McIntosh MC-275-CE Owners manual

STEREO POWER

AMPLIFIER

MC275

MC275

MC275 STEREO POWER AMPLIFIER

INTRODUCTION

The MC275 Commemorative issue
amplifier and owner's manual are faithful
reproductions of the originals, produced
from May, 1961 through July, 1973. A few
changes were made to the original amplifier
design to reflect technology advances and
contemporary system requirements.

Gold plated Balanced Audio Input con-

nectors are added to take advantage of the

greater noise reduction capabilities of

Balanced Cables. High current capacity

Gold Plated Output terminals are also in-

GENERAL DESCRIPTION

Over one million watts of amplifier output
power capacity have been manufactured by
Mclntosh since 1949. In this 1,000,000* watts
of audio power there are less than 10 watts
of distortion capacity and less than 1/100
watt of noise capacity!

The dramatic difference in the quality of
music reproduction when you listen through
Mclntosh instruments is directly due to low

distortion performance. Careful, devoted
research is a way of life at Mclntosh. The
world's finest amplifier is the creation of
persevering, resourceful Mclntosh engineers.

Mclntosh Laboratory is the only manufac-

turer in the entire industry to guarantee the

lowest distortion at all audio frequencies, at

full power. The U.S. Patent Office has
recognized the advanced technology of the
Mclntosh circuit by granting 6 patents.

Long life, flexibility, highest quality con-

struction are characteristic designs in every
Mclntosh instrument. Wide electrical and

cluded. Close tolerance film resistors,
polypropelene capacitors and fiberglass
printed circuit boards are used.

This amplifier is dedicated in remem­brance of the late Gordon J. Gow, coinven­tor of the original Mclntosh Unity Coupled
circuit. Mr. Gow was Vice President of Mcln­tosh Laboratory Inc. from its inception in
1949 until 1977 when he was elected Presi­dent. He held the position of President un-

til

1989.

thermal margins of safety for all com-

ponents and tubes, advanced engineering,

and cool operating design add to the long

life built into every Mclntosh product.
Reliability prolongs your investment without

expensive maintenance costs.

The MC275 has on one chassis two 75
watt power amplifiers. In addition to its use
as a stereo amplifier, the flexibility of the

MC275 permits it to be used as a
monophonic amplifier that delivers 150
watts, or as two separate 75 watt amplifiers
with each channel amplifying completely
separate programs, or as two amplifiers for
use with an electronic crossover network.
Such flexibility permits maximum use for
greatest return from your investment.

*1,000,000 Watts was the total power output produced in
12 years from 1949 through 1961. In 1992 alone, Mclntosh
will produce amplifiers with a total power output of more
than 5,000,000 watts with less than 1/100 watt of distor­tion capacity.

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U.S. Patent Nos.: 2,477,074; 2.545,788; 2,646,467; 2,654.058; 2,860,192; 2.929,028.

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TECHNICAL DESCRIPTION

The patented Mclntosh Unity Coupled cir­cuit and output transformer have establish­ed Mclntosh amplifiers as the unchalleng­ed leaders in the audio field.

Before 1949, low distortion at high power
and high efficiency was impossible. A com­pletely new engineering approach resulted

in an amplifier that for the first time permit­ted high power with distortion below 1%.
That new engineering produced the Mcln­tosh Unity Coupled circuit and the Mclntosh
bifilar wound output transformer. With the

introduction of the Mclntosh amplifier, new
standards for distortion-free performance
were established.

PRIMARY NO. 1

Fig. 1 —Representation of bifilar winding.

PRIMARY NO. 2

The Mclntosh output transformer is
unique. It has two primary windings which
are wound bifilarly. In the bifilar technique
both primary wires are wound side by side.
Each turn of primary number one is next to
the same turn of primary number two. There
is almost complete magnetic coupling be­tween the two wires. The magnetic coupl­ing is reinforced by the capacitance be-

tween the two wires.

INPUT

Fig. 2—Representation of Mclntosh output circuit.

OUTPUT

In the Mclntosh Unity Coupled circuit one
of the bifilar primary windings is connected
through the power supply to the plate and
cathode of one of the output tubes. The
other bifilar primary winding is similarly con­nected to the other tube.

All low distortion high power amplifiers
use push-pull output circuits known as
Class AB1AB2 or B. Two tubes are arranged
in a balanced circuit. This permits each tube
to operate alternately somewhat over half
the time. Compared to full time operation of
the tubes, the push-pull method reduces
heating and permits more power from a

given type of tube. Despite this advantage
of the conventional push-pull circuit one pro­blem in particular remained to be solved.
When current in each tube is cut off to begin
the idle period, distortion is produced at the
instant when current flows. This form of
distortion is known as Notch Distortion as

was well illustrated by Mr. Pen Tung Sah in
the "Proceedings of the I.R.E." Volume 24,
pp 1522-1541 in 1936.

Imperfect coupling between the primary
windings found in all conventional output
transformers produces the condition which
permits notch distortion. Trying to improve
coupling in a conventional transformer
decreases the power response at both low

and high frequencies, heating the output
tubes and lowering the available power
output.

The Mclntosh Unity Coupled output cir­cuit and bifilar transformer is the first com­mercial breakthrough that eliminates notch
distortion by coupling both output tubes
almost to perfection. In the Mclntosh
transformer, the extremely close coupling of
the bifilar windings removes the condition
which permits notch distortion. Furthermore

the two output tubes are arranged as par­tial cathode followers. Half of the output cir­cuit is in the cathode and half in the plate
of each tube. The output tubes now are
operating in a local feedback loop which
reduces their distortion, reduces their inter-

nal generator resistance, and reduces their

balance requirements. The Mclntosh circuit

in reality perfects push-pull high efficiency

output circuits.

The MC275 uses an advanced design of

the Mclntosh output transformer. The new

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