Specifications

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"Reprinted with permission from the June 1982 issue of STEREO REVIEW magazine.

Mclntosh MC 2255 Power Amplifier

• Mclntosh MC 2255 Power Amplifier

• Power Rating: 250 watts per channel

• Size: 16¼ x 14¾ x 7¼ inches

• Weight: 82 pounds

• Price: $2,750

HE Mclntosh MC 2255 basic power am-

plifier is rated to deliver its output into

loads of 1, 2, 4, or 8 ohms, from 20 to

20,000 Hz, with no more than 0.02 per cent

harmonic or intermodulation distortion. Its

stereo outputs may be paralleled or bridged

to drive a mono load with a maximum out-

put of 500 watts at 0.02 per cent distortion.

Depending on the connection used, the

mono load impedance can be from 0.5 to 16

ohms.

The unusual load capabilities of the MC

2255 derive from the use of large autotrans-

formers to match the output transistors to

their loads. Like vacuum-tube amplifiers,

the MC 2255 has output terminals desig-

nated for 1, 2, 4, or 8 ohms. Thus, regard-

less of the speaker impedance, the output

transistors are optimally loaded and can de-

liver

their

full

power

without

excessive

dis-

tortion or overheating.

The output stages of the MC 2255 oper-

ate in class-B, but a unique biasing system

completely eliminates the crossover distor-

tion usually associated with class-B opera-

tion. This being the most efficient mode of

linear amplifier operation, the total power

consumption of the MC 2255 from the 120-

volt a.c. line is only 0.7 ampere at idle (or

normal playing volume) and 12 amperes at

full

output.

The

input

and

driver

stages

form a complete class-AB low-power ampli-

fier which drives the front-panel headphone

jack as well as the power stages. Switches

connect the input sections for mono opera-

tion.

In the

MONO/PARALLEL

mode

the

right-channel input drives both output sec-

tions

phase,

and for the

MONO

BRIDGE

mode the other input section is used as a

phase inverter so that the outputs can be

driven 180 degrees out of phase.

The power stages are protected by a novel

Power Guard circuit that makes it impossi-

ble to clip the amplifier output by overdriv-

ing it. A waveform comparator monitors the

input and output signals of the amplifier,

and if the output waveform differs from the

input by an amount corresponding to about

0.5 per cent harmonic distortion, a red LIM-

IT light glows on the panel (there are sepa-

rate lights for the two channels). Any fur-

ther increase in the drive level causes the

signal to be attenuated ahead of the output

section. This prevents the output from ever

exceeding its linear operating range (ac-

cording to McIntosh, the amplifier can be

overdriven by 20 dB before distortion

reaches 2 per cent).

Internally, the McIntosh MC 2255 is a

very complex amplifier, containing some

eighty-five transistors, forty-seven diodes,

and fourteen integrated circuits. Many of

its components are involved in the protec-

tive systems and in its novel self-test fea-

ture. Each time the amplifier is turned on,

an automatic seven-step test sequence

checks the key operating voltages for cor-

rectness. As each step is executed, the cor-

responding numeral lights up on a front-

panel display and a green light signals that

it has been passed. If any stage of the test is

not satisfactory, its number remains lit to

indicate the problem area. Two different

test speeds can be selected, and one can

choose to have a "beep" sound after each

step or to have the tests proceed in silence.

Two large meters are calibrated logarith-

mically from less than 2.5 milliwatts to 500

watts output (because of the output trans-

formers, these readings are equally applica-

ble to any of the load impedances for which

the amplifier is designed). Another scale

reads in decibels from -20 to + 2 (the lat-

ter corresponding to about 250 watts out-

put). Knobs below the meters control LEFT

GAIN, RIGHT/MONO GAIN, METER RANGE

(-20 dB, -10 dB, 0 dB, WATTS, HOLD),

the SPEAKERS outputs, and POWER. The

HOLD position of the METER RANGE switch

causes the meters to retain their highest

readings. The meter-driving circuits allow

them to respond to very short program

peaks, although they are calibrated in aver-

age watts.

At the right side of the panel are the two

indicator groups.

The

POWER GUARD

display

shows the number of the SYSTEM TEST se-

quence step as it is executed, and pairs of

red and green LEDs show either that the

LIMIT

(of

output power)

has

been exceeded

or that the amplifier operation is NORMAL.

Above this group, a meter group illuminates

the words WATTS, HOLD, or DECIBELS, ac-

cording

to the

setting

of the

METER RANGE

switch.

On the rear of the chassis are two sets of

barrier terminal strips for the speaker out-

puts, a single unswitched a.c. outlet, and the

holder for the 15-ampere line fuse. A three-

position MODE switch selects STEREO, MONO

BRIDGE,

MONO PARALLEL

operation. Next

STEREO REVIEW