McIntosh MAC 4200 Owner's Manual page 24

SENTRY MONITOR circuit does not limit the power output
available from the amplifier nor does it have any effect on
signals passing through the amplifier when operating con-
ditions are normal.
A power amplifier which does not use output
transformers to bypass DC (direct current), can destroy
loudspeakers should there be a failure within the
amplifier. Safety circuits are necessary to protect
loudspeakers. The MAC 4200 has a DC detecting circuit
connected to the output of each channel. When DC is
present, this circuit reacts in milliseconds to open the
speaker relay. The speakers remain disconnected until the
cause has been corrected. Under normal operating condi-
tions the DC protective circuit has no effect on the opera-
tion of the output circuit.
POWER GUARD
POWER GUARD (US Patent #4048573), a unique feature
of Mclntosh amplifiers, assures that each channel of the
MAC 4200 will deliver full power free of clipping distor-
tion. Clipping is caused when an amplifier is asked to pro-
duce more power output with low distortion than its
design characteristics permit. Amplifiers when driven to
clipping can deliver large quantities of power that will
have more than 40% harmonic distortion which causes the
sound to be grossly distorted. The extra energy content of
the clipped signal will damage most loudspeakers, par-
ticularly high frequency speakers. The Mclntosh POWER
GUARD circuit protects your ears and your speakers from
this kind of damage.
The Mclntosh patented POWER GUARD circuit
eliminates amplifier clipping due to overdrive. It also il-
luminates the red Power Guard indicators at the top of
the POWER indicator when the amplifier is driven beyond
its maximum output capacity.
The POWER GUARD circuit compares the wave shape
of the amplifier input and output signals. Normally there is
no disparity between these signals and the comparison
produces no output. When the amplifier is driven beyond
its maximum power capacity a difference will develop. If
the disparity exceeds 0.5% (equivalent to 0.5% total har-
monic distortion) the difference will cause the Power
Guard bar indicators to light. If there is a further increase
in the disparity the difference output controls a fast acting
(microseconds) electronic attenuator at the amplifier input
to reduce the amplifier gain, thus holding the amplifier
output to its maximum undistorted value regardless of the
degree of overdrive to the amplifier. The amplifier must
be overdriven by 20 dB before the output distortion ex-
ceeds 2%.
The comparison is achieved in an especially compen-
sated operational amplifier integrated circuit. Its output is
22 TECHNICAL DESCRIPTION
detected by a full wave bridge that feeds signals to the
control circuitry for the Power Guard indicators and to the
electronic attenuator at the amplifier input. The attenuator
is a light emitting diode/light dependent resistor network
selected for its low distorion and time constant
characteristics.
TURN ON DELAY
The MAC 4200 has transient-free turn on/turn off
characteristics, time-controlled by a transistor switch that
operates a heavy duty relay which connects the outputs
to the speakers. The control to the transistor switch is
derived from a long time constant capacitor charging net-
work which energizes the relay approximately one second
after the AC power switch is turned on. The same circuit
has a short turn-off time constant which causes the relay
to drop out before the receiver's main power supply has
a chance to discharge.
POWER SUPPLY
Two high current power supplies, a positive 48 volt and
a negative 48 volt DC are used to drive the output power
amplifier. Four large filter capacitors, 6800 microfarads
each, are used to store the large amount of energy
necessary for good filtering and precise voltage regula-
tion. A stable well regulated power supply is required for
proper low frequency response and negligible low fre-
quency distortion.