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
AB
1
, AB
2
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 conven-
tional push-pull circuit one problem in par-
ticular 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 de-
creases the power response at both low and
high frequencies, heating the output tubes
and lowering the available power output.
The Mclntosh Unity Coupled output cir-
cuit and bifilar transformer is the first com-
mercial breakthrough that eliminates notch
distortion by coupling both output tubes
almost to perfection. In the Mclntosh trans-
former 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 wind-
ings 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 inter-
leaving is accomplished by winding groups of
primary layers, then secondary layers alter-
nately 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 regula-
tion, 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 tem-
perature. When the amplifier is off the therm-
istor has a high resistance value (about 79
ohms). Just after the amplifier is turned on
the current which flows through the therm-
istor 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 con-
tinuous per channel
Monopnonic—80 watts continuous
Harmonic Distortion:
Less than 0.5% at rated output or less,
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.
2
