Macintosh MC2200 Owner's Manual - page 17
The input waveform and the output waveform are
compared in an integrated circuit differential amplifier.
Any differences, due to distortion of the output waveform,
are converted to a control voltage. This control voltage is
applied differentially to a high gain operational amplifier
where it is amplified about 100 times. The amplified
control voltage is detected by a full wave bridge rectifier
then enters a Schmidt trigger which is programmed to
sequence the front panel indicators from NORMAL (green)
to LIMIT (red} at a preset distortion level. Also built into
the trigger circuit is a timing capacitor which holds the
LIMIT indicator on long enough to be visible even when the
duration of the overload is much shorter. The LIMIT indica-
tor will be illuminated for an overload condition as short
as one cycle of a signal at 20,000 Hz.
The same control voltage used to activate the LIMIT
indicator also actuates the POWER GUARD system. If the
control voltage is greater than is needed to illuminate
the LIMIT indication then the POWER GUARD begins op-
erating. The control voltage is used to operate a light
emitting diode/light dependent resistor (LOR) network.
The LDR is part of an attenuator between the volume con-
trol and the input amplifier. The input to the power am-
plifier can be attenuated to as little as one tenth, if neces-
sary, in order to maintain low distortion in the event of
excessive input signal.
The POWER GUARD circuit is activated when the rear
panel switch is in the NORMAL position. POWER GUARD
protection is disabled in the OUT position. The LIMIT indi-
cator operates regardless of the position of the POWER
GUARD switch.
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