Specifications

STEREO REVIEW
test reports
to the two input phono jacks is a switch that
sets the input sensitivity to either 0.75 or 2.5
volts for full output depending upon the as-
sociated equipment. (The latter is the nor-
mally preferred setting for most setups.)
The MC 2255 is a handsome and rugged
amplifier, following a long-standing McIn-
tosh tradition in its styling and construc-
tion. The pane! and most of the top metal-
work are finished in black, with front ac-
cents of silver and softly lit blue-green me-
ters. The chassis is chrome-plated. Also fur-
nished with the amplifier are side brackets
and hardware for the McIntosh Panloc sys-
tem for custom installations.
• Laboratory Measurements. Precondi-
tioning the MC 2255 at one-third rated
power made the heat sinks very hot, but the
rest of the amplifier remained comfortably
cool throughout our tests. In normal opera-
tion the MC 2255 is no more than faintly
warm and has no need of a cooling fan or
any unusual ventilation precautions
With both channels driving 8-ohm loads
at 1,000 Hz the distortion was undetectable
(well below the noise level) until we reached
10 watts output, when it measured 0.00056
per cent. It increased very gradually with
higher power to 0.0032 per cent at 250
watts and 0.0045 per cent at 300 watts. The
maximum power (corresponding to "clip-
ping power," except that the waveform
could not be made to clip) was about 357
watts, with distortion reading 0.24 per cent
at the limiting point. The output into 4
ohms (using the appropriate output termi-
nals) was also 357 watts, and we were able
to develop 420 watts per channel into 2-ohm
loads.
At the rated 250 watts output into 8
ohms, the maximum distortion was 0.01 per
cent at 20 Hz. Over most of the audio range
it was about 0.004 per cent, rising to 0.009
per cent at 20,000 Hz. At lower power out-
puts the distortion was substantially lower.
The amplifier sensitivity (using the 2.5-volt
switch setting) was 0.15 volt for a 1-watt
reference output, and the A-weighted noise
and hum level was 86 dB below 1 watt. The
frequency response of the amplifier was
within +0. - 0 1 dB from 20 to 20,000 Hz
and was down 0.9 dB at 5 Hz and 3 dB at
150 kHz.
The amplifier rise time was about 3 mi-
croseconds, and its IHF slew factor was
about 10 The IHF intermodulation distor-
tion, measured with 18- and 19-kHz sig-
nals, was — 94 dB for the second-order com-
ponent at 1,000 Hz and -67 dB for each of
the third-order products at 17 and 20 kHz,
all being referred to a 250-watt level.
The clipping headroom of the amplifier
was 1.55 dB for 4- and 8-ohrn loads and
2.55 dB for 2-ohm loads. The dynamic pow-
er output was 455 to 466 watts, depending
on the load impedance, giving dynamic-
headroom ratings of 2.65, 2.7, and 2.5 dB
for loads of 8, 4, and 2 ohms, respectively.
The meters read quite accurately (about
0.2 dB high at full power) and responded to
very brief transient signals. They are driven
from the class-AB input amplifier instead
of from the output stages as in most ampli-
fiers, so the meter readings are unaffected
by switching off the speakers. We found the
headphone volume to be only marginally
useful with 600-ohm phones. It was ade-
quate with conventional-impedance phones.
Mclntosh MC 2255
Power Amplifier
• Comment. Mclntosh (one of the old-
est names in hi-fi, and perhaps the only
firm from its time to remain under the
original ownership) has earned an im-
pressive reputation for their continued
support of their products, their excep-
tionally conservative design and specifi-
cations, and generally outstanding qual-
ity The MC 2255, the first McIntosh
product we have reviewed in many
years, is a perfect example of the contin-
uation of those policies.
In its circuitry and operating features,
the MC 2255 is quite unlike any other
basic power amplifier we have seen. By
using autotransformers to match the
load impedance to the transistor require-
ments, McIntosh has made an amplifier
capable of delivering its full potential
performance into almost any load im-
pedance it might encounter. That per-
formance, as our tests have shown, is ab-
solutely first-rate. It is difficult to imag-
ine any home installation needing more
power than the MC 2255 delivers with
such apparent ease. Its noise, distortion,
stability, and any other quality one
might name are quite literally "state of
the art."
The Power Guard system is most ef-
fective in making it impossible to hard-
clip the output of the amplifier. Regard-
less of how hard it is driven, it simply
cannot develop an audible amount of
distortion on musical program material
(2 per cent is well below the probable
threshold of audibility of distortion in a
music system being driven to 350-plus
watts). This feature should also mean a
greatly reduced likelihood of blowing
out a speaker, since clipping is a com-
mon cause of tweeter damage. For the
nontechnical user, the self-test feature is
mostly a "security blanket," although
we can appreciate that it would also sim-
plify troubleshooting and servicing.
LEARLY, no effort has been spared in
the design and construction of this am-
plifier. This sort of perfectionism carries
a considerable price, both in dollars and
pounds (avoirdupois, not sterling!). Con-
sidering the probable long life of the MC
2255, that price does not seem at all
unreasonable -Julian D. Hirscti
7
Reprinted through the kind permission of Stereo Review
c