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16
and damaging distortion
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 the
amplifier is overdriven,
there will be a difference
between the two signal
waveforms. When the
difference exceeds 0.3%,
the Power Guard acti-
vates the PG light and
a dynamic electronic
attenuator at the ampli-
fier input reduces the
input volume just enough
to prevent any further in-
crease in distortion. The
Power Guard circuit acts
so fast that there are ab-
solutely no audible side
effects and the sonic purity of the music reproduction
is perfectly preserved. The MC611 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.
Protection Circuits
The MC611 incorporates the McIntosh 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 MC611 insure a long and safe
operating life. This is
just one of the many
characteristics of McIn-
tosh Power Amplifiers
that make them world
famous.
The MC611 also
includes the unique
patented McIntosh
Power Guard circuit.
Power Guard eliminates
the possibility of ever
overdriving the ampli-
fier into clipping. Refer
to figures 20, 21 and 22. 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
undesirable, since it can damage valuable loudspeaker
system tweeters. You will never experience the harsh
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
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 manu-
facturing
of Auto-
formers is
legendary
in the high
fidelity
industry.
McIntosh
engineers
know how
to do it
right.
Power Output Meter
The McIntosh MC611 has a large 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 measured, multiplied
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 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
highest power output of the source material is thus
recorded.
Technical Description, con’t
Figure 18
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