The input waveform and the output waveform are com-
pared 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 gam operational amplifier where it
is amplified about 100 times. The amplified control volt-
age 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 indicator will be illumi-
nated 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 operating.
The control voltage is used to operate a light emitting
diode/light dependent resistor (LDR) network. The LDR is
part of an attenuator between the volume control and the
input amplifier. The input to the power amplifier can be
attenuated to as little as one tenth, if necessary, 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 m the OUT position. The LIMIT indi-
cator operates regardless of the position of the POWER
GUARD switch.
15
www.freeservicemanuals.info
5/10/2017
Digitized in Heiloo Netherland
