McIntosh MC-240 Owners manual

STEREO POWER

AMPLIFIER

CONTENTS

MC24O

GENERAL DESCRIPTION 1
TECHNICAL DESCRIPTION 1

PANEL FACILITIES 4

INSTALLATION 5

CONNECTIONS 5

Input—Stereo 5
Input—Twin Amp 5
Input—Mono 6

Output—Stereo or Twin Amp 6

Output—Mono 6
Line Output—Stereo or Twin Amp 6
Line Output—Mono 7

AC Power 7

OPERATION 7
ADJUSTMENTS 7

GUARANTEE 8

OWNER'S MANUAL

MC24O

MC240

STEREO

GENERAL DESCRIPTION

POWER

AMPLIFIER

Over one million watts of amplifier output

power capacity have been manufactured by
Mclntosh since 1947. In this 1,000,000watts
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 manufac­turer 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.

TECHNICAL DESCRIPTION

1

Long life, flexibility, highest quality con­struction are characteristic designs in every
Mclntosh instrument. Wide electrical and

thermal margins of safety for all components
and tubes, advanced engineering, and cool
operating design add to the long life built

into every Mclntosh product. Reliability pro-

longs your investment without expensive
maintenance costs.

The MC240 has on one chassis two 40

watt power amplifiers. In addition to use as a

stereo amplifier, the flexibility of the MC240
permits it to be used as a monophonic

amplifier that delivers 80 watts, or as two

separate 40 watt amplifiers with each chan­nel amplifying completely separate pro-

grams, or as two amplifiers for use with an

electronic crossover network. Such flexi­bility permits maximum use for greatest
return from your investment.

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

Before 1947, low distortion at high power

and high efficiency was impossible. A com-

pletely 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.

Fig. 1 —Representation of bifilar winding.

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

1

U.S. Patent No's: 2,477,074; 2,545,788; 2,646,467; 2,654,058; 2,860,192; 2,929,028.

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 between the
two wires.

INPUT

Fig. 2 —Representation of Mclntosh output circuit.

OUTPUT

1

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

AB1, AB2 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 of cut
off and again at the instant when current
flows. This form of distortion is known as
Notch Distortion and 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
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 internal generator

resistance, and reduces their balance

requirements. The Mclntosh circuit in
reality perfects push-pull high efficiency
output circuits.

Leakage inductance (lack of coupling)
between the primary and secondary wind­ings of the output transformer limits the high
frequency response of an amplifier. The
primary and secondary windings of the
Mclntosh output transformer are interleaved

five times to improve coupling. The inter­leaving is accomplished by winding groups of

primary layers, then secondary layers alter­nately until the total transformer is wound.
Interleaving helps to extend the Mclntosh
power band width to over 100,000 cycles.

Since 1947 this new technology has been
built into every Mclntosh power amplifier.

Good voltage regulation in the power
supply permits overloads without overshoot
or blocking, good transient response, and
complete stability. To improve regulation a
silicon rectifier power supply is used in the
MC240. In addition to better voltage regula­tion, the silicon rectifier allows even higher
operating efficiency, cooler operation, and

longer amplifier life.

To greatly extend tube and component
life a thermistor in the MC240 limits current
surges produced when the equipment is
turned on. The thermistor is a special type of
resistor. Its resistance depends on its tem­perature. When the amplifier is off the therm­istor has a high resistance value (about 79
ohms). Just after the amplifier is turned on
the current which flows through the therm­istor heats it and causes its resistance to
decrease to a low value (less than .1 ohms).
Current is thus limited when the MC240 is
first turned on but is not limited as the unit
warms.

ELECTRICAL SPECIFICATIONS

Power Output:

Stereo or twin amplifier—40 watts con­tinuous per channel
Monopnonic—80 watts continuous

Harmonic Distortion:

Less than 0.5% at rated output or less,

2

20 cycles through 20,000 cycles. Typical
performance is 0.3% or less at 20 cycles
and 20,000 cycles at full power. Typical
performance in the mid frequencies is

0.1% or less at full power.