are used in the feedback loop to define the logarithmic function.
The output from the logarithmic amplifier stage feeds a full wave rectifier
circuit made up of U401B and U401C, which produces a positive-going
signal at pin 8 of U401C. The THRESHOLD Control imparts a DC bias
voltage onto this rectified signal prior to feeding a peak-hold filter made up
of U401A circuitry. The THRESHOLD Control provides a DC bias that sets
the point where the signal from the logarithmic circuit triggers D403 on. This
action charges capacitor C401 up as it acquires the peak value of the incom
ing signal. R405 and R418 help to determine the fast attack and slow release
times of the peak-hold circuit.
U401A is a difference amplifier that outputs a negative DC voltage in the
absence of an input signal. For this condition, the output of U401A pulls the
VC bias voltage lower, resulting in another 20 dB of attenuation. When a
signal exceeds the THRESHOLD setting and reaches U401 A's input, the
voltage at U401A's output rises and VCAtt attenuation decreases. U401D, a
voltage comparator, also senses the output signal from U401A to turn on a
fast release mode for any signal that is decaying towards silence. The fast
release occurs through Q402 which switches R417 in parallel with R418 into
the peak-hold circuit. The result is a quicker discharge of C401 and a faster
attenuation of VCAtt for elimination of noise tail.
U405A inverts the control voltage from the expander and then feeds U408, a
ten segment bar driver circuit, to activate the green, yellow, and red thresh
old indicators located on the front panel.
The front panel NR MODE button, SW401, selects A or B noise reduction
modes. A mode is always active, and selecting B mode adds an opposite
polarity feed-forward signal to the final MIX stage. A and B LED's indicate
the chosen mode through closure of remaining switch contacts on SW401.
6.2.4 Noise Reduction (NR) MODE B
When NR MODE B is selected via SW401, a balanced signal from U403A and
U403B is summed with the VCAtt output signals at the input terminals of
U405B through capacitors C414 and C415. This feed-forward is signal is
opposite polarity to VCAtt ouputs, resulting in a complete cancellation of
signals when the VCAtt signal is attenuated 8 dB. Above this point, the
VCAtt output signals predominate to provide normal Exciter enhancement.
Below the 8 dB attenuation threshold, the feed-forward signal predominates
with its opposite polarity condition. Mixing this signal back with the origi
nal audio through the MIX control produces a high frequency cancellation
effect within the final composite signal. The net result of NR MODE B is that
source noise can be reduced by 5 dB below the attenuation threshold through
the use of the described high frequency shelving.
6.2.5 Voltage Controlled Attenuator (VCAtt)
The signal leaving the state variable filter is transformed into a differential
signal by U403A and U403B. The differential output feeds INI and IN2 of
6-6
Circuit Description
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