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14
Power Supply Circuits
To compliment the design of the MC255 Power Am-
plifier Circuitry, there is a high current high voltage
power supply. Refer to figures 24 and 27.
The very large Power Transfomer can supply over 13
amps of current. Refer to figure 25 (golf ball is for size
comparsion).
Figure 22
Without Power Guard
Figure 21
Input Test Signal
(14kHz & 15kHz)
Figure 20
Normal Operating Area
Sentry Monitor
Safety Area
Output
Transistor
Failure
Technical Description, con’t
Figure 23
With Power Guard
Figure 25
Figure 27
Protection Circuits
The MC255 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
ensures safe opera-
tion of the ampli-
fier under even
the most extreme
operating condi-
tions. The different
types of protection circuits incorporated in the MC255
insure a long and safe operating life. This is just one
of the many characteristics of McIntosh Power Ampli-
fiers that make them world famous.
The MC255 also includes the unique patented
McIntosh Power Guard circuit. Power Guard elimi-
nates the possibility of ever overdriving the ampli-
fier 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
these waveforms. If the
amplifier is overdriven,
there will be a difference
between the two signal
waveforms. When the dif-
ference exceeds 0.3%, the
Power Guard activates
the PG light and a dy-
namic electronic attenua-
tor 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 MC255 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.
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