16
Figure 21
Without Power Guard
Figure 20
Input Test Signal
(14kHz & 15kHz)
Figure 22
With Power Guard
Figure 19
Normal Operating Area
Sentry Monitor
Safety Area
Output
Transistor
Failure
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 MC1.25KW has a huge hand built
Output Watt Meter that responds 95% full scale to a
single cycle tone burst at 2kHz. Refer to figure 18.
Voltage and current outputs are electronically mea-
sured, multiplied and fed to a special circuit that accel-
erates the pointer movement in the upward direction.
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 provided to change the
meter to the Watts Hold Mode of operation. This al-
lows fast upward movement of the pointer but greatly
increases Hold Time at the peak of its travel. The
Figure 18
highest power output of the source material is thus
recorded.
Protection Circuits
The MC1.25KW incorporates a version of the Mc-
Intosh Sentry Monitor Output Transistor Protection
Circuit. Refer to Figure 19. There is absolutely no
compromise in
sonic performance
with this circuit,
and it ensures safe
operation of the
amplifier under
even the most
extreme operating
conditions. The
different types of
protection circuits
incorporated in the
MC1.25KW insure a long and safe operating life. This
is just one of the many characteristics of McIntosh
Power Amplifiers that make them world famous.
The MC1.25KW also includes the unique patented
McIntosh Power Guard
circuit. Power Guard
eliminates the possibility
of ever overdriving the
amplifier into clipping.
Refer to figures 20, 21
and 22. An overdriven
amplifier can produce
both audible and inau-
dible 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
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
electronic 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
effects and the sonic purity of the music reproduction
is perfectly preserved. The MC1.25KW Power Ampli-
fier with Power Guard is not limited to just the rated
power output, but will actually produce distortion free
output well above its rated power due to the McIntosh
philosophy of conservative design.
