Specifications

The impedance ratio required between
primary and secondary has been re-
duced by a factor of four-to-one com-
pared with the conventional arrange-
ment. It is now 125 to 1 (1000/8).
The turns ratio is therefore only half
of what it was before. Leakage induc-
tance is therefore much lower, and so
is the shunt capacitance across the
windings. The use of a bifilar winding
technique completely eliminates the
leakage inductance problem of cou-
pling between the sections of the pri-
mary windings. It was the develop-
ment of the Unity Coupled Circuit
by McIntosh (the circuit is patented)
way back in 1947 that enabled us to
produce amplifiers which were a whole
order of magnitude lower in distor-
tion than the competition of those
days. Typically, we were able to pro-
duce power output circuits with total
harmonic distortion of under 1.0%
even before the distortion-reducing
negative feedback loop was added.
With just 20 dB of feedback applied,
the THD was further reduced to under
0.1%!
What About Transistor Amplifiers
The audio industry welcomed the
power output transistor as the solution
to all its problems. After a few falter-
ing starts (early germanium power
output transistors were notoriously
unreliable and easily destroyed by high
operating temperatures), silicon power
transistors became the standard power
device in power amplifiers.
Since power output transistors ex-
hibit a low output impedance, it was
possible to design output circuits to
match 8-ohm loads directly—without
the need for a matching audio output
transformer. Indeed, most OTL ampli-
fiers, when coupled to 8-ohm resistive
loads for test purposes, can deliver full
rated power to those loads for long
periods of time without overheating or
exceeding safe thermal dissipation
limits. The trouble is that we don't
listen to resistors we listen to loud-
speakers. It will come as no surprise to
you to learn that speakers having a
"nominal" impedance of 8 ohms often
measure lower and higher impedance
values at different audio frequencies.
Then, too, consider the fact that many
popular speaker systems have nominal
impedances of 4 ohms, and the imped-
ance of 4 ohm speakers can easily dip
down to as low as 2 ohms at certain
frequencies. What happens to an OTL
amplifier with such low impedances
connected to it? In theory, if an out-
put stage is designed to match an 8
ohm impedance, its power "output
capability should double when it's
connected to a 4 ohm impedance. But
as this mismatch occurs, thermal dis-
sipation increases rapidly. In fact,
operating into a 4 ohm load, heat dis-
sipation is double what it would be
when operating at 8 ohms, as illustrat-
ed in Fig. 4. Unfortunately, if the
amplifier was designed for 8-ohm oper-
ation, its thermal dissipation limits
were designed with some safety factor
Heat dissipation
capacity required to
meet FTC rating
at 8 ohms.
Load impedance in ohms (1000%=Heat
produced at rated output into 8 ohms.)
Fig. 4 - Heat produced by transformerless
amplifier at various load impedance
for 8 ohm operation, so as to meet the
new FTC preconditioning require-
ments. These call for the amplifier to
be able to deliver one-third rated
power at rated impedance for one
hour. But, as you can see from Fig. 4,
the safety margin is not nearly great
enough to permit operation at 4-ohms
—or 2-ohms-or 1-ohm impedances.
Remember, too, that many amplifiers
and receivers have provisions for con-
nection of more than one pair of
speakers for use in different listening
rooms, so that even if 8-ohm speakers
are selected, using double pairs of
them results in a 4-ohm net nominal
impedance even before allowing for
downward variations in impedance
at specific frequencies in the audio
spectrum. So, unless manufacturers are
willing to resort to disproportionately
massive heat sinks, cooling fans or
combinations of both, designing power
amplifiers that can deliver their maxi-
mum powers at both 8 ohms and im-
pedances of 4 ohms and lower be-
comes physically impractical in the
case of the OTL amplifiers.
Fig. 6 - Performance of MC 2300
The Answer-Output Transformers!
If a transistorized amplifier were
equipped with an output transformer,
you could move up or down in load
impedance and maintain full power
ratings without over-dissipating any-
thing, since the amplifier's output
stages would always be working into
an ideal load.
To many hi-fi enthusiasts, output
transformers tend to create visions of
compromised design. That is just not
the case today. Technology in mater-
ials and transformer design methods
have advanced significantly in recent
years and, remember, we're dealing
with low impedance devices-not tubes.
It's no longer necessary to translate
impedances from a "plate circuit" to a
speaker-a step down of several hun-
(* - - - Continuous operation not possible
due to overheating. Protection circuit is as-
sumed to current limit when load falls be-
low 4 ohms, in actuality the output into 4
ohms and lower impedances will fall below
the values shown.)
Fig. 7 - Performance of non McIntosh
transformerless amplifier rated for
300 watts in 8 ohm load
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