connect

Caution: Do not connect the AC Power Cord to the

MC255 Rear Panel until after the Loudspeaker
Connections are made. Failure to observe this
could result in Electric Shock.

The connection instructions below, together with the
MC255 Connection Diagram located on the separate
folded sheets “Mc1A and Mc2A”, are examples of a
typical audio systems. 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 Information” on page 3.

1. For Zone A Operation, connect a power control

How to Connect Input Connections and Output Terminals Connections

cable from the Audio Preamplifier or A/V Control
Center Power Control (Trigger) Output 1 to the
MC255 POWER CONTROL ZA INPUT.

2. For Zone B Operation, connect a power control
cable from the Audio Preamplifier or A/V Control
Center Power Control (Trigger) Output 2 to the
MC255 POWER CONTROL ZB INPUT.

Notes: 1. When a Power Control Cable is connected

between the MC255 and a Preamplifier
(or A/V Control Center), the AUTO OFF
Feature is bypassed. Refer to page 11.

2. When a Subwoofer Loudspeakers is being
used together with the MC255, connect
a Power Control Cable from the MC255
POWER CONTROL Output ZA to the
Power Control Input on the Subwoofer

Loudspeaker.

3. The MC255 has connections for Balanced Audio
XLR Connection Cables that come from the con­nections on the Audio Preamplifier or A/V Control
Center. Refer to the Connections on the separate
folded sheets “Mc1A and Mc2A”. UnBalanced
Audio Cables can be used instead of the Balanced
XLR cables.

The MC255 Power Amplifier is designed for Loud­speakers with an impedance of 4 ohms or 8 ohms.
Connect a single Loudspeaker to each of the Output
Ter m inals.

When connecting Loudspeakers to the MC255 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:

Loudspeaker Cable Distance vs Wire Gauge Guide

Loudspeaker

Impedance

4 Ohms

8 Ohms

25 feet

(7.62 meters)

or less

14AWG 12AWG 10AWG

16AWG 14 AWG 12 AWG

50 feet

(15.24 me ters)

or less

100 fe et

(30. 48 me ters)

or less

4. Prepare the Loudspeaker Hookup Cable for attach­ment to the MC255 Power Amplifier:

Bare wire cable ends:
Carefully remove sufficient insulation from the
cable ends, refer to figures 2, 3 & 4. If the cable
is stranded, carefully twist the strands together
as tightly as
possible.

Figure 2

Notes: 1. If desired, the twisted ends can be tinned

with solder to keep the strands together.

2. The prepared bare wire cable ends may be
inserted into spade lug connectors.

3. Banana plugs are for use in the United
States and Canada only.

Figure 3 Figure 4

Banana Plugs are for use in the United States and
Canada only:

5. Attach the previously prepared bare wire cable ends
into the banana plugs and secure the
connections. Refer to figure F.

6. Rotate the top of the Output Terminal

Post clockwise until it is nger tight.

Figure F

Refer to gure G. Then using the Mc­Intosh Wrench, rotate the top of the
Output Terminal one quarter of a turn
(90° ). Do not over tighten. Refer to

gure E.

Figu re G

7

How to Connect Output Terminals Connections, con’t

7. Referring to figure H, connect the Loudspeaker
hookup cables with
banana plugs into the
MC255 hole at the top
of the terminal to the

Figure H

Negative Output Termi­nal and the Positive Output Terminal indenti-

fied as 4Ω (ohm) or 8Ω (ohm) connection to match

the impedance of the Loudspeaker, being careful
to observe the correct polarities. Place the LOAD
Switch on the rear panel of the MC255 to either
the 4 or 8 Ohm Setting, to match the impedance of
the majority of the loudspeakers connected.
If the Loudspeaker’s impedance is in-between the
available connections, use the nearest lower im­pedance connection. Refer to “General Informa­tion” Note 3 on page 2 for additional information.

WARNING: Loudspeaker terminals are hazard-

ous live and present a risk of electric
shock. For additional instruction on
making Loudspeaker Connections con­tact your McIntosh Dealer or McIn­tosh Technical Support.

8. Connect the MC255 power cord to an active AC
outlet.

the lugs or wire cannot slip out. Refer to figures 7
and 8.

Figure 7

Figure 8

If the Loudspeaker’s impedance is in-between
the available connections, use the nearest lower
impedance connection. Refer to “General Informa­tion” Note 3 on page 2 for additional information.

WARNING: Loudspeaker terminals are hazard-

ous live and present a risk of electric
shock. For additional instruction on
making Loudspeaker Connections con­tact your McIntosh Dealer or McIn­tosh Technical Support.

10. Connect the MC255 power cord to an active AC
outlet.

Spade Lug or Wire Connections:

9. Connect the Loudspeaker hookup cables to the
MC255 COM (Negative) Output Terminal and

Positive Output Terminal indentified as 4Ω (ohm)
or 8Ω (ohm) connection to match the impedance

of the Loudspeaker, being careful to observe the
correct polarities. 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 terminals so

8

9

Front Panel Displays and Controls

Meter indicates the Front
Left Channel Output of
the amplifier

Meter indicates the Front
Center Channel Output
of the amplifier

The LEDs indicate when the POWER GUARD
Circuitry has activated for Amplifier Channels,
Left Front (LF1) and Left Surround (LS2)

The LED indicates when the POWER
GUARD Circuitry has activated for the
Amplifier Center Channel (C4)

The LEDs indicate when the POWER GUARD
Circuitry has activated for Amplifier Channels,
Right Front (RF5) and Right Surround (RS4)

Meter indicates the Front
Right Channel Output of
the amplifier

10

METER LIGHTS Switch
on the Illumination of the
Left, Center, Right Channel
Meter Display

Standby Power
On Indicator

POWER Switch Turns AC Power
Off, Remote, AC Power On

How to Operate

2

3

2

3

How to Operate

Power On

There are two ways to have AC Power switched ON to
the MC255:

REMOTE Setting

When the MC255 Power Control Con­nection (ZA and/or ZB) receives a Power
Control Signal from a Preamplifier or
A/V Control Center, it will turn On
automatically when the MC255 POWER

Figure 9

Control is set to the REMOTE Setting.
Refer to figure 9.

ON Setting

When the MC255 POWER Control is
set to the ON position, it will become
active Zone A only. Refer to figure 10.
To switch the MC255 OFF, place the
POWER Control in the OFF position.

Fig u re 10

MC255 Channel Operation

There are two Channel Operation Modes. The first
operation mode is referred to as Zone A. This occurs
when the POWER Control is positioned to ON or set
to the REMOTE position and is receiving a Power
Control Signal in the ZA connector. When the MC255
is in the Zone A operation mode, the following is a list
of Channel Operations:

MC255 ZONE A CHANNEL OPERATION

Channel Number Channel Function

1 Left Front

2 Left Surround

3 Center (Front)

4 Right Surround

5 Right Front

The other operation mode is referred to as Zone A/B
when the POWER Control is in the REMOTE posi-

tion and both the ZA and ZB connectors are receiving
Power Control Signals.

MC255 ZONE A and ZONE B OPERATION

Zone Channel Number Channel Function

A 1 Left Front

A 3 Center (Front)

A 5 Right Front

B 2 Left

B 4 Right

Meters

Rotate the METER LIGHTS Switch to the ON posi­tion to Illuminate the three Front Panel Meters for the
Left, Center and Right Channels. Refer to figure 11.

Lights Off - Meter lights are turned off, the Me-

Watts- The meters respond to all the musical infor-

mation being produced by the amplifier. They
indicate to an accuracy of at least 95% of the
power output with only a single cycle of a
2000Hz tone burst. Refer to figure 12.

ters will continue to indicate the power
output.

Note: When Power Control

Input of the MC255 is
connected to an Audio
Preamplifier or A/V
Control Center with
Remote Meter Illumi­nation Control, the
Meter Illumination will
automatically be remotely controlled
(On/Off).

Fi g u r e 11

Figure 12

Auto Off Switch

The MC255 incorporates Power Save Circuitry to
automatically place the MC255 into the power saving
Standby Mode approximately 30 minutes after there
has been an absence of an audio input signal.

When there is a Power Control Connection be­tween the MC255 and a Preamplifier or A/V Control
Center with Power Save Circuitry, the
AUTO OFF Switch in the ENable position
is bypassed (located on the Rear Panel of
the MC255). Refer to figure 13.

In the event there is no Power Control
Connection and the Power Save Circuitry is activat­ing inappropriately relative to your specific use of the
MC255, place the AUTO OFF Switch in the DISable
position.

Note: If the Power Save Circuitry has switched Power

to the MC255 OFF, place the POWER in the OFF
Position and then in the ON position to reset the
circuitr y.

Load Switch

Place the Load Impedance Switch to the 4 Ohms posi-

tion if the Loudspeakers Connected to the
MC255 are not 8 Ohms but 4 Ohms. Refer
to figure 14.

Figure 13

Figure 14

11

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 created a Power Amplifier without compro­mise, using the most advanced McIntosh circuit design
concepts.

The MC255 has a continuous average power output
rating of 250 watts and with a peak output current of
25 amperes per channel; making this one of the most
advanced amplifiers available today. The distortion
limits for the MC255 are no more than 0.005% at
rated power output for all frequencies from 20Hz to
20,000Hz. Typical performance at mid frequencies is
less than 0.002%. The true distortion readings on the
MC255 are so low, it takes special measuring tech­niques to make accurate readings. The MC255 can
deliver the best possible performance from any type of
high quality loudspeaker system. Refer to figure 15.

Creating an amplifier with this level of perfor­mance did not come easily. Many months of design,
testing and measuring were required. Extensive con­trolled listening tests, the ultimate form of measuring,
were made before the final design was accepted.

Design Philosophy

The design philosophy incorporated in the MC255
involved several different techniques, all based on
sound scientific logic. 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.Refer to figure 16.

12

Fig ure 15

UNBALANCED

INPUT

BALANCED

INPUT

Technical Description

Block Diagram of the Power Amplifier (one channel shown)

4Ω/8Ω DISPLAY

CONTROL

ILUMETER

POWER OUTPUT

INDICATOR

AUTO SENSE

CONTROL

ThermalTrak
DC BIAS

THERMAL
SENSORS

+

-

POWER GUARD

AMP

ATTENUATOR

MUTE

AMP

TO POWER

SAVE CONTROL

All transistors are selected to have nearly constant
current gain over the entire current range they must
cover. Output transistors in particular,
have matched uniform current gain,
high current bandwidth product and
large active region safe operating area.
These Power Transistors are the very
latest in semiconductor technology and
incorporate a new design known as
ThermalTrak™. Refer to figure 17. This
allows for the instantaneous and accu­rate monitoring of the Power Transistor
Temperature. The MC255 Power Output

Fi g ure 17

Figu r e 16

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 range of music conditions with
the benefits of lower distortion and cooler operation.
Precision metal film resistors and low dielectric ab­sorption film capacitors are used in all critical circuit
locations.

The high efficiency circuit design of the MC255
contributes to low operating temperatures. More than
2800 square inches of heat sink area keep the MC255
operating safely with convection cooling. No fans are
needed. Refer to figure 18.

Figure 18

Power Output Meter

The McIntosh MC255 has a large three Meter
Wattage Output Meter that responds 95% full scale
to a single cycle tone burst at 2kHz. Refer to figure

19. Voltage and current outputs are electronically
measured, multiplied and fed to a special circuit that
accelerates the pointer movement in the upward direc­tion. When the pointer reaches its peak it pauses only
long enough for the human eye to perceive its position,
then drops. It is almost 10 times faster than a profes­sional VU meter.

Fig u re 19

13

Technical Description, con’t

Protection Circuits

The MC255 incorporates the McIntosh Sentry Moni­tor Output Transistor Protection Circuit. Refer to
Figure 20. There is
absolutely no com­promise in sonic
performance with
this circuit, and it

Normal Operating Area

ensures safe opera­tion of the ampli­fier under even
the most extreme

Sentry Monitor
Safety Area

Output
Transistor
Failure

operating condi­tions. The different

Figure 20

types of protection circuits incorporated in the MC255
insure a long and safe operating life. This is just one
of the many characteristics of McIntosh Power Ampli­fiers that make them world famous.

The MC255 also includes the unique patented
McIntosh Power Guard circuit. Power Guard elimi­nates the possibility of ever overdriving the ampli­fier into clipping. Refer to figures 21, 22 and 23. An
overdriven amplifier can
produce both audible

Input Test Signal

(14kHz & 15kHz)

and inaudible distortion
levels exceeding 40%.
The audible distortion is
unpleasant to hear, but
the inaudible ultrasonic
distortion is also undesir­able, since it can damage
valuable loudspeaker
system tweeters. You
will never experience the
harsh and damaging dis-

Figu re 21

tortion due to clipping.

The Power Guard

Without Power Guard

circuit is a waveform
comparator, monitoring
both the input and output
waveforms. Under normal
operating conditions,
there are no differences
between the shape of
these waveforms. If the
amplifier is overdriven,
there will be a difference
between the two signal
waveforms. When the dif-

Figure 22

ference exceeds 0.3%, the
Power Guard activates

With Power Guard

the PG light and a dy­namic electronic attenua­tor at the amplifier input
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

Figure 23

is perfectly preserved. The MC255 Power Ampli­fier with Power Guard is not limited to just the rated
power output, but will actually produce distortion free
output well above its rated power due to the McIntosh
philosophy of conservative design.

Power Supply Circuits

To compliment the design of the MC255 Power Am­plifier Circuitry, there is a high current high voltage
power supply. Refer to figures 24 and 27.

Figure 27

The very large Power Transfomer can supply over 13
amps of current. Refer to figure 25 (golf ball is for size
comparsion).

Figure 25

14

Technical Description, con’t

AC POWER

CIRCUIT

BREAKER

POWER

RELAY

Block Diagram

of the

Power Supply

It is enclosed in the legendary McIntosh Potted Enclo­sures and weighs 30 lbs. The six super size main filter
capacitors can store over 350 Joules of energy which
is necessary for the wide dynamic range that “Digital
Audio” demands. Refer to figure 26.

POWER SAVE

CONTROL

Figu re 24

The power amplifier draws high current from the AC
power line. Therefore, it is important that they plug
directly into the wall outlet.
Also, most owners desire one power switch for the
whole audio system. The MC255 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 MC255. The MC255 also has two
remote Power Control Out Jacks. The Power Control
signal from these jacks is delayed by a fraction 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
high power Home Systems employing multiple MC255
Power Amplifiers.

Figure 26

15

16

Photos

17

Specications

Specications

Power Output (All Five Channels)

Minimum sine wave continuous average power output

per channel, all channels operating is:

200 watts into a 4 ohm or 8 ohm load

Power Output (Three Front Channels)

Minimum sine wave continuous average power output

per channel operating is:

250 watts into a 4 ohm or 8 ohm load

Output Load Impedance

8 and 4 ohms

Rated Power Band

20Hz to 20,000Hz

Total Harmonic Distortion

0.005% maximum harmonic distortion at any power
level from 250 milliwatts to rated power, 20Hz to
20,000Hz

Dynamic Headroom

3.6d B

Frequency Response

+0, -0.25dB from 20Hz to 20,000Hz
+0, -3dB from 10Hz to 100,000Hz

Input Sensitivity (for rated output)

2.2 Volt Balanced with a 4 ohm Loudspeaker

1.1 Volt Unbalanced with a 4 ohm Loudspeaker

3.0 Volt Balanced with an 8 ohm Loudspeaker

1.6 Volt Unbalanced with an 8 ohm Loudspeaker

Signal To Noise Ratio (A-Weighted)

108dB below rated output

Intermodulation Distortion

0.005% maximum, if the instantaneous peak power
output does not exceed twice the rated power output
or less per channel with all channels operating for any
combination of frequencies from 20Hz to 20,000Hz

Wide Band Damping Factor

Greater than 70 at 4 ohms
Greater than 140 at 8 ohms

Input Impedance

20,000 ohms Balanced
10,000 ohms Unbalanced

Voltage Gain

29dB

Power Guard

Less than 2% THD with up to 14dB overdrive at
1,000Hz

Power Control Inputs ZA and ZB

5-15VDC, less than 1mA

Power Control Output ZA and ZB

12VDC, 50mA maximum total
Output is delayed 0.2 seconds from turn On

Power Requirements

Field AC Voltage conversion of the MC255 is not
possible. The MC255 is factory configured for one of
the following AC Voltages:

100 Volts, 50/60Hz at 14.4 Amps
110 Volts, 50/60Hz at 13.0 Amps
120 Volts, 50/60Hz at 12.0 Amps
127 Volts, 50/60Hz at 12.0 Amps
220 Volts, 50/60Hz at 7.5 Amps
230 Volts, 50/60Hz at 6.5 Amps
240 Volts, 50/60Hz at 6.5 Amps
Standby: less than 0.5 watt

Note: Refer to the rear panel of the MC255 for the cor-

rect voltage.

Overall Dimensions

Width is 17-1/2 inches (44.5cm)
Height is 9-7/16 inches (23.9cm) including feet
Depth is 21 inches (53.3cm) including the Front Panel
and Cables

Weig ht

89.5 pounds (40.6 kg) net, 122.5 pounds (55.6 kg) in
shipping carton

Shipping Carton Dimensions

Width is 29-1/2 inches (74.9cm)
Height is 17 inches (43.2cm)
Depth is 29 inches (73.7cm)

18

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 plastic
feet are attached to the bottom of the equipment. Four
1/4 - 20 x 2-1/4 inch screws and washers must be used
to fasten the unit securely to the bottom pad and ship­ping skid. This will ensure the proper equipment loca­tion on the bottom pad. Failure to do this will result in
shipping damage.

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 Laboratory. Refer to page 2. Please
see the Part List for the correct part numbers.

Quantity Part Number Description
1 034052 Shipping carton top
1 034051 Shipping carton bottom
2 034054 Foam Pad (top and bottom)
2 034186 Foam Pad (front and rear)
2 034187 Foam Pad (sides)

Packing Instructions

1 034136 Inner carton top
1 034137 Inner carton bottom
1 034188 Foam Pad (inner carton)
1 034479 Shipping skid
4 101212 1/4 - 20x2-1/4 cap screw
4 104058 Flat washer

19

McIntosh Laboratory, Inc.

2 Chambers Street
Binghamton, NY 13903
www.mcintoshlabs.com

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. 24105100