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16
Refer to figure 18.
There is abso-
lutely no perfor-
mance limitation
with an Autoformer.
Its frequency re-
sponse exceeds that
of the output circuit
itself, and extends
well beyond the
audible range. Its
distortion level is
so low it is virtu-
ally impossible to
measure.
In the rare event
of a power ampli-
fier output circuit
failure, the McIn-
tosh Autoformer
provides absolute
protection from possible damage to your valuable
loudspeakers. The unequaled expertise of McIntosh
in the design and manufacturing of Autoformers is
legendary in the high fidelity industry. McIntosh engi-
neers know how to do it right.
Power Output Meter
The McIntosh MC312 has large Output Watt Me-
ters that respond 95% full scale to a single cycle tone
burst at 2kHz. Refer to figure 17. The Voltage Output
is electronically measured and fed to a special circuit
that accelerates the pointer movement in the upward
direction. Refer to figure 25 on the next page. When
the pointer reaches its peak it pauses only long enough
for the human eye to perceive its position, then drops.
It is almost 10
times faster than
a professional
VU meter.
A front panel
switch is pro-
vided to change
the meter to the
Watts Hold Mode
of operation.
This allows fast
upward move-
ment of the pointer but greatly increases Hold Time at
the peak of its travel. The highest power output of the
source material is thus recorded.
Protection Circuits
The MC312 incorporates the McIntosh Sentry Monitor
Output Transistor Protection Circuit. Refer to Figure
18. There is absolutely no compromise in sonic perfor-
mance with this circuit, and it ensures safe operation
of the amplifier under even the most extreme operat-
ing conditions. The
different types of
protection circuits
incorporated in
the MC312 insure
a long and safe
operating life. This
is just one of the
many character-
istics of McIntosh
Power Amplifiers
that make them world famous.
The MC312 also includes the unique patented Mc-
Intosh Power Guard circuit. Power Guard eliminates
the possibility of ever overdriving the amplifier into
clipping. Refer to figures 19, 20 and 21. An overdriven
amplifier can produce both audible and inaudible dis-
tortion levels exceeding 40%. The audible distortion is
unpleasant to hear, but the inaudible ultrasonic distor-
tion is also undesirable,
since it can damage valu-
able loudspeaker system
tweeters. You will never
experience the harsh and
damaging distortion due
to clipping.
The Power Guard
circuit is a waveform
comparator, monitor-
ing both the input and
output waveforms.
Under normal operating
conditions, there are no
differences between the
shape of these waveforms.
If an amplifier channel is
overdriven, there will be
a difference between the
two signal waveforms.
When the difference
exceeds 0.3%, the Power
Guard activates the PG
light and a dynamic elec-
tronic attenuator at the
amplifier input reduces
the input volume just
enough to prevent any
further increase in distortion. The Power Guard circuit
acts so fast that there are absolutely no audible side ef-
fects and the sonic purity of the music reproduction is
Figure 22
Without Power Guard
Figure 21
Input Test Signal
(14kHz & 15kHz)
Figure 20
Normal Operating Area
Sentry Monitor
Safety Area
Output
Transistor
Failure
Technical Description, con’t
Figure 19
Figu re 18
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