McIntosh MC-2255 User Manual

THE

MclNTOSH

MC

2255

SOLID STATE STEREO POWER AMPLIFIER

Reading Time: 31 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 2255. 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 2255 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: [phone]

Take Advantage of 3 years
of Contract Service...

Fill in the Application NOW.

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 authoriz-

ed service agency or the factory.

2. Any Mclntosh authorized service agency
will repair Mclntosh instruments at nor-

mal 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 mishandl-

ing 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

Contents

SERVICE 1

INSTALLATION 2

HOW TO CONNECT 4

FRONT PANEL INFORMATION 9

REAR PANEL INFORMATION 11

PERFORMANCE LIMITS AND RATINGS 12

PERFORMANCE CHARTS 13

TECHNICAL DESCRIPTION 15

BLOCK DIAGRAM 17

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 post­marked 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 con-

tract or warranty may be available locally.

Copyright 1981 (c) by Mclntosh Laboratory Inc.

1

How to Connect

INPUT
STEREO OPERATION

Use shielded cables to connect the signal from
the preamplifier or signal source to the power
amplifier. To minimize the possibilty 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 2255.

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.

In stereo operation the MODE SWITCH must be in
the STEREO position.

MONOPHONIC OR SINGLE CHANNEL OPERATION

A shielded cable from the signal source is plug­ged into the Right (Mono) input jack of the MC 2255
only. The MODE SWITCH on the back panel of the
amplifier must be placed in one of the MONO posi­tions. In the MONO positions the output of the right
channel input amplifier is fed to both left and right
power amplifiers. The Left INPUT is disconnected.

Only the signal fed into the Right (MONO) input will

be amplified.

Be certain that the MC 2255 is never operated in

the stereo mode with the outputs connected for

monophonic operation. Should the MODE SWITCH

be left in the STEREO position and the outputs re­main strapped for a mono parallel load, one channel
will attempt to drive the other which causes high cir­culating currents and overheating.

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 depreciation of damping characteris­tics, modification of frequency response and reduc­tion in power output.

Use lamp cord or wire with similar 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 distances of
under 30 feet between the amplifier and speaker, #
18 wire or larger can be used. For distances over 30
feet use larger diameter wire. Select the correct size
wire from the chart below. It is recommended that
the DC resistance of the speaker leads be less than
5% of the speaker impedance. Resistance of the
leads should be computed for the length of wire both
to and from the speaker or speakers.

For multiple speaker operation, run separate
leads from the amplifier to the speakers.

MAXIMUM WIRE LENGTHS

Wire

Gauge

18

16
14
12
10

Wire lengths above represent the wire resistance

equal to 5% of the speaker impedance.

For 4 Ohm Load

Feet

15
25
40
60

100

Meters

4.57

7.62

12.19

18.29

30.48

For 8 Ohm Load

Feet

30

50

80

120
200

Meters

9.14

15.24

24.38

36.58

60.96

OUTPUT

Be certain the loudspeakers connected to the MC
2255 are capable of handling the power output of the
amplifier.

Selection of the proper gauge wire to connect the

loudspeakers preserves the quality of sound repro-

STEREO OPERATION

Check the impedance of the speaker which is
usually identified on the speaker itself or in the
owner's manual. Connect a lead from the common
terminal of the left speaker to the amplifier LEFT
OUTPUT terminal strip COMmon screw. Connect

4

another lead from the other terminal of the
loudspeaker to the left output terminal marked for
the impedance of the speaker on the LEFT OUTPUT
terminal strip. The right channel speaker is con­nected in the same manner to the RIGHT OUTPUT
terminal strip.

When multiple speakers are to be connected to
either or both outputs, the combined load im­pedance must be caluclated. The load must be con­nected to the appropriate impedance tap. Use this
table to aid in selecting the correct impedance
match:

Load
impedance
in ohms

0.8 to 1.6

1.6 to 3.2

3.2 to 6.4

6.4 and up

Connect for:

1 ohm output
2 ohm output
4 ohm output
8 ohm output

For monophonic operation using the MONO

PARALLEL mode, output impedances of ½, 1, 2, and
4 ohms are accomodated. Connect as listed below.
The common output terminal is at ground potential.

Load Impedance
In Ohms

½

1
2
4

Connect - Speaker
Lead To:

Either Left or Right

Common Terminal

Either Left or Right

Common Terminal

Either Left or Right

Common Terminal

Either Left or Right

Common Terminal

Connect + Speaker
Lead To:

Both Left and Right

1 Ohm Terminals

Both Left and Right

2 Ohm Terminals

Both Left and Right

4 Ohm Terminals

Both Left and Right

8 Ohm Terminals

FOR MONOPHONIC CONSTANT VOLTAGE LINE

OPERATION

For output
voltage of

Connected for:

If a load impedance is used that is lower than the
output impedance tap, then reduced power and pos­sible distortion will result. If a load impedance is us­ed that is higher than the output impedance tap,
then neither the signal nor the amplifier will be
harmed but the power available is reduced.

FOR STEREO CONSTANT VOLTAGE OPERATION:

For output

voltage of

25 volts

Connect for:

4 ohms

MONOPHONIC OR SINGLE CHANNEL OPERATION

When the MC 2255 is used as a monophonic or
single channel power amplifier the two channels are
combined to produce output up to 500 watts. The
outputs must be connected as described below.

For monophonic operation using the MONO
BRIDGE mode, output impedances of 2, 4, 8, and 16
ohms are accomodated. Output connections are
made by connecting to the output terminals as
listed below. Note that neither output terminal is at
ground potential.

25 volts

2 ohm output (mono)

Be certain that the MC 2255 is never operated in

the stereo mode with the outputs connected for
monophonic operation. Should the MODE SWITCH

be left in the STEREO position and the outputs re-

main strapped for a mono parallel load, one channel
will attempt to drive the other which causes high cir­culating currents and overheating.

AC POWER

The MC 2255 operates on 120 volts 50/60 Hz. The
auxiliary AC OUTLET on the MC 2255 is not fused or
switched.

Load Impedance
In Ohms

2
4
8

16

Connect - Speaker
Lead To:

Left 1 Ohm Terminal
Left 2 Ohm Terminal
Left 4 Ohm Terminal
Left 8 Ohm Terminal

Connect + Speaker
Lead To:

Right 1 Ohm Terminal
Right 2 Ohm Terminal
Right 4 Ohm Terminal
Right 8 Ohm Terminal

5

Stereophonic

Connections

PROGRAM SOURCE

LEFT
SPEAKER

MUST BE IN STEREO

POSITION FOR

STEREO PROGRAMS

6

RIGHT

SPEAKER

Mono-Bridge

Connections

PROGRAM SOURCE

(CONNECTIONS FOR

8 OHM SPEAKER

ILLUSTRATED)

SPEAKER

CONNECTIONS SHOWN FOR MONO

WHEN MODE SWITCH

IS IN THE MONO-BRIDGE POSITION

7

Mono-Parallel
Connections

PROGRAM SOURCE

(CONNECTIONS FOR

4 OHM SPEAKER

ILLUSTRATED)

SPEAKER

CONNECTIONS SHOWN FOR MONO

IS IN THE MONO-PARALLEL POSITION

8

WHEN MODE SWITCH

The Front Panel Controls and
How to Use Them

METERS

Output power monitor meters indicate the output
power of each channel. Each meter has two primary
scales: WATTS and DECIBELS. When the METER
RANGE switch is in one of the decibel (dB) posi­tions, peak signal readings are indicated on the
lower DECIBEL scale. When the METER RANGE
switch is in the WATTS position, direct power in
watts is read from the upper watts scale. The meters
are calibrated to read average watts. The in­termediate markings between the calibrations repre­sent, beginning with 250 watts, 100 watts, 50 watts,
the indicated 25 watts, 10 watts, 5 watts, the in­dicated 2.5 watts, 1 watt, .5 watt, the indicated 0.25
watt, 0.1 watt, 0.05 watt, the indicated 0.025 watt,

0.01 watt, 0.005 watt, the indicated 0.0025 watt, 0.001
watt and 0.0005 watt. Although the meter calibra­tions are in average watts for a sine wave signal, the
meters electrically respond to signal peaks. The
meters are voltage actuated and indicate power ac­curately when the amplifier is operated into rated
output load impedances.

The meters respond to the peak output of each
channel. Ordinary meters lack the capability of in­dicating the short interval information in a sound
wave. The mass of the meter movement is too great
to respond to the nearly instantaneous changes in

music program material. Short interval information
can have a duration as short as half a thousandth of

a second. Ordinarily, a meter pointer moving over its
scale in such a short time would not be seen.
Mclntosh has developed circuits that drive the
meters to respond to the short interval information
in a sound wave to an accuracy of 90%. The elec­trical pulse that drives the meter pointer is time
stretched long enough so that the peak position of
the pointer can register in the persistence of vision
characteristic of the retina of the human eye.

LEFT GAIN

The LEFT GAIN control adjusts the volume in the

left channel to the desired listening level. Turn the
control clockwise to increase the volume.

RIGHT/MONO GAIN

The RIGHT/MONO GAIN control adjusts the
volume in the right channel to the desired listening
level. Turn the control clockwise to increase the
volume. When the output of MC 2255 is connected
for monophonic operation and the rear panel MODE
SWITCH is in the MONO position the volume is con­trolled by the RIGHT/MONO GAIN control only.

METER RANGE

The METER RANGE switch has five positions.

9

WATTS

POWER

In the WATTS position the meter's primary calibra­tion is from .0025 watts (two and a half milliwatts),
up to 250 watts, the rated power output of the MC

2255. The meter is calibrated for 500 watts at the
right hand end of the meter scale. While the MC 2255
cannot reach this power level continuously, it is
possible for short interval peaks to exceed, con­siderably, the 250 watt continuous rating.

HOLD

In the HOLD position, the meter indicates WATTS
and locks to the highest power peak in a sequence
of peaks. The meter will be driven to maximum
power and electronically held there until a higher
peak passes through the amplifier. If no further
peaks are reached the meter needle will very slowly
return to its rest position (decay rate: 6 dB per
minute).

DECIBELS

In the other three positions of the METER RANGE
switch the meters will indicate the output of each
channel in DECIBELS relative to 250 watts or any
other chosen reference.

0dB In this position of the switch, if the

amplifier delivers 250 average watts, the
meter indicates 0 dB; at 125 average watts
the meter indicates - 3 dB. If the amplifier
is overdriven to +2 dB the indicated out­put would be 396 watts.

-10 dB In this position of the switch, if the
amplifier delivers 25 average watts, the
meter indicates 0 dB; at 12.5 average watts
the meter indicates -3 dB.

-20dB In this position of the switch, if the
amplifier delivers 2.5 average watts, the

meter indicates 0 dB; at 1.25 average watts

the meter indicates -3 dB.

HEADPHONES

The output of the front panel HEADPHONE jack
has been designed to feed low impedance dynamic
stereo headphones.

The HEADPHONE output is not affected by the

SPEAKER switch.

SPEAKERS

OFF: The loudspeakers are turned off when the
SPEAKER switch is in the OFF position. You can lis-

ten to headphones in private.

THE SPEAKER SWITCH MUST BE IN THE "ON" PO-

SITION TO HEAR MUSIC FROM THE LOUDSPEAK-

ERS.

ON: Music will be heard through the loudspeakers.
Use this as the normal listening position.

The power switch turns the MC 2255 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 2255 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

POWER GUARD assures that the MC 2255 cannot
be overdriven, thus amplifier output clipping is
eliminated. Clipping is caused when an amplifier is
asked to produce more power output than it can
deliver with low distortion. Amplifiers are capable of
delivering large quantities of highly distorted power
when they are driven to clipping. The extra energy
content of the clipped signal will damage most
speakers. Mclntosh's Power Guard circuit protects
your speakers from this kind of damage. The MC
2255 has a built in "waveform comparator" that
compares the wave shape of the output signal to the

input signal. If the disparity between the two
signals, due to overdrive, 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 operates to
limit the amplifier input dynamically so that the
amplifier cannot be overdriven. Power Guard
eliminates amplifier output clipping. As long as the

amplifier operates without
overload the NORMAL in­dicator illuminates.

NEW AUTOMATIC TEST
SYSTEM

The MC 2255 contains a
new automatic test circuit.

When AC line power is ap­plied, an LED digit illuminates to indicate which test
is being performed. Starting with the numeral 7, it
makes the required measurement and verifies if it is
within tolerance by lighting the "normal" Power
Guard green lights. It then counts down to six, per­forms and verifies the next test, then five, four, three,

etc. until it reaches "1" and then the speakers turn
on. A "beep" tone is heard each time a test is per­formed. If a circuit should fail, the red "limit" lights
will come on and the sequence will stop at that
point. Speaker turn on will not occur until the fault

has been corrected; thereby protecting your system

from any further damage.

There are two user controls associated with the
auto test circuit. They are located on the amplifier
top panel. Two switches control the speed of the
countdown (SLOW or FAST) and the presence of the
beep tone (ON-OFF).

10

Rear Panel information

MODE SWITCH

The MC 2255 will operate in three modes, Stereo,
Mono Bridge, and Mono Parallel. The Mono modes
differ in the loads they will drive and the connection
sequence to the speaker terminals.

LEFT and RIGHT OUTPUT TERMINALS

For stereo operation, output impedances of 1, 2, 4
and 8 ohms have been provided on a secure, screw
type barrier strip. For monophonic operation proper
interconnection provides 0.5, 1, 2, 4, 8, and 16 ohms
from the same barrier strips. See page 5 for connect­ing instructions.

INPUT JACKS

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 2255 is 0.75 volts or

2.5 V depending on the position of the INPUT LEVEL
switch. 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
Mclntosh, set the switch in the position nearest to
the stated output rating of the source equipment.

AC POWER

The input to the MC 2255 is 120 volts 50/60 Hz at
up to 12 amps. The primary circuit is protected by a
15 Amp Fuse.

11

Performance Limits

PERFORMANCE GUARANTEE

Performance Limits are the maximum deviation

from perfection permitted for a Mclntosh instru-

ment. We promise you the MC 2255 you buy must be
capable of performance at or exceeding these limits
or you get your money back. Mclntosh is the only

manufacturer that makes this guarantee.

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
250 watts minimum sine wave continuous average
power output, per channel, both channels operating
into 1 ohm, 2 ohms, 4 ohms, or 8 ohms load im-

pedance, which is:

15.8 volts RMS across 1 ohm

22.4 volts RMS across 2 ohms

31.6 volts RMS across 4 ohms

44.7 volts RMS across 8 ohms

MONO
500 watts minimum sine wave continuous average
power output in to 0.5 ohm, 1 ohm, 2 ohms, 4 ohms, 8
ohms, or 16 ohms load impedance, which is:

15.8 volts RMS across 0.5 ohm

22.4 volts RMS across 1 ohm

31.6 volts RMS across

44.7 volts RMS across

63.2 volts RMS across

89.4 volts RMS across

OUTPUT LOAD IMPEDANCE

STEREO
1 ohm, 2 ohms, 4 ohms, and 8 ohms; separate ter­minals are provided for each output.
MONO-PARALLEL

0.5 ohm, 1 ohm, 2 ohms, and 4 ohms; obtained by
connecting together the appropriate terminals of
both channels.
MONO-BRIDGED
2 ohms, 4 ohms, 8 ohms, or 16 ohms, balanced to
common connections.

RATED POWER BAND

20 Hz to 20,000 Hz

TOTAL HARMONIC DISTORTION

STEREO

0.02% maximum harmonic distortion at any power
level from 250 milliwatts to 250 watts per channel
from 20 Hz to 20,000 Hz, both channels operating.
MONO

0.02% maximum harmonic distortion at any power

2 ohms
4 ohms
8 ohms

16 ohms

level from 250 milliwatts to 500 watts from 20 Hz to

20,000 Hz.

INTERMODULATION DISTORTION

STEREO

0.02% maximum if instantaneous peak power out­put is 500 watts or less per channel with both chan-

nels operating for any combination of frequencies,
20 Hz to 20,000 Hz.
MONO

0.02% maximum if instantaneous peak power out-

put is 1000 watts or less for any combination of fre­quenices, 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 + 0.25 -1 dB.

NOISE AND HUM

95 dB below rated output.

RATINGS

DAMPING FACTOR

Greater than 30

INPUT IMPEDANCE

50,000 ohms.

INPUT SENSITIVITY

Switchable: 0.75 volt or 2.5 volts—level control pro­vided for higher input voltages.

POWER GUARD

Clipping is prevented and THD does not exceed 2%
with up to 20 dB overdrive at 1 KHz.

GENERAL INFORMATION

POWER REQUIREMENTS

120 volts 50/60 Hz 0.7 to 12 amps.

SEMICONDUCTOR COMPLEMENT

85 silicon transistors
47 silicon rectifiers and diodes
14 integrated circuits

12

Performance Charts

FREQUENCY IN Hz

FREQUENCY IN Hz

13

FREQUENCY IN Hz

PERFORMANCE CHARTS ARE
FOR ONE STEREO CHANNEL
WITH BOTH CHANNELS OPERATING.
MONO OPERATION IS IDENTICAL
EXCEPT POWER IS DOUBLED
FOR THE SINGLE MONO OUTPUT.

FREQUENCY IN Hz

POWER IN WATTS

14

Technical Description

INPUT AMPLIFIER

Each channel input contains a complete seven
transistor low power amplifier. A differential tran­sistor pair provides high input impedance and low
noise. The differential signals are combined in a cur­rent mirror circuit which drives a class A amplifier
stage. The following output stage is driven by a com­plimentary pair of transistors with class AB biasing.

The output signal drives the metering circuit, head-

phone jack, and the high power output amplifier.

This discrete transistor amplifier design was
selected for low noise, low distortion, adequate

power output capability to drive headphones, and

freedom from turn on and turn off transients.

The INPUT LEVEL selector and GAIN controls are
passive attenuators which precede the input
amplifier circuitry. Therefore, the input system to the
amplifier cannot be overloaded when the controls
are correctly set.

In the MONO mode of operation the input signal

feeds only the right input amplifier via the

RIGHT/MONO GAIN control. The output of the right

input amplifier feeds both output power amplifier
sections. When MONO BRIDGE MODE is selected
the left channel input amplifier is used as a phase in­verter before the left output amplifier. The output
channels are therefore 180° out of phase with
respect to each other which is the correct arrange­ment for bridge output connections. When MONO
PARALLEL MODE is selected the channels operate
in phase. The channels, of course, also operate in
phase for the STEREO MODE.

OUTPUT POWER AMPLIFIER

The power amplifier inputs are coupled to the in­put amplifier outputs through an electronic switch.
The switch eliminates turn-on and turn-off transients
and is used for speaker switching. A junction FET
and LED/light dependent resistor network make up
the switch. This combination allows the lowest

possible distortion when the amplifier is on and high

isolation when the output power amplifier is off. The
control signal to the switch is held off for the auto
test countdown time. Therefore, transients that may
come into the amplifier from source equipment will
not be amplified or reach the loudspeaker. Since the

HEADPHONE output and meters are powered by the

input amplifier, their operation is not affected by the
SPEAKER switch or turn on delay system.

The first stage of the output power amplifier is a
differential transistor pair biased for best linearity.
The offset to the differential pair is adjustable. Cor-

rect adjustment allows the lowest possible distor-

tion at low frequencies. A current mirror circuit com­bines the differential outputs into one signal which

is then amplified by a following class A voltage
amplifier. Both the differential transitors and the
voltage amplifier are supplied by active current
sources. The results are lower distortion and cleaner
turn on characteristics.

The driver stage consisting of a complimentary
pair of power transitors biased class AB follow the
voltage amplifier. Next, three complimentary pairs
of rugged power transistors make up the power out­put stage. All power transistors are mounted on con­servatively sized anodized aluminum heat sinks.
Because of a unique connection of the bias network,
the output transistors operate class B and exhibit no
crossover distortion often associated with class B
operation. The heat sinks, therefore remain cool
when there is no output.

The turn-on and turn-off characteristics of the
Power Amplifier are smooth and silent. No noisy
relay is required to disconnect the loudspeakers dur-

ing turn-on time. Relays can cause noise and in­creased distortion if the contacts aren't perfectly
clean.

The amplifier output signal is fed to the output ter­minals through the output autotransformer. The
Mclntosh designed interleaved multifilar wound
autotransformer is used to properly match the
amplifier to stereo output load taps for 1, 2, 4 and 8
ohms. The MC 2255 will deliver full power over the
entire audio frequency range at any of these im­pedances. The autotransformer also protects
speakers from damage in the event of amplifier
failure. Should a direct current component appear in
the output it is shunted by the autotransformer and
DC cannot damage the speaker.

A Mclntosh patented Sentry Monitoring circuit

constantly monitors the output signal and instantly

reacts to prevent overload of the output transistors.

At signal levels up to rated output this circuit has
high impedance and has no effect upon the output.

If the power output exceeds design maximum, the

Sentry Monitoring circuit operates to limit the signal
to the output transistors. In the event of a short cir­cuit across the amplifier output or severe im­pedance mismatch the Sentry Monitoring circuit will
protect the output transistors from failure. Both
positive and negative halves of the output signal are
monitored and protected independently.

POWER GUARD PROTECTION CIRCUIT

The Mclntosh patented Power Guard circuit

eliminates amplifier clipping due to overdrive. The

15

circuit also illuminates red LIMIT indicator lamps
when the amplifier is driven beyond its maximum
output capacity. Power Guard prevents loudspeaker
damage and eliminates harsh output distortion
caused by amplifier clipping.

The Power Guard circuit consists of a waveform
comparator which monitors the wave shape of the
amplifier input and output signals. Normally there is
no disparity between these signals and the com­parator produces no output. When the amplifier is
driven beyond its maximum power capacity a dif­ference will develope. If the disparity exceeds 0.5%

(equivalent to 0.5% total harmonic distortion) the
comparator output causes the red LIMIT indicators
to light. If there is a further increase in the disparity
the comparator output controls an electronic at­tenuator at the amplifier input to reduce the
amplifier gain, thus holding the amplifier output to

its maximum undistorted value regardless of the

degree of overdrive to the amplifier. The amplifier

may be overdriven by 20 dB before the output distor-

tion exceeds 2%.

The comparator is an especially compensated

operational amplifier integrated circuit. Its output is
detected by a full wave bridge that feeds signals to
the control circuitry for the LIMIT and NORMAL in­dicators and to the electronic attenuator at the
amplifier input. The attenuator is a light emitting
diode/light dependent resistor network selected
especially for its low distortion and time constant
characteristics.

SYSTEM TEST CIRCUIT

When AC line power is applied to the MC 2255 the

System Test circuit functions to verify operating

potentials at 7 test locations within the amplifier.

Upon applying power a digital display lights with the
number 7. The digit is driven by a counter that steps
down at a 1 second or .4 second rate. Each time the
digit changes electronic switches select the outputs
of various voltage comparators connected to test
points in the amplifier. If voltages are normal the
green NORMAL Power Guard indicator lights, there
is an audible beep produced by an internal sounder,
and the digit decrements to the next test number.
When the count passes number 1 the loudspeakers
connect and the display blanks off. If a fault condi­tion exists the counter stops. The digit displayed in­dicates the circuit at fault. Servicing the amplifier is
thereby simplified.

this amplifier and goes directly to the full wave rec­tifiers through an attenuator which is controlled by
the METER RANGE switch.

The logarithmic amplifier consists of a high gain
operational amplifier with a biopolar connected sili­con diode pair as feedback elements. These diodes
have a uniform logarithmic characteristic over an 80
dB range. Only 60 dB of this logarithmic range is us­ed in the MC 2255.

The full wave rectifier circuit uses an operational
amplifier with silicon diode feedback networks. This
amplified diode circuit has nearly perfect rectifica­tion characteristics. One rectifier detects only posi­tive signals. The other responds only to negative

signals and produces a positive output. The outputs
of the rectifiers are combined at the operational
amplifier output, so the highest signal, either

positive or negative, is the one that is indicated by
the meters. Gate diodes are used to charge a low

leakage capacitor which attains and holds a charge
during signal peaks. The operational amplifier pro­vides a large amount of current so the capacitor can
charge suddenly. The charge on the peak holding
capacitor is amplified in a two transistor DC
amplifier which is used to drive the meter. From the
output of this amplifier there is a DC feedback net­work that connects back to the detector to assure
excellent overall linearity and frequency response.
The current drive to the meters has a peaking
capacitor to accelerate the upscale response of the
meter needle. The meters also have a parallel shunt
resistor to correctly damp their action. In the WATTS

mode the discharge of the peak holding capacitor is
controlled by a resistor current source. In WATTS

HOLD, the resistor is disconnected so the peak

reading is retained. The rate of decay is about 6 dB
per minute.

POWER SUPPLY

The power supply is a conventional full wave
bridge rectifier arrangement providing plus and
minus 50 volts DC. Electronic regulators step down
and regulate plus and minus 15 volt sources for low
level circuits and plus 5 volts for the logic circuits.

Thermistors are used in the power transformer

primary circuit to limit the turn-on current.

The test circuit uses logic counter, BCD decoder,
lamp driver, and voltage comparator integrated cir­cuits.

METER CIRCUIT

The meter circuit has three basic sections: a loga­rithmic amplifier, a full wave rectifier, and a DC amp­lifier. In the WATTS ranges, the logarithmic amplifier
is used. In the DECIBEL ranges, the signal bypasses

16

Block
Diagram

17

MclNTOSH LABORATORY INC.

2 CHAMBERS ST., BINGHAMTON, N.Y. 13903

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.

039258