10
Sentry Monitor
Sentry Monitor provides protection for the MC901 by
monitoring Output. In the event of a large impedance
mismatch, a short circuit at the Output Terminals
or Loudspeaker Terminals, or Tube failure, Sentry
Monitor will activate to prevent potentially destructive
levels of current from flowing.
When Sentry Monitor is activated, the MC901 will
power off, and the three smaller tubes on the side of
the problem will flash red. Since the Vacuum Tube
Amplifier actually consists of two Amplifiers out of
phase with each other, an issue can occur on either
side as indicated by the flashing red lights.
To reset the Sentry Monitor Protection Circuitry, turn
the Power Control Knob to OFF. (See “Power Control
Knob” on page 14.) Wait one minute and then turn
the Power Control Knob to ON.
If Sentry Monitor continues to engage, the problem
must be fixed before continuing. Check that
wires connected to the Output Terminals and the
Loudspeaker Terminals are not shorted. If those
connections are fine, and the Impedance of the
Loudspeakers is close to the value of the connected
Output terminals, the issue may be a defective tube.
Vacuum Tubes on the side indicated by the flashing
red lights should be tested. Contact an Authorized
McIntosh Dealer or Authorized McIntosh Service
Agency for service.
Sentry Monitor has absolutely no effect on the
performance of the MC901 while it is monitoring the
MC901.
Quad Balanced
McIntosh’s Quad Balanced design solves the age-
old Amplifier trade off that more power equals more
noise. Properly designed high-power solid state
Amplifiers use many transistors connected in parallel
to share the power load. The transistors share the load
to avoid becoming stressed. The downside is that each
transistor added raises the noise floor a small amount.
Quad Balancing eliminates this problem.
In the MC901, the 600 watt Solid State Amplifier
contains two identical signal paths or modules. The
audio signal is fed into one module, but the other
module receives the same music signal, inverted,
so the electrical waves are traveling in the opposite
direction (up where it was going down, down where
the wave had been going up). During amplification,
some small amount of noise is produced by each
amplification module. Module #1, therefore, produces
amplified music plus a bit of noise, while Module
#2 produces amplified inverted music, plus a small
amount of noise. The two signals enter McIntosh’s
patented Output Autoformer, with dual windings
going opposite directions, which invert the previously
inverted signal, putting it back to its original form.
The power from Module #1 (regular music) is
combined with the power from Module #2 (inverted
once before amplification and once again inside the
transformer, so it’s now back to its original form),
but with twice the power. The noise, on the other
hand, gets cancelled when the noise from Module #1
is added to the noise from Module #2, since those
noise waves are going opposite directions. The Output
Autoformer doubles the power and cancels noise at
the same time.
The Vacuum Tube side of the MC901 also takes full
advantage of Quad Balanced architecture. With the
Vacuum Tube implementation of Quad Balanced
design, all the benefits of Vacuum Tubes are preserved
while unwanted noise is eliminated.
Circuit Breaker
The MC901 is protected by a Circuit Breaker. The
Circuit Breaker can be reset by pressing down (see
Figure 12). If the Circuit Breaker continually trips,
resolve the underlying issue before continuing to use
the MC901.
Figure 12– Circuit Breaker
INPUT MODE
BALANCED
UNBALANCED
SERIAL
NUMBER
CAUTION
RISK OF ELECTRIC SHOCK
DO NOT OPEN
ATTENTION:
RISQUE DE CHOC ELECTRIQUE-NE PAS
OUVRIR
120V 50/60Hz
12 AMPS
CIRCUIT
BREAKER
