McIntosh MC152 Owner's Manual page 16

Technical Description, con't
In the rare event of a power amplifier output circuit
failure, the McIntosh 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 MC152 has large Output Watt
Meters responding to 95% full scale to a single cycle
tone burst at 2kHz. Refer to figure 19. The Ampli-
Figure 19
fier Outputs are electronically measured and fed to a
special circuit that accelerates the pointer movement
in the upward direction. Refer to figure 26 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 profes-
sional VU meter.
16
Protection Circuits
The MC152 incorporates the McIntosh Sentry Moni-
tor Output Transistor Protection Circuit. Refer to
Figure 20. There is
absolutely no com-
promise in sonic
performance with
this circuit, and it
Normal Operating Area
ensures safe opera-
tion of the ampli-
fier under even
Sentry Monitor
the most extreme
Safety Area
operating condi-
tions. The different
types of protection
circuits incorporated in the MC152 insure a long and
safe operating life. This is just one of the many char-
acteristics of McIntosh Power Amplifiers that make
them world famous.
The MC152 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 21, 22 and 23.
An overdriven amplifier
can produce both audible
and inaudible distortion
levels exceeding 40%.
The audible distortion is
unpleasant to hear, but
the inaudible ultrasonic
distortion is also undesir-
able, since it can damage
valuable loudspeaker
system tweeters. You
will never experience the
harsh and damaging dis-
tortion due to clipping.
The Power Guard
circuit is a waveform
comparator, monitoring
both the input and output
waveforms. Under normal
operating conditions,
there are no differences
between the shape of
Output
these waveforms. If the
Transistor
Failure
amplifier is overdriven,
Figure 20
there will be a differ-
ence between the two
signal waveforms. When
the difference exceeds
0.3%, the Power Guard
activates the PG light
and a dynamic electronic
attenuator at the ampli-
fier input reduces the
input volume just enough
Input Test Signal
to prevent any further
(14kHz & 15kHz)
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
MC152 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.
Figure 21
Without Power Guard
Figure 22
With Power Guard
Figure 23