Specifications

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
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