Mcintosh MC 2500, MC 2255, MC 2250, MC 2155, MC 2120 catalog

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The World Wide Leader
In State Of The Art
High Fidelity Technology
Engineered and Manufactured
in the United States of America
Mclntosh Laboratory Inc.
2 Chambers Street
Binghamton, New York, 13903-9990
YOU SHOULD OWN
McIntosh
BECAUSE
TABLE OF CONTENTS
Power Amplifiers:
MC 2500, MC 2255, MC 2250, MC 2155, MC 2120,
MC 502
2-16
PreAmplifiers:
C 33, C 29, C 27, C 504
14-24
Tuners:
MR 80, MR 78, MR 75 25-37
Tuner-Preamplifier:
MX 117 38-45
Receiver:
MAC4100 AM/FM Receiver 46-50
Preamplifier- Amplifier:
MA6200 51
Loudspeakers: 52-65
The Mclntosh Story 66-67
Equalizers:
MQ107, MQ 104 68
FM Guide. . . 69-76
Mclntosh instruments are design-
ed and manufactured for long life.
Mclntosh instruments have always
been designed for long life with low
maintenance costs and high quali-
ty performance. Mclntosh in-
struments have been and are the
LABORATORY STANDARD
for the world.
Until 1949 the performance re-
quirements for a Mclntosh had
long been an engineering dream.
They became a reality with the in-
troduction of the first Mclntosh
amplifier. Through all these years
Mclntosh has produced instru-
ments that have exceptionally long
life. Clinics held ail over North
America have shown that most of
the Mclntosh instruments ever
manufactured still meet or exceed
their original exacting specifica-
tions.
Used McIntosh instruments have
the highest resale value. Retailers
report that customers are constant-
ly searching for used Mclntosh in-
struments. A Mclntosh does not re-
main on the "Used" display long.
You'll get more when you trade-in
your Mclntosh assuring you of
maximum return on your invest-
ment,
Mclntosh dedication, not only to
improvements, but also to fun-
damentals, has justified many
patents on basic circuit structures
as well as refinements.
Doesn't it make good sense to deal
with a company that wants to do as
much for you as it possibly can?
1
THE NEW
MCINTOSH AMPLIFIERS
Mclntosh leadership in research and bipolar
epitaxial transistor technology has produced
startling new advances in safe, cool,
superior performance and protection.
Mclntosh engineering continues to advance
power amplifier technology and protected perfor-
mance. Experience and knowledge are the found-
ation on which the engineering superstructure is
built that supports the Mclntosh recognized reputa-
tion as Laboratory Standard for the world. A new
level of technology and a higher level of amplifier
performance is realized in the all new Mclntosh
Amplifiers.
LEADERSHIP
Mclntosh life testing selects only components
that give the most trouble free performance. Add-
ed care in engineering, design and manufacturing
produces long product life at the peak of perfor-
mance.
LEADERSHIP
Mclntosh POWER GUARD assures maximum am-
plifier power without clipping distortion.
LEADERSHIP
Mclntosh engineers developed a unique output
circuit configuration that is temperature stable
and that delivers clean output power at any level
without a trace of crossover distortion.
LEADERSHIP
Mclntosh Automatic System . Test provides
positive protection and extends the long trouble
free life of an amplifier. Each time an amplifier is
turned on, seven tests are completed that
measure and verify accurate performance.
LEADERSHIP
Mclntosh Output Autotransformers deliver full
power output and multiple feedback loops assure
lowest distortion at all power levels and all
speaker impedances.
LEADERSHIP
Mclntosh designed mute circuits give positive
protection from transients due to turn on, turn off
power supply voltage changes.
MC 2255 Shown in optional walnut veneer cabinet
2
YOU ARE PROTECTED FROM
LISTENING TO AMPLIFIER
PRODUCED DISTORTION
WITH MCINTOSH
POWER GUARD
Plus:
Mclntosh Output Autotransformer delivers full
power output. Multiple feedback loops assure
lowest distortion at all power levels and all
speaker impedances
Mclntosh engineers developed a unique output
stage circuit arrangement that is completely
temperature accurate, that delivers clean output
power at any level without crossover distortion
Mclntosh life testing of components permits com-
ponent selection for trouble-free performance; ad-
ded care in production engineering and manufac-
turing results in long product life
Mclntosh designed "turn-on/mute" circuits pro-
vide positive protection from "turn-on transients"
and other potentially damaging noises
Mclntosh POWER GUARD assures maximum
amplifier power without clipping distortion
Higher power demands on amplifiers have
presented music listeners with a form of unpleas-
ntness in listening, amplifier overload (hard clipping)
that looks and acts like square waves. Clipping is
caused when the amplifier is asked to produce more
power output with low distortion than it can deliver.
Clipping of a complex wave form is largely compos-
ed of odd order harmonics and intermodulation pro-
ducts. High order odd harmonics and intermodula-
tion products are dissonant and are not musically
related to the signal being amplified. They are heard
as great and disappointing discordance and distor-
tion.
In most acoustical events we may listen to sur-
prisingly low average power output but the peak
power requirements can be very high. Consider
these graphs of the power demanded of an amplifier
reproducing the piano, the pipe organ, and the bass
saxophone. The charts show that the peak power de-
mand is almost 1000 times (30 dB) the average power
demand. Since it is necessary that these short inter-
val power spikes be reproduced with low distortion,
it means the average power output of the power
amplifier must be limited to 1/1000th of its capability
or the listener must accept the discordant distortion
of clipping.
Amplifiers when driven to clipping are capable of
delivering up to twice the heat load to the
loudspeaker. In addition, they can have more than
40% harmonic distortion. The extra heat energy con-
tent of the clipped signal will damage most
speakers. Mclntosh leadership in engineering has
developed a new circuit that...(1) dynamically
prevents power amplifiers from being overdriven in-
to hard clipping. ..(2) which reduces the heat
developed in the loudspeakers. ..(3) assures that the
amplifier will produce its maximum output without
increased distortion. That circuit we call "POWER
GUARD."
20 100 1K 10K
HERTZ
-30
0
30
PIANO
PIPE ORGAN
30
0
-30
20 100 1K 10K
HERTZ
BASS SAXOPHONE
30
0
-30
20 100 1K 10K
HERTZ
3
THE MCINTOSH POWER GUARD
The Power Guard waveform comparison circuit
detects minute amounts of waveform difference be-
tween the output signal and the input signal. A
sampling of the program material at the output of
the amplifier is constantly compared with the pro-
gram material at the amplifier input. Should the dif-
ferences reach 1%, Power Guard goes to work.
THE MCINTOSH POWER METERS
Mclntosh developed output monitoring meters
add to your operating flexibility. Ordinary meters are
incapable of indicating the short interval informa-
tion in a sound wave. The mass of the meter move-
ment is too great to respond to the instantaneous
changes in music program material. That short inter-
val information can have a duration as brief as one-
half of one thousandth of a second. Even should the
meter be capable of the high velocity movement the
human eye could not perceive the information.
Oscillogram of output waveform with and without Power Guard.
Input overdriven for each trace 20 dB.
In only a fraction of a millisecond Power Guard
dynamically reduces input level to prevent amplifier
overload yet permits the amplifier to deliver its ab-
solute maximum power output without extra distor-
tion. In addition, the output of the "waveform com-
parator" activates the front panel NORMAL and
LIMIT indicators.
The Power Guard circuit provides a precise visual
indication when the amplifier has reached full power
output. Any time that the input circuit is fed ex-
cessive amounts of signal causing waveform dif-
ferences through the amplifier of 0.5%, the output
mode indicators change from green NORMAL to red
LIMIT automatically and instantaneously. This warn-
ing persists long enough for positive visual indica-
tion of clipping for a pulse that is so infrequent or
short that it would be impossible to be seen even on
an oscilloscope. The indicators will illuminate on
clipping for a pulse as short in time as 100 microsec-
onds. You are always assured that the power of your
amplifier is as clean and distortion free as it can be.
Mclntosh engineering pursued both problems
electrically by developing new electronic circuits
that cause the meters to respond to short interval in-
formation with an accuracy of 98%! To permit the
eye to see such high speed motion the electronic cir-
cuits that drive the meter pointer are time stretched
so the meter pointer position can register in the per-
sistence of vision characteristics of the human eye.
The meters indicate directly in watts, or can be
made to hold the highest reading and continuously
update on higher power or can be switched to be
peak reading peak locking decibel meters.
When used as a watt indicating meter all the infor-
mation is direct reading, without conversions or
complicated mathematics. In addition, as direct
reading meters they are calibrated in average watts
for a sine wave signal but respond to signal peaks.
The meters indicate direct power in watts. They
are calibrated in average watts for a sine wave
signal but respond to signal peaks. So, a 200
average watt indication also means 400 instan-
taneous peak watts. The meters are voltage ac-
tuated and indicate power accurately when the
amplifier is operated into rated output load im-
pedances.
Watts Hold, permits the meter to lock to and in-
dicate 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 a
lower peak or to its rest position at a decay rate of 10
dB per minute.
4
The meters have extremely fast rise time, about
500 microseconds for 90% accuracy. A tone burst of
500 microseconds is almost inaudible even at full
power.
The meters are protected from damage in the
event of overloading in the wrong meter range.
AUTOMATIC TEST SYSTEM
The Automatic Test System provides positive pro-
tection and extends the long trouble free life of an
amplifier. Each time the amplifier is turned on an
Automatic Test System measures and verifies ac-
curate performance at seven critical points in the
amplifier's circuits. The Automatic Test System
verification assures operational readiness before
operation starts and limits any damage should there
be component malfunction. Each time a test is
verified an LED number indicator shows which test
is being performed. An adjustable "beep" tone can
be heard with each test.
If in the testing an unacceptable voltage is en-
countered, the LED numeric designation locks to
isolate the faulty circuit. Faulty circuit identification
permits the service technician more efficient repair.
Without the Mclntosh Automatic Test System at-
tempts to locate a fault by the probing and testing
needed, will often create additional problems by put-
ting undue mechanical and electrical stress on the
circuit components. The Automatic Test System pro-
tects your investment.
THE MCINTOSH OUTPUT CIRCUIT DESIGN
To achieve long trouble free life in an amplifier it
is essential to have cool operation. Cool operation
results from the careful design of the output circuit,
matching of the output circuit to the loudspeakers
with an autotransformer and a mechanical design
that permits the use of generous sized heat sinks
providing adequate ventilation without the use of
fans.
The bipolar eptaxial output transistors and the
Mclntosh output circuit allows the amplifier to
operate as cool as possible. When there is limited
program demand on the amplifier only the optimum
number of output devices operate. When there is no
signal no output device is conducting. Conservative
Mclntosh engineering keeps operating temperatures
low assuring long life.
The interleaved multifilar wound Mclntosh design-
ed autotransformer transfers all the power you paid
for to all impedance taps. You are not power penaliz-
ed for operating at an output impedance of less than
8 ohms. The Mclntosh autotransformer does its out-
standing job without adding phase shift, limiting fre-
quency response or power output. The distortion
through the autoformer is 0.003% at 20 Hz and
unmeasurable at higher frequencies. In short, the
Mclntosh autotransformer is the ideal answer to a
difficult problem.
Heat sinks must be large and they must have ade-
quate ventilation for effective cooling. For example
the MC 2255 has 1100 square inches (7.64 square
feet) of radiating surface. In addition, the chassis
has been designed to permit the maximum amount
of air to flow over the heat sinks to conduct away the
life limiting heat.
Mclntosh amplifiers provide the correct connec-
tion impedance to drive numbers of speakers simul-
taneously. For instance the 1 ohm output will drive
eight 8 ohm speakers and deliver full power without
overheating.
MC 2155 shown in optional walnut veneer cabinet.
5
test reports
"Reprinted with permission from the June 1982 issue of STEREO REVIEW magazine.
Copyright © 1982 Ziff-Davis Publishing Company. All rights reserved."
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
T
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
in
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,
or
MONO PARALLEL
operation. Next
STEREO REVIEW
6
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
Output Transformers
in Transistor
Power Amplifiers
by Sidney Corderman*
Output transformers can make tran-
sistor power amplifiers more reliable,
more flexible, and more powerful. At
the same time output transformers
offer the best continuous protection
to loudspeakers against the hazards of
avalanche failure of output transistor
devices.
Time has shown that output trans-
formers make transistor amplifiers
operate cool and safe. The output
transformerless amplifier (OTL)
becomes less exciting when amplifiers
must give long, consistent and predict-
able operation.
Let's take a look at transformers in
general at their past and present
use in amplifiers - - - and at why
Mclntosh Laboratory continues to be
the leader in the amplifier field with
the use of transformers.
Remember Vacuum Tube Amplifiers?
Until the early 1960's, McIntosh
and just about everyone else in the
high fidelity component manufactur-
ing business produced vacuum tube
power amplifiers exclusively. The
familiar push-pull circuit of Fig. 1
reigned supreme. In that circuit we
had a pair of tetrode or pentrode tubes
with their high output impedance try-
ing to deliver power to low impedance
loudspeaker systems. A transformer
was needed to provide the necessary
impedance match between them. But
there were problems in trying to
achieve an optimum transfer of power
between tubes and speakers. Typically,
using a pair of 6L6 output tubes in
push pull, we had a tube load imped-
ance of 4000 ohms trying to deliver
Fig. 1 Typical push pull output circuit
(see story tor dashed line information)
power to, say, an 8 ohm speaker load.
The impedance ratio was 500 to 1,
and the necessary transformer had to
have a turns ratio of around 23 to 1
(turns ratio varies as the square root of
the impedance ratio). The required
turns ratio created problems at both
ends of the audio frequency spectrum.
Leakage inductance and shunt capaci-
tance (represented as dashed lines in
Fig. 1) caused high frequency roll-off.
The primary inductance of the trans-
former together with its inherent non-
linear characteristics placed limits on
low-frequency response. And the
energy stored in the unwanted leakage
inductance caused notch distortion, as
illustrated in Fig. 2.
Fig. 2 - Notch distortion in a typical Class B
output circuit
The McIntosh Unity Coupled Circuit
Long before the advent of tran-
sistorized power amplifiers, McIntosh
found an effective way to solve these
problems. We called it the Unity Cou-
pled Circuit. The basic configuration
is illustrated in the diagram of Fig. 3.
Fig. 3 — McIntosh "notch free" low
distortion Unity Coupled Circuit
*
Vice President of Research and Development, McIntosh Laboratory Inc.
8
The impedance ratio required between
primary and secondary has been re-
duced by a factor of four-to-one com-
pared with the conventional arrange-
ment. It is now 125 to 1 (1000/8).
The turns ratio is therefore only half
of what it was before. Leakage induc-
tance is therefore much lower, and so
is the shunt capacitance across the
windings. The use of a bifilar winding
technique completely eliminates the
leakage inductance problem of cou-
pling between the sections of the pri-
mary windings. It was the develop-
ment of the Unity Coupled Circuit
by McIntosh (the circuit is patented)
way back in 1947 that enabled us to
produce amplifiers which were a whole
order of magnitude lower in distor-
tion than the competition of those
days. Typically, we were able to pro-
duce power output circuits with total
harmonic distortion of under 1.0%
even before the distortion-reducing
negative feedback loop was added.
With just 20 dB of feedback applied,
the THD was further reduced to under
0.1%!
What About Transistor Amplifiers
The audio industry welcomed the
power output transistor as the solution
to all its problems. After a few falter-
ing starts (early germanium power
output transistors were notoriously
unreliable and easily destroyed by high
operating temperatures), silicon power
transistors became the standard power
device in power amplifiers.
Since power output transistors ex-
hibit a low output impedance, it was
possible to design output circuits to
match 8-ohm loads directly—without
the need for a matching audio output
transformer. Indeed, most OTL ampli-
fiers, when coupled to 8-ohm resistive
loads for test purposes, can deliver full
rated power to those loads for long
periods of time without overheating or
exceeding safe thermal dissipation
limits. The trouble is that we don't
listen to resistors we listen to loud-
speakers. It will come as no surprise to
you to learn that speakers having a
"nominal" impedance of 8 ohms often
measure lower and higher impedance
values at different audio frequencies.
Then, too, consider the fact that many
popular speaker systems have nominal
impedances of 4 ohms, and the imped-
ance of 4 ohm speakers can easily dip
down to as low as 2 ohms at certain
frequencies. What happens to an OTL
amplifier with such low impedances
connected to it? In theory, if an out-
put stage is designed to match an 8
ohm impedance, its power "output
capability should double when it's
connected to a 4 ohm impedance. But
as this mismatch occurs, thermal dis-
sipation increases rapidly. In fact,
operating into a 4 ohm load, heat dis-
sipation is double what it would be
when operating at 8 ohms, as illustrat-
ed in Fig. 4. Unfortunately, if the
amplifier was designed for 8-ohm oper-
ation, its thermal dissipation limits
were designed with some safety factor
Heat dissipation
capacity required to
meet FTC rating
at 8 ohms.
Load impedance in ohms (1000%=Heat
produced at rated output into 8 ohms.)
Fig. 4 - Heat produced by transformerless
amplifier at various load impedance
for 8 ohm operation, so as to meet the
new FTC preconditioning require-
ments. These call for the amplifier to
be able to deliver one-third rated
power at rated impedance for one
hour. But, as you can see from Fig. 4,
the safety margin is not nearly great
enough to permit operation at 4-ohms
—or 2-ohms-or 1-ohm impedances.
Remember, too, that many amplifiers
and receivers have provisions for con-
nection of more than one pair of
speakers for use in different listening
rooms, so that even if 8-ohm speakers
are selected, using double pairs of
them results in a 4-ohm net nominal
impedance even before allowing for
downward variations in impedance
at specific frequencies in the audio
spectrum. So, unless manufacturers are
willing to resort to disproportionately
massive heat sinks, cooling fans or
combinations of both, designing power
amplifiers that can deliver their maxi-
mum powers at both 8 ohms and im-
pedances of 4 ohms and lower be-
comes physically impractical in the
case of the OTL amplifiers.
Fig. 6 - Performance of MC 2300
The Answer-Output Transformers!
If a transistorized amplifier were
equipped with an output transformer,
you could move up or down in load
impedance and maintain full power
ratings without over-dissipating any-
thing, since the amplifier's output
stages would always be working into
an ideal load.
To many hi-fi enthusiasts, output
transformers tend to create visions of
compromised design. That is just not
the case today. Technology in mater-
ials and transformer design methods
have advanced significantly in recent
years and, remember, we're dealing
with low impedance devices-not tubes.
It's no longer necessary to translate
impedances from a "plate circuit" to a
speaker-a step down of several hun-
(* - - - Continuous operation not possible
due to overheating. Protection circuit is as-
sumed to current limit when load falls be-
low 4 ohms, in actuality the output into 4
ohms and lower impedances will fall below
the values shown.)
Fig. 7 - Performance of non McIntosh
transformerless amplifier rated for
300 watts in 8 ohm load
9
Fig. 5 - McIntosh MC 2205 output stage and transformer
dred to one. With transistor output
stages, a ratio of only about 4 to 1 is
required. In tube amplifiers, extremely
good balance in the push-pull primary
was required if notch distortion was to
be avoided. Now, using a single ended
push-pull transistor output stage the
transformer can be driven in a single
ended fashion. One end of the winding
Is returned to ground potential. With
the transformer at ground, no isolation
is required between the input and out-
put and therefore a simple auto-
transformer can be used.
Fig. 5 shows a typical arrange-
ment used in our new MC 2205 ampli-
fier. The output stages are designed to
work optimally into a load impedance
of 2.1 ohms and it becomes a simple
matter to "tap into" the auto-trans-
former for that precise impedance
match. Taps for 1 ohm, 2 ohm, 4 ohm
and 8 ohm operation are arranged so
that the output transistors continue to
work into their optimum impedance.
The result: full power output at any of
these impedances, with no possibility
of thermal over-dissipation.
Our popular MC 2300 amplifier
also uses an auto-transformer and
Fig. 6 shows how that amplifier is able
to deliver its full rated power (300
watts RMS per channel) into any
impedance from 0.5 ohms to 16 ohms,
as well as to 25 volt and 70 volt multi-
speaker system taps on the transform-
er. If we compare these results with
those obtained with a similarly rated
Fig. 8A - Typical of phase shift in
McIntosh auto-transformer at 8 ohms
OTL amplifier (Fig. 7) we see that at
all but 8 and 16 ohms, continuous
operation at theoretical maximum
power is impossible because of over-
heating and protection circuit limit-
ing. Operation at 16 ohms, though
possible, is limited to a maximum
power output of 150 watts, in this
case, while operation into a 70-volt
line is impossible because of limita-
tions in power supply voltages.
What About Phase Shift?
Critics opposed to the use of trans-
formers in output circuits of audio
amplifiers arc quick to point out that
"transformers introduce phase shift"
at the low and high frequency ex-
tremes. As a matter of fact, a properly
designed transformer (and we'll get
into some of the factors that are in-
volved in designing McIntosh output
transformers in a moment) can intro-
duce about 3 degrees of phase shift at
20 kHz (Fig. 8A), which is certainly
insignificant. The typical volume con-
trol used on amplifiers (both those
that are OTL and those equipped with
transformers) introduce more shift
than that about 20 degrees in fact
(Fig. 8B). Since an output transformer
is driven from an extremely low im-
pedance, there is actually more low-
frequency phase shift caused by the
usual input coupling capacitor at these
low frequencies than by the trans-
former.
So, why haven't more manufactur-
ers used output transformers on solid-
state amplifiers? Possibly they are not
aware of the technology, but more
likely they don't want to spend the
extra cost. A good transformer is an
expensive component. It is heavy,
takes up a fair amount of space and
contradicts the audiophile's notion
that transistorized equipment must be
small and lightweight. Be that as it
may, the FTC regulations suggest that
output transformers are the only logi-
cal solution to rating audio amplifiers
honestly at 4, 8, 16 or any other
impedance required.
1 watt, 8 ohms
Volume Control Clockwise
Volume Control 12 o'clock
Fig. 8B - Typical phase shift
in a complete Mclntosh amplifier
Not Just Any Transformer!
At Mclntosh, we wind all our own
output auto-transformers. Of course,
we could purchase them from any one
of a number of transformer companies
who do nothing but wind transformers
(our power transformers are, in fact,
purchased from other suppliers), but
we have long since found that trans-
formers can't always be made success-
fully "according to the book". A great
deal of experimentation is required be-
fore a new design of a transformer can
10
be mated to a specific amplifier cir-
cuit. We went through dozens of de-
velopmental samples in the case of our
new MC 2205 amplifier. What we
ended up with it shown schematically
in the diagram of Fig. 9. The trans-
former is trifilar wound to provide
coupling between sections. It takes 23
individual windings to make this out-
put transformer. There are five differ-
ent winding sections, all five of which
are connected in parallel. We use
grain oriented silicon steel core lamin-
ations because that kind of core means
less iron-and less iron in turn means
tighter coupling. It also means lower
winding resistance for a given size
wire. The grain oriented silicon steel
means that it has a higher magnetic
saturation point-about 17 kilogauss
versus 12 to 13 kilogausses for the
non-oriented variety. There is there-
fore less core loss, or, to put it simply,
we end up with a more efficient trans-
former-one which couples more of
the available amplifier power to the
speaker loads. To further improve cou-
pling, we don't use any interlayer in-
sulating paper in a power transformer
that might pose a breakdown problem.
But since our polyurethane insulated
wire is rated at 4000 volts per mil (and
since the highest voltage we're talking
about for an audio transformer is
about 56 volts), this really is no prob-
lem at all. All of our output trans-
formers arc potted with material
which has especially high thermal
conductivity. Besides helping to keep
operating temperatures within the
transformer down, this compound re-
duces lamination buzz to inaudible
levels. We figure you'd rather listen to
your speakers than to our transformers!
Our Transformers Are Only Part
of The Story
Whether an amplifier uses an out-
put transformer or not, its output de-
vices must be designed to work into an
optimum load so that maximum cur-
rent delivered by the output transis-
tors never exceeds the safe operating
area specified for the transistor. Fig.
11 shows current versus voltage limita-
tions for the epibase type of output
transistor used in our MC 2205 ampli-
fier. If all amplifier loads were purely
resistive, staying within the safe oper-
ating area would be relatively simple,
but the fact is that speakers often pre-
Fig. 9 - MC 2205 output
auto-transformer schematic diagram
Output volts
Fig. 10-Load and limiting data of
the McIntosh MC 2205 measured at 8 ohm output
11
provided for reactive loads.
To Sum It All Up
The points we've tried to make are
relatively few, but they spell the differ-
ence between a McIntosh output-
transformer equipped amplifier and
every other kind of amplifier around.
1. A transformer equipped ampli-
fier will deliver rated power at any im-
pedance for which a transformer tap
is provided.
2. An OTL amplifier designed for
8-ohm operation cannot operate safely
(according to the FTC rules) when
driving lower impedances (4 ohms,
2 ohms, etc.), yet such loads common-
ly occur either because of speaker im-
pedance variations with frequency or
because of paralleling of multiple
speaker systems across one channel
of an amplifier.
3. The new FTC power rule regard-
ing audio amplifiers has forced many
manufacturers to omit 4-ohm ratings -
even though 4-ohm speakers arc in
common use. McIntosh transformer-
equipped audio
amplifiers
deliver
full
power at any impedance for which a
transformer output tap is provided.
4. Because of their design, McIntosh
transformers introduce less series leak-
age inductance than is commonly en-
countered with OTL amplifiers which
require a series inductance between
the output circuit and the speaker con-
nection for amplifier stability. At the
8 ohm tap of our MC 2205, leakage in-
ductance is a low 3.5 microhenries.
This represents an impedance of only
2.2 ohms at a frequency of 100 kHz.
5. Properly designed output trans-
formers impose no limitations on fre-
quency response. At the 8-ohm tap of
the MC 2205, response is down 0.3 dB
at 50 kHz. With a 4-ohm toad con-
nected, response is down 0.1 dB at
50
kHz.
6. Phase response of the MC 2205
amplifier, using its specially designed
output transformer, is accurate to
within 9 degrees at the 8 ohm tap at
a frequency of 50 kHz and undergoes
zero degrees of phase shift at 20 Hz.
At the 4-ohm tap, phase shift at 50 kHz
is only 7.2 degrees.
Next time anyone gets into an argu-
ment with you concerning the attri-
butes of an OTL amplifier versus a
McIntosh transformer-equipped ampli-
fier, you might let your adversary read
this story.
Output volts
Fig. 12 Load and limiting data of
a non McIntosh high-powered transformerless amplifier measured at 8 ohms output.
12
fiers. In Fig. 10 we have combined the
safe operating diagram of Fig. 10 with
load and limiting characteristics at the
8-ohm tap of our MC 2205. As you
can see, even when the load is totally
reactive, every possible voltage and
current condition falls within the
safe operating area of the output de-
vices used. Compare this diagram with
Fig. 12 derived from data concerning
the output transistors used in a cur-
rently available high-powered amplifier.
Note that inadequate protection is
Fig. 11 - Current versus voltage
limitation epibase type
output transistor
Collector volts
You are Mclntosh protected
six ways with the new
Mclntosh amplifiers.
PROTECTION
1. The patented Mclntosh Sentry Monitoring circuit
constantly monitors the output signal. At signal
levels up to rated output this circuit has high im-
pedance 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
independently.
PROTECTION
2. Should the temperature of the heat sinks rise
above normal through restricted ventilation or other
causes, the AC Power is disconnected by an
automatic heat sensing relay. The AC power will be
restored when the temperature returns to normal.
PROTECTION
3. Any loudspeaker damaging DC component in the
output circuit, from whatever cause, is shunted to
ground through the Mclntosh autotransformer. You
and your speakers are protected completely from
this kind of amplifier failure.
PROTECTION
4. Mclntosh gives you a money back guarantee of
performance. Your Mclntosh instrument must be
capable of meeting its published performance limits
or you get your money back. No other manufacturer
offers you this money back guarantee of perfor-
mance.
PROTECTION
5. The Mclntosh 3 Year Service Contract protects
you from the cost of repair for three full years
because Mclntosh will provide the service materials
and labor needed to return the measured perfor-
mance to the original performance limits. The SER-
VICE CONTRACT does not cover any shipping costs
to and from the authorized service agency or the fac-
tory.
PROTECTION
6. The Automatic Test System provides positive pro-
tection and extends the long trouble free life of an
amplifier. Each time the amplifier is turned on seven
tests measure and verify accurate performance.
Automatic Test System protects by verifying circuit
readiness before operation starts. Each time a test
is verified a numeric indicator turns on to indicate
the test being performed. If in the test countdown an
unacceptable voltage is encountered, the numeric
designation locks to isolate the faulty circuit.
13
PERFORMANCE GUARANTEED
Performance limits are the maximum
deviation from perfection permitted for a
Mclntosh instrument. We promise you that
when you purchase a new Mclntosh pro-
duct from a Franchised Mclntosh dealer it
will be capable of performance at or ex-
ceeding these limits or you can return the
unit and get your money back. Mclntosh is
the only manufacturer that makes this
statement.
Mclntosh audio power ratings are stated
in accordance with the Federal Trade
Commission Regulation of November 4,
1974 concerning power output claims for
amplifiers used in home entertainment
products.
14
POWER AMPLI
MC 2500
POWER OUTPUT STEREO
Minimum Watts,
Both Channels Operating
POWER OUTPUT MONO
Minimum Watts
POWER BAND WIDTH
TOTAL HARMONIC DISTORTION
Maximum, 250 mW to Rated
Power, 20 Hz to 20 kHz
OUTPUT LOAD IMPEDANCE
Stereo
Mono
INTERMODULATION DISTORTION
Maximum, 250 mW to Rated Power
FREQUENCY RESPONSE
20 Hz to 20 kHz
(at 1 Watt)
NOISE AND HUM
Below Rated Output
OUTPUT VOLTAGES FOR
DISTRIBUTION SYSTEMS
DAMPING FACTOR
INPUT IMPEDANCE
INPUT SENSITIVITY
POWER REQUIREMENT
AC Line Voltage - Frequency
Watts at No Signal
Watts at Rated Output
SEMICONDUCTOR COMPLEMENT
Transistors
Diodes
Integrated Circuits
SIZE
Panel Height
Panel Width
Depth
FINISH
WEIGHT
Net
In Carton
HEAT SINK AREA
SPECIAL FEATURES
Power Guard
Output Autoformers
Automatic Test System
Output Meters: Calibrated in Watts
Output Meters: Calibrated in dB
Thermal Turn-Off
Sentry Monitor
D.C. Speaker Protection
Output Limit Indicator
Panloc Mounting
Cooling: Convection
500 Watts Per
Channel
1000 Watts
20 Hz to 20 kHz
0.02%
1, 2, 4, 8 W
1/2,
1, 2, 4, 8, 16 W
0.02%
+ 0.
-0.25
dB
-95 dB
25 Volts
greater than 30
50.000 W
0.75 or 2.5 Volts
120V, 50-60 Hz
75 Watts
1800 Watts
91
35
6
10-1/2" (26.7cm)
19" (48.3cm)
17" (43.2cm)
Gold and Black
Panel, Black
Knobs
129#(58.5kg}
144#(65.3kg)
1990
sq. in.
X
N/A
X
X
X
X
X
X
N/A
N/A
FIER PERFORMANCE LIMITS
MC 2255
MC 2250
MC
2155
MC
2120
MC 502
250 Watts Per
Channel
500 Watts
20 Hz to 20 kHz
0.02%
1, 2, 4, 8 W
1/2,
1, 2, 4, 8, 16 W
0.02%
+ 0,
-0.25
dB
-95 dB
25 Volts
greater than 30
50,000 W
0.75 or 2.5 Volts
120V, 50-60 Hz
70 Watts
1440 Watts
85
47
14
7-1/8" (18.1cm)
16-3/16" (41.1cm)
14-1/2" (36.8cm)
Black Glass Panel,
Gold/Teal Nomen-
clature, Gold and
Black Knobs
82# (37.2kg)
96# (43.5kg)
1080
sq. in.
X
X
X
X
X
X
X
X
X
X
X
250 Watts Per
Channel
500 Watts
20 Hz to 20 kHz
0.02%
1, 2, 4, 8 W
1/2,
1, 2, 4, 8, 16 W
0.02%
+ 0,
-0.25
dB
-95 dB
25 Volts
greater than 30
50,000 W
0.75 or 2.5 Volts
120V, 50-60 Hz
84 Watts
1440 Watts
76
37
9
6-31/32" (17.7cm)
16" (40.6cm)
14-1/2" (36.8cm)
Gold Panel
Gold and Black
Knobs
80# (36.3kg)
94# (42.6kg)
1080
sq. in.
X
X
X
N/A
N/A
X
X
X
X
N/A
X
150 Watts Per
Channel
300 Watts
20 Hz to 20 kHz
0.02%
1, 2, 4, 8 W
1/2,
1, 2, 4, 8, 16 W
0.02%
+ 0,
-0.25
dB
-95 dB
25 Volts
greater than 30
50,000 Si
0.75 or 2.5 Volts
120V, 50-60 Hz
84 Watts
720 Watts
81
47
14
5-7/16" (13.8cm)
16" (40.6cm)
14-1/2" (36.8cm)
Black Glass Panel,
Gold/Teal Nomen-
clature, Gold and
Black Knobs
65# (29.5kg)
77# (35kg)
772 sq. in.
X
X
X
X
X
X
X
X
X
X
X
120 Watts Per
Channel
240 Watts
20 Hz to 20 kHz
0.1%
2, 4, 8, 16 W
1, 2, 4, 8 W
0.1%
+ 0,
-0.25
dB
-95 dB
25 Volts
14 to 50
100,000 W
0.75 or 2.5 Volts
120V, 50-60 Hz
50 Watts
460 Watts
39
24
2
5-7/32" (13.3cm)
16" (40.6cm)
14-1/2" (36.8cm)
Gold Panel,
Gold and Black
Knobs
57# (26kg)
70# (32kg)
772 sq. in.
X
X
N/A
N/A
N/A
X
X
X
X
N/A
X
75 W/Chan. 2.7 to 4 W
50W/Chan. 8 W
150 Watts
into 8 Ohms
20 Hz to 20 kHz
0.02%
2.7 to 8 W
8 W
0.02%
+ 0,
-0.25
dB
-95 dB
25 Volts (Stereo Only)
>50
75,000 W
0.75 or 2.5 Volts
120, 50-60 Hz
20 Watts
400 Watts
39
20
4
3-5/8" (9.2cm)
16" (40.6cm)
14-1/2" (36.8cm)
Black Glass Panel,
Gold/Teal Nomen-
clature, Gold and
Black Knobs
27# (12kg)
38#(17kg)
513 sq. in.
X
N/A
N/A
N/A
N/A
X
X
X
X
X
X
" 5
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