McIntosh MC303 - Manual (Page 14)

14
increase in temperature will result in a reduced life
expectancy for the amplifier.
The Autoformer creates an ideal match between the
Power Amplifier Output Circuitry and the Loudspeaker.
There is absolutely no performance limitation with an
Autoformer. Its frequency response from below 20Hz to
well beyond 20,000Hz exceeds the Power Output Circuitry
Frequency Response, and extends well beyond the audible
range. Its distortion level is so low it is virtually impossible
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 Loud-
speakers.
Protection Circuits
The different types of protection circuits incorporated in
the MC303 insure a long and safe operating life.
The MC303 incorporates the McIntosh Sentry Monitor
output transistor protection circuit. Refer to figure 16.
There is absolutely no
compromise in sonic
performance with this
circuit, and it ensures
safe operation of the
amplifier under even
the most extreme op-
erating conditions.
The MC303 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 inaudible distortion
levels exceeding 40%. The audible distortion is unpleas-
ant to hear, but the inaudible ultrasonic distortion is also
undesirable, since it can
damage valuable loudspeaker
system tweeters. You will
never experience the harsh
and damaging distortion due
to clipping.
The Power Guard circuit is a
waveform comparator, moni-
toring both the input and out-
put 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 dif-
ference between the two signal
waveforms. When the differ-
ence exceeds 0.3% the Power
Guard activates the PG light
and a dynamic electronic at-
tenuator 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 effects and
the sonic purity of the music
reproduction is perfectly
preserved. The MC303 Power
Amplifier with Power Guard
is not limited to just the rated
power output, but will actu-
ally produce distortion free
output well above its rated
power due to the McIntosh
philosophy of conservative
design.
Power Supply Circuits
To compliment the design of the MC303 Power Amplifier
Circuitry, there is a high current high voltage power supply
for the three channels. Refer to figure 20 on page 15. The
power amplifiers draw high current from the AC power
line. Therefore, it is important that they plug directly into
the wall outlet.
The MC303 incorporates a very large high current
Main Power Transformer. Refer to figure 21. It is enclosed
in the legendary McIntosh Potted Enclosures and weighs
38 lbs. The super size multiple large main filter capacitors
Figure 17
Input Test Signal
Figure 18
Without Power Guard
Figure 19
With Power Guard
Figure 21
Figure 16
Normal
Operating
Area
Normal
Operating
Area
Sentry Monitor
Safety Area
Sentry Monitor
Safety Area
Output Transistor Failure Output Transistor Failure