Mcintosh ma6100 schematic

AMBIENT SOUND

By Len Feldman

A postscript to MR's product-testing philo­sophy or "The Great McIntosh Controversy."

50

LAB REPORT

By Norman Eisenberg
and Len Feldman

McIntosh MA-6100 Preamp-Amplifier

Reprinted with the kind permission of

MODERN RECORDING

52

15 Columbus Circle New York, N.Y. 10023 [phone]

YOU SHOULD BUY

MclNTOSH BECAUSE

• McIntosh instruments are designed and manufactured for long life.

• McIntosh instruments have always been designed for long life with
low maintenance costs and high quality performance. McIntosh in­struments have been and are the LABORATORY STANDARD for
the world. Until 1949 the performance requirements for a McIntosh
had long been an engineering dream. They became a reality with the
introduction of the first McIntosh amplifier. Through all these years
McIntosh has produced instruments that have exceptionally long life.

Regardless of age, most McIntosh instruments are still in use today.
Amplifier clinics held all over North America have shown that most
of the McIntosh instruments ever manufactured are still in service
today and still meet or exceed original exacting requirements for
performance that were required of them when new.

• Used McIntosh instruments enjoy the highest resale value in this
field. Retailers report that customers are constantly searching for
used McIntosh instruments. A McIntosh does not remain on the
"Used" display long. You'll get more when you trade-in your McIn-

tosh assuring you of maximum return on your investment.

• McIntosh dedication, not only to improvements, but also to funda­mentals, has justified many patents on refinements as well as basic

circuit structures.

• The size of the McIntosh investment in professional testing instru-

ments bears eloquent testimony to the commitment to acquire
knowledge for the benefit of our customers. On a percentage basis
McIntosh probably invests more of its sales dollars in testing facilities
than anyone else in a like business. As new testing instruments are
produced that update the McIntosh ability to know, McIntosh invests
in them . . . wave form analyzers-real time analyzers-low distortion
signal generators, etc. Only through this impressive investment,
through continuous testing and research, through product analysis,
and endless measurement can McIntosh promise and deliver to you
reliability, long life, performance, highest value, and freedom from
service.

• Since its beginning in 1949, McIntosh Laboratory has had the same
ownership and management. McIntosh is the only company in this
industry that has had this kind of history. McIntosh has been and is
still dedicated to the best possible values.

• Doesn't it make good sense to deal with a company that wants to do
as much for you as it possibly can?

BY LEN FELDMAN

Postscript to Our Product

Testing Philosophy ...

In the very first issue of Modern Recording, I went
into some detail regarding MR's approach to product
reviews and test reports. I discussed the importance of
lab measurements versus listening tests, and the for­mat that Norman Eisenberg and I would use in trying
to fairly report the merits (or demerits) of a variety of
audio products. I did not detail how we would "sam­ple" products for testing. A recent series of episodes
took place in our labs which points up some of the
problems and conflicts inherent in attempting to test
electronic equipment, and I'd like to share the exper­ience with you.

Basically, we obtain equipment in one of two ways.
Either we ask a manufacturer to send us a representa­tive sample of the unit to be tested, or we obtain one on
temporary loan from a dealer. In rare cases, if we can­not obtain a sample by either of these approaches we
will go out and purchase the required unit in hopes
that we might sell it at a minimum loss after our tests
are completed. Personally, I prefer getting a unit di­rectly from the manufacturer, and the story I'm about
to tell will show why.

As many readers may know, the Mclntosh Company
has, for many years, refused to "lend" equipment to
publications for review purposes. That is one of the
main reasons why MR, believing that the consumer
should have the right to know, wanted to test a piece
of Mac equipment. Mac's reasons are many - some of
which I cannot argue with, others of which I feel are
not justified. For one thing, the people at McIntosh
feel that if a manufacturer supplies equipment directly
to a magazine for review, the manufacturer has an op­portunity to "tweak up" the sample (or even possibly
modify it or improve it) for better results. On this
point, I can only speak from experience dating back
some six years, during which time I have probably
tested several hundred components sent to me directly
from manufacturers. I cannot remember a single in­stance in which a manufacturer "phonied up" a circuit
so that the unit would yield better measurements. Of
course, I cannot deny that many manufacturers prob­ably "double-check" such samples before sending
them to me, but, frankly, I see nothing wrong with a
manufacturer making doubly sure that a unit he sends

for review performs in accordance with specs and is
not the occasional "lemon" that any manufacturer of
complex electonic gear is liable to turn out once in a
while. In fact, I wish more manufacturers would dou­ble-check the units they send for evaluation. If 1 re­ceive a faulty unit from a manufacturer (and, indeed, I
have in the past received several defective pieces of
equipment) that I know to be defective (not poorly de­signed - just defective), I don't have the same war­ranty privilege of having it fixed up by a local service
agency. I will have wasted the better part of a day (it's
usually the last measurement that shows up the fault-

- hardly ever the first; a variant of Murphy's law, you
know) only to have to start all over again with a re­quest for a second unit from the manufacturer. I see no
point in printing a report based upon a defective unit
that does not represent what the model can do!

McIntosh's other argument against supplying
equipment to "reviewers" for test purposes is that
they have no way of knowing whether the particular
tester or reviewer has the necessary test equipment or
knowledge with which to properly evaluate that equip­ment. I certainly can't argue with that one-though I
believe that a simple bit of investigation on the part of
McIntosh (or any other company that may feel the
same way) could insure that samples are sent only to

qualified reviewers possessing reasonably up-to-date
test equipment.

Be all that as it may, MR decided that we wanted to

do a report on a piece of Mclntosh equipment for this
issue. Knowing that McIntosh would not supply
one, MR decided to borrow a unit from one of their

authorized dealers for that purpose. We did just that,
and that's how we obtained our first MA-6100 Inte­grated Amplifier for testing. We were all prepared for
an exciting "first" at our lab. Preamp section tests
went smoothly enough, with all published specs met or
exceeded by far. Construction was excellent, as the
famed McIntosh reputation had led us to believe.
Then, we came to the power output measurements
and-sure enough-we realized at once that the unit

was defective. Output sinewaves were being clipped se-

verely on one side only before rated power was at­tained. What to do? We couldn't go back to the manu­facturer-they hadn't supplied the unit in the first

place. We asked the dealer if this were, indeed, a unit

50

from a factory-sealed carton and were told, at first,
that it was. (It had not been sent to us in a McIntosh
carton which, at once, raised our suspicions.)

I agonized over this situation for some time, along
with the editors and publisher, and I finally decided to
confer with the manufacturer as to what might be the
trouble. A bit of telephone diagnosis led both McIn­tosh and myself to the conclusion that something
must have happened in one "side" of the power supply
and that, indeed, the unit was not functioning cor­rectly. The very next day, a representative of
McIntosh was in my laboratory with a second unit for
comparison. We did a little trouble-shooting and found
that one diode of a four-diode rectifier bridge circuit
was open and that the required negative 41 volts DC
supply was therefore running at about 37.5 volts (caus­ing the one-sided clipping). McIntosh volunteered to
take the defective unit back up to their plant, repair it
and ship it back to the dealer. Since I was responsible
for the unit, I felt I had to call the dealer once more to
tell him what we had found. It was only then that the

dealer admitted that this particular unit had been sent
to a local authorized McIntosh service station earlier
for a power supply repair. Seems a diode had shorted
and the service agency had replaced the diode and sent
it back to the dealer. What the service outfit did not
discover was that when the first diode had shorted, it
had "opened" a second diode. No listening test in the
world could have disclosed the fact that the "repaired"
unit still had another open diode, for the difference be­tween 64.5 watts and rated 70 watts is just over one

third of one dB (0.355 dB to be exact). Nor would a user

have been able to tell that the unit had not been fully
repaired, for at all but maximum power output, all
other specifications were met and exceeded.

I relate this story in full for several reasons. First, to
illustrate the dangers of testing "sample" units whose
origins are not known. How much better it would have
been for all of us (readers, manufacturer and your
obedient product reviewer) if we could have started

with a "known entity," produced and tested under
controlled conditions. Secondly, what would it have
proven to readers if we had simply "put on the
blinders" and reported that the McIntosh MA-6100
doesn't make rated power output? All but that "odd­ball" or improperly repaired unit of the three we
tested, did meet specs-and then some.

All of which brings us back to the tabulations which

you will find in our test report of the McIntosh MA-

6100. You will note that we ended up reporting on two
units. The first is the unit which the people from Mac
brought to my lab when the problem with the bor­rowed unit arose. We could have stopped there, but we
felt that if we did so, without telling the full story here,
we would be doing our readers (and McIntosh) an in-

justice. On the other hand, if we simply told what hap-

pened, and then reported that the second unit more
than lived up to expectations, we would be doing
exactly what McIntosh feels is improper in equipment
reporting. The only solution, therefore, was to find
another dealer who would supply us with yet a third
unit, and that is exactly what we did. A comparison of
the results obtained from the second (Mac-supplied)
and third (off the dealer's shelf, this time in a sealed
carton) proves, we think, what I said earlier. It is ob­vious that McIntosh didn't have to do anything spe­cial to the unit that they hastily brought down to my

lab. The third unit delivered virtually the same power

as the unit brought down by McIntosh personnel.

Perhaps now, McIntosh-and any other companies
who are reluctant to have their products reviewed by
audio-involved publications-will take another look at
the whole problem of test reports. Perhaps readers will
be less prone to "climb on our backs" for testing prod­ucts supplied by manufacturers. And perhaps, as a re­sult of this unnerving experience (the episode ran for
three days, and the phone bills are yet to be received),
the lives of magazine reviewers (myself included) will
be made just a little easier in the future. 9

51

NORMAN EISENBERG AND LEN FELDMAN

McIntosh MA-6100

Preamp-Amplifier

General Description: The McIntosh MA-6100 is a

stereo preamp/power amp combined on one chassis, or,
as custom terms such a unit, an "integrated ampli­fier." Its features and controls are intended not only to
appeal to the serious audiophile but to the advanced
hobbyist too, including the owner more than casually
interested in tape-recording.

It is possible to connect, all at once, to the MA-6100
two turntables, two tape recorders, two tuners, and
any auxiliary (line-level) source. In addition to the nor­mally expected inputs for tape (line-level) there is a
stereo input for connection directly from a tape head

-a feature once offered on de luxe preamps but not to
be found on units of recent vintage. Provision also is
made for "interrupting the circuit" between the pre­amp and power amp section via stereo preamp-out and

power amp-in jacks, which permits the insertion of

special accessory units such as speaker equalizers or

other signal-shaping devices. It also permits the use of
either section by itself for special applications.

Two sets of stereo speakers (nominally designated as
"main" and "remote") may be hooked up and turned
off or on from the front panel, where there also is a
headphone jack. The headphone signal is not affected
by the speaker selector switches. An unusual and use­ful feature in connection with the speaker options is a
switch that selects the load impedance depending on

what combination of speakers is being used-the
switch is found at the rear and has two positions, one

for 8 to 16 ohms and the other for 4 ohms. There are
four AC convenience outlets for powering other equip­ment; of these one is always "hot" and three are con­trolled by the unit's main power switch.

Nothing seems to have been overlooked in the way of

front-panel controls. Bass and treble controls are dual­concentric types that permit separate adjustment on
each channel. The channel balance control shares a
dual-concentric arrangement with the loudness
compensation control which itself also has a

"presence" position. Power off/on is combined with the
master volume control.

The input selector has six positions to correspond to
the input sources connected at the rear. The mode
selector offers an unusually broad range of possi­bilities: left input to left and right outputs; right input
to left and right outputs; reverse stereo; normal stereo;
mono (left plus right); left plus right to left output
only; left plus right to right output only. While the
average home listener probably will find little or no
need for such versatility, the advanced hobbyist or
semi-pro will appreciate the options they represent for
special purposes, not to mention their serving as a
built-in trouble-shooting tool to help isolate problems
to source, amp or speakers.

In addition to these knob-controls is a series of push­buttons. Two control the tape-monitor function for the
two recorders that may be connected at the rear. Two
more facilitate tape-copying in an easy and ingenious

52

way: the Tl-to-T2 button connects the output jacks
from tape recorder no. 1 to the input jacks of tape
recorder no. 2 without affecting the program being
monitored over the speakers. In this position, a copy of
the program being played on tape recorder no. 1 can be
made on tape recorder no. 2. To monitor this copy, you
simply press the Tape 1 monitor switch.

The T2-to-Tl button does the same thing the other
way round, permitting a copy of the material on tape
recorder no. 2 to be made directly on tape recorder no.

1. This time, to monitor the copy, you would press the

Tape 2 monitor button.

In addition, there are two more buttons for low and

high frequency filters, and yet another for phase rever­sal-this last is another feature once found on many
control panels and for some years not offered on most
amplifiers or preamps. Needless to say, the phase
switch not only is useful in initially setting up stereo
speakers but it can provide a check on the proper phas­ing of stereo program material. It also comes in handy

when adding a rear-channel amplifier and speakers to
an existing stereo set-up to convert to quadraphonic
sound. Proper phasing of all four speakers is germane
to the full quadraphonic effect generally; in matrix de­coding, it is especially important that all four channels
remain in phase.

The speaker selector buttons and the stereo head­phone jack complete the picture at the front. The panel
itself is neat enough to fit into any home decor and is
"business-like" enough to fit into the more workaday

environment of a studio. Somehow, "Mac" stylists
have managed to design a look that may well appeal to
both types of user, and this opinion dovetails with the
unit's general feel as a product.

was on the dealer's shelf, we must conclude it was a
freak accident (and not a serious one at that since the
fraction of a decibel less output on one channel prob­ably would never have bothered anyone just listening
to the amplifier), but one which McIntosh, in any
event, has been apprised of and which they presum­ably will guard against happening in the future.

As to the new, normally functioning samples, both
produced better than 85 watts per channel (McIntosh
claims 70). Power bandwidth extended from 14 Hz to
35,500 Hz in one, from 13 Hz to 35,000 Hz in the other
(McIntosh claims 20 to 20,000 Hz). For McIntosh's
claim of 0.2% harmonic distortion, we measured in
either sample the extremely low level of 0.03% har­monic distortion.

And that's the way it went through all our tests of
both the power amp section and the preamp section
which, by the way, has very accurate RIAA equaliza­tion and excellent overload signal characteristics com­bined with a very low noise level-exactly what you
look for in a preamp. Control and filter characteristics
all were fine and well-suited to their intended use; sig­nal sensitivities and output levels were well within the
normal range for easy mating with other equipment of
contemporary design. All controls and switches per­formed with professional smoothness to lend the unit a

Test

Results:

and because this company offers a money-back guar­antee that its products will meet or exceed published
performance specs, MR raised its critical eyebrow
when the first sample of this amplifier, which had been
obtained off a dealer's shelf, fell somewhat short of
making 70 watts in one output channel. The details of
this are explained elsewhere in this issue (see L.F.'s
"Ambient Sound" column on p. 50), but briefly, inves­tigation revealed that the unit we had started to test

was a used model that had been less-than-expertly re­paired prior to our getting it for testing. We ac­cordingly discontinued tests of this unit and got hold
of two new units, both "factory-carton fresh," and the
test results reported here are based on those two units.

To state the story briefly, the second and third sam­ples did indeed exceed published specifications, went
through our test swimmingly, and most assuredly
document that the MA-6100 is a superb amplifier in

every respect. As for the less-than-perfect sample that

Because

of

McIntosh's reputation,

53

McIntosh MA-6100: Top view.

"quality feel" which was substantiated by our lab
tests and further borne out when we opened the beast
and studied the extra-careful layout of quality parts
that make up its innards and circuitry. All told, MR's
view of the MA-6100 is that it is a superb integrated

amplifier that should interest a wide range of users.

General Info: Front panel is 16 inches by 57/

inches. The chassis itself is 15 inches wide and 13

inches deep (including Panloc buttons for optional

panel mounting) and connectors at the rear apron.

Front-panel knobs require clearance of 1½ inches hi
front of mounting panel. Unit weighs 34 pounds net
(46 pounds in shipping carton). Owner's manual is
excellent, with detailed explanations and good illus­trations. Price: $699.

Individual

Comment

by

L.F.:

The

full

story

of

my adventure with the MA-6100 preamp-amplifier will

be found in my "Ambient Sound" column on page 50.

In terms of panel controls, the Mac unit gives a user the
kind of input and output flexibility which has long
since been abandoned by other manufacturers.
Consider, for example, the flexible mode switch which
lets you feed just about any mono or stereo combi­nation of channels to the pair of outputs. Or the
cleverly incorporated compensation control (tandemly
mounted to the balance control to maintain an unclut­tered look) which incorporates both a loudness
compensation switch and a fixed mid-frequency

presence emphasis that is neither too exaggerated nor

insufficiently effective. It's been a long while since we
saw a "tape head" input on an integrated amplifier (or

on a preamplifier, for that matter) and we doubt
whether even a small percentage of purchasers would
ever have a need for this low-level input, but the flex­ible tape copy arrangement and the dual tape monitor
circuits will find favor with serious tape recordists.

Action of the bass, treble and loudness controls,
shown in the accompanying 'scope photos recorded
directly using a slow-sweeping audio generator and a

spectrum analyzer are just what you would expect and
correspond almost exactly with McIntosh's own pub­lished curves, as they appear in the owner's manual.
High and low cut filters have 12 dB per octave slopes,
and the "presence boost" at midrange frequencies
peaks center frequencies by just under 5 dB when this
feature is switched in.

Readers may be curious to know why the MA-6100 is
rated at 60 watts per channel under 4-ohm load condi­tions and at 70 watts per channel when operating into
8-ohm loads. Normally, one would expect higher power
output into 4-ohm loads. In the case of this unit, a
special rear panel switch has been added, which limits
output when placed in the 4-ohm position in order to
meet early interpretations of the FTC Audio Amplifier
Power rule. That interpretation required that the
amplifier be able to sustain one hour's operation at
one-third of full power output without overheating or
cycling of thermal protection circuits. Since this
change was made, the FTC has re-interpreted the rule
to permit repeated thermal cycling while making this
test,

so

long

as a

full

hour

of

"on" time

was

accumu­lated. Of course, under musical listening conditions the
test is academic and, in fact, with the switch set to the
"8- or 16-ohm" position, the amplifier would deliver

16

much more than its rated power when loaded with 4­ohm loads.

In addition to listening tests conducted at levels
which produced peaks at or near full power output, we
had occasion to run the amplifier at full power output
under steady signal conditions for extended periods of
time. This test puts a severe strain on the power trans­former, and few consumer-type amplifiers could take
such a beating for a very long period. The McIntosh
MA-6100 stood up to this test nobly, with transformer
temperatures such that we could comfortably rest our
fingers on the surface of the transformer.

As for the listening tests themselves, we used three
different types of loudspeakers for musical listening,
including a pair that normally require an input of
about 60 watts to produce reasonably loud sound
levels. The amplifier contributed no audible distortion

during

these

tests.

Sound quality

was

strictly a func-

tion of the speakers used-as it should be.

Dead shorts maintained across the output terminals
while operating the amplifier at high power output
levels triggered the protection circuits (which

McIntosh calls a Sentry Monitoring Circuit) almost
instantaneously, and return to normal operation of the
system took place equally rapidly when the offending
short was removed.

One gets the feeling that the McIntosh MA-6100

was built to last a very long time. During the course of
our investigations, we had occasion to examine the in­sides of the amplifier and were impressed with the
company's conservatism in choice of components,
many of which are of much more than necessary qual­ity. It is difficult to equate power output with respect
to selling price for this unit, for certainly there are inte­grated amplifiers around which produce as much
power for less cost. McIntosh stresses long-term reli­ability for all of their products, along with a dedication
to service and customer satisfaction that cannot be
adequately judged in the short tune we had to live with
these units. The large and devoted group of audio­philes who own Mac equipment suggests that many
people are willing to pay a premium for this reliability
and service.

Individual

Comment

by

N.E.: This

was a

more­than-usually interesting test project for me since it re­sulted in two things, both of which I deem positive and
constructive: (1) it proved out a piece of equipment from
a company about which there has been considerable
speculation, and (2) it resulted in the first published
test report in years on a piece of McIntosh equipment
in any magazine in this field that accepts advertising.
It is well known that for years McIntosh has main­tained an aloofness regarding editorial coverage of its
products which not only precluded their making test
samples available but which also extended to their not
sending information, even when requested by editors.

I am sure that the idea of simply going to a dealer

and getting a McIntosh unit has occurred to more

54

McIntosh MA-6100: Response when set to
"flat" (lower curve) and "pres" (upper curve)
positions.

than one editor, but apparently this idea never has
been translated into action. We, at Modern Recording,
can thus take credit for an editorial first and for some­thing of a scoop.

That in itself might be enough to be happy about.
But what our little adventure also produced was some
information that should be of value both to the reader/
consumer and to the manufacturer-and that is the
fact that it is entirely possible for a unit to have been
serviced in a less-than-perfectly competent way and
then returned to the dealer who sent it out for repair,
presumably for sale to a customer. To be sure, the
dealer had no way of knowing about the faulty repair

job and it is certainly true that the nature of the flaw

was such that in normal use of the amplifier it prob­ably would never show up. But it was the kind of flaw
that would show up in careful lab tests such as ours.
And I suppose it could be argued that even a minor
discrepancy would be objectionable to the kind of
buyer who is willing to pay a premium price for equip­ment that is reputed to be built with more than

average care and precision.

To guard against this sort of thing happening in the
future, it would seem that McIntosh might more rigor­ously monitor its in-field servicing set-up. And a word

to whomever it was who replaced that shorted diode
and sent it back to the dealer: whenever you find a bum
circuit part, especially in high-grade equipment, you
always suspect and check out neighboring circuit
parts which may have caused the defect or which may
have been damaged as a side effect of the defect.

End of sermon. Now everyone can be happy and

enjoy Mac equipment.

McIntosh MA-6100: Tone control range and low

and high filter response.

McIntosh MA-6100: Loudness control charac-

teristic at various volume control settings.

McINTOSH MA-6100 INTEGRATED AMPLIFIER: Vital Statistics

PERFORMANCE CHARACTERISTICS

Power output per channel, 8 ohms, 1 kHz

Power output per channel, 4 ohms, 1 kHz

Power band for rated 70-watts output

Harmonic distortion for rated output, 1 kHz
IM distortion for rated output
Harmonic distortion at 1-watt output, 1 kHz
IM distortion at 1-watt output
Damping factor, 8 ohms
Frequency response, 20 Hz-20 kHz,

high-level Input
Signal-to-notso,
Frequency response, RIAA equalization

Max. Input before overload, preamp

Signal-to-nolse

Input sensitivity, phono 1

Output level, tape

ret.

ratio,
at 10 mV Input
at 2 mV input

headphone jack

full

output

preamp,

phono 2
high-level inputs

tape inputs

CIRCLE 11 ON READER SERVICE CARD

ref.

20 Hz

20 kHz

20 Hz
20 kHz

lull

output

LAB MEASUREMENT
Sample A

85. 7 watts
76 watts
81 watts

80 watts
66 watts
72 watts

14 Hz to

35.5

0.03%

0.12%

0.05%

0.05%

58

±0.3

92 dB
±1 dB

135 mV

79 dB
65 dB

2 mV

2 mV

280 mV
280 mV

280 mV

0.84V,
8 ohms

kHz

dB

Sample B

85.9 watts
76 watts

80.7 watts

81.3 watts
68 watts
72 watts

13 Hz to
35 kHz

0.03%

0.10%

0.04%

0.04%

56

±0.25dB

92 dB

MR

137 mV

79 dB
65 dB

2.1 mV

2 mV

290 mV
290 mV

290 mV

0.8V,

8 ohms