Gain Reduction Amplifier - Universal Audio 1178 Operating Instructions Manual

uses a
special transformer designed by UREI for low phase shift,
flat response, and excellent overload characteristics.
The amp-
lifier provides approximately 17
dB of
gain, and an additional
3
dB
is
achieved in the
output transformer when terminated with
a
600 ohm load
.
4.6
GAIN REDUCTION AMPLIFIER
The audio signal
is
coupled through
C31 to the GR circuit
IC
7D
and
IC 7A.
Resistor R31 sets the
threshold for the 4:1
compression
ratio.
Depending on the position of
S3,
different resistors are
switched in parallel with R31,
determining the threshold for the
other compression ratios:
8:1,
12:1 or 20:1.
The audio signal
is
full
wave rectified
and a
negative dc
voltage
is
developed at the
junction
of
diodes CR5 and CR6.
This
voltage
is
inverted in
amplifier
IC
7B,
and
then drives transistor
Q3,
which supplies the GR control voltage across capacitor
C37.
The compression ratio
is
set with different resistors selected
by
S3.
Depending on the resistor value,
a
current of opposing
polarity
is
summed into the inverting input of IC 7B.
Corresponding
resistors
are:
R38 for 4:1,
R39 for
20:1,
voltage divider
R35,
R36,
R37 for 8:1 and 12:1 ratios.
Diode CR7
around the inverting
amplifier, prevents the output voltage from assuming
a
negative
value
.
During conduction, transistor Q3 functions
as a
current source
for the
timing capacitor C37.
The charging time
is
variable with
R42,
which
is
the Attack Control of the limiter.
When transistor
Q3
is
turned off,
the discharge of C37 occurs through the large resis-
tance of R43. It
too
is
variable, and
is
the Release Control of the
limiter.
During discharge the relatively small value of R42 can be
neglected.
The stop resistors, R44 and R45,
are selected to deter-
mine the fastest attack and release time of the unit.
When the
voltage across C37 is zero,
no limiting occurs.
With
increasing positive value, the control voltage drives FET Q7A propor-
tionately into conduction, thus providing
a
voltage divider for the
audio signal.
During STEREO operation the control voltages of the
two limiter channels are connected together.
The gain reduction function
is
disabled with switch
S5.
(This
is
part of the Attack Control and is
engaged
in the full CCW
posi-
tion.)
A sufficiently large positive current
is
driven into the
input of
IC
7,
effectively overriding any negative dc voltages
generated by the rectifier circuit.
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