McIntosh MC 2250, Owner'S Manual

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no difference in wave shape between these signals
and the comparator produces no output. When the
amplifier is driven beyond its maximum power
capacity a difference will develope. If this exceeds
0.5% (equivalent to 0.5% total harmonic distortion)
the comparator output causes the red LIMIT in-
dicators to light. If there is a further increase the
comparator output activates an electronic at-
tenuator at the amplifier input to reduce the
amplifier gain, this automatic attenuator will hold
the amplifier output to its maximum undistorted
value regardless of the degree of overdrive to the
amplifier. The amplifier may be overdriven by 20 dB
before the output distortion exceeds 2%.
The comparator is an especially compensated
operational amplifier integrated circuit. Its output is
detected by a full wave bridge that feeds signals to
the control circuitry for the LIMIT and NORMAL in-
dicators and to the electronic attenuator at the
amplifier input. The attenuator is a light emitting
diode/light dependent resistor network selected
especially for its low distortion and proper time con-
stant characteristics.
SYSTEM TEST CIRCUIT
When AC line power is applied to the MC 2250 the
System Test circuit functions to verify operating
potentials at 7 test locations within the amplifier.
Upon applying power a digital display lights with the
number 7. The digit is driven by a counter that steps
down at a 1 second or .4 second rate. Each time the
digit changes, electronic switches select the out-
puts of various voltage comparators connected to
test points in the amplifier. If voltages are normal
the green Power Guard lights flash, there is an audi-
ble beep produced by an internal sounder, and the
display decreases to the next test number. When the
count passes number 1 the loudspeakers connect
and the display blanks off. If a fault condition exists
the counter stops and both red Power Guard limit
lights go on. The digit displayed indicates the circuit
at fault. Servicing the amplifier is thereby simplified.
The test circuit uses logic counter, BCD decoder,
lamp driver, and voltage comparator integrated cir-
cuits.
POWER SUPPLY
The power supply is a conventional full wave
bridge rectifier arrangement providing plus and
minus 50 volts DC. Electronic regulators step down
and regulate plus and minus 15 volt sources for low
level circuits and plus 5 volts for the logic circuits.
Thermistors are used in the power transformer
primary circuit to limit the turn-on current.
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