McIntosh MC252 - Manual (Page 18)

18
Figure 19
With Power Guard
Figure 18
Without Power Guard
Figure 17
Input Test Signal
Figure 16
lower than optimum, causes more output current to flow,
which results in extra heat being generated in the power
output stage. This increase in temperature will result in a
reduced life expectancy for the amplifier.
The Autoformer creates an ideal match between the
power amplifier output stage and the loudspeaker. Refer to
figure 15. There is absolutely no performance limitation
with an Autoformer. Its frequency response exceeds that of
the output circuit itself, and extends well beyond the au-
dible range. Its distortion level is so low it is virtually im-
possible to measure. In the rare event of a power amplifier
output circuit failure, the McIntosh Autoformer provides
absolute protection from possible damage to your valuable
loudspeakers.
Protection Circuits
The MC252 incorporates its version of the McIntosh Sen-
try Monitor out-
put transistor
protection cir-
cuit. Refer to
figure 16. There
is absolutely no
compromise in
sonic perfor-
mance with this
circuit, and it
ensures safe op-
eration of the
amplifier under even the most extreme operating condi-
tions. The different types of protection circuits incorpo-
rated in the MC252 insure a long and safe operating life.
The MC252 also
includes the unique
patented McIntosh
Power Guard circuit.
Power Guard elimi-
nates the possibility of
ever overdriving the
amplifier into clipping.
Refer to figures 17, 18
and 19. An overdriven
amplifier can produce
both audible and inau-
dible distortion levels
exceeding 40%. The
audible distortion is
unpleasant to hear, but
the inaudible ultra-
sonic distortion is also
undesirable, since it
can damage valuable
loudspeaker system
tweeters. You will
never experience the
harsh and damaging
distortion due to clip-
ping.
The Power Guard
circuit is a waveform
comparator, monitor-
ing both the input and
output waveforms.
Under normal operat-
ing 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 sig-
nal waveforms.
When the difference
exceeds 0.3%
(equivalent to 0.3%
harmonic distortion),
the Power Guard ac-
tivates the PG light
and a dynamic elec-
Figure 15