1C
207
(as
well
as
all the other
71
1
IC's)
requires
a
power supply o
l
approximately
+12
and
-6
volts.
-*-12.6
volts is supplied
by dropping the
+15
volts
through
3
silicon diodes, CR232-234.
C231,
269
bypass
the fast
risetime
oulses
produced
by
IC207
to
ground.
A
bypassed source
of
-14
volts for
the pulse-handling
transistor Q2II
is provided by dropping the
-15
volt sup-
ply through
CR22
-
,
bypassed by
C264,
265.
-5.5
volts
is derived
by dropping the
-15
volts
through
a
9 volt
zener diode, CR235.
Program-Controlled
Pre-emphasis:
In
order to obtain control
of
excess high-frequency
energy,
a
6 dB/octave high-frequency pre-emphasis
is
applied
to the output of the
broadband
AGC.
This
pre-emphasis continues
to 25 kHz,
and is then
rolled
off.
A
comparator
at
the output of the pre-emphasis filter
cre-
ates
an error signal
if
the pre-emphasized
signal attempts
to exceed
a
fixed threshold.
This error signal, by feed-
back,
reduces
the pre-emphasis until the
overload is
e-
I
iminated
.
The
pre-emphasis is created by
summing
the
output
of
a
bandpass filter centered at
35
kHz
(R233,
235,
237
C233,
235,
IC209A)
with
the
flat signal.
The sum of the
two yields
a
6 dB/octave
pre-emphasis to beyond
20
kHz.
The amount
of
oandpassed signal summed in determines the
constant of the
pre-emphasis. This gain
is
determined by
two factors: (I) A voltage divider consisting
of
resistors
RI07,
109,
III,
113,
115
across terminals II and
13,
and
switched by the
front-panel
TIME CONSTANT
switch;
along
with
R245,
and
(2)
a
voltage-controlled attenuator,
realized with IC2I7A, which
shunts R245 to reduce the
bandpass
gain
as
necessary
to eliminate
overloads due
to
excessive
high-frequency energy.
IC211 is
a 30.1 dB non-inverting
amplifier
which
makes up for losses in the
previously mentioned
voltage
divider.
The summing of tne
fiat
and bandpassed signals is
done in
IC209B. The bandpass
filter
is inverting;
there-
fore, the overall
bandoass path
is
inverting.
In
order to
sum the flat
signal
with
correct
phase, the
flat
signal
must also be inverted.
This
is accomplished by intro-
ducing
the
flat
signal through
R239 to
IC209B 's
inverting
input;
the bandpassed
signal is introduced to
IC209B's
non-inverting input.
In addition,
a
small amount of
bandpassed signal
is
introduced to
IC209B’s inverting input through
R241
.
This is because
IC217A can never reduce the
main band-
pcfss
gain to zero.
This may be
required if the pre-em-
phasis is to be totally
defeated
because
the
program ma-
terial consists
of
a
high-frequency
sine wave, for
exam-
ple. Therefore, the
extra out-of-phase
bandpassed
com-
ponent means that there
is some low, but achievable
gain
through the main
bandpass
path that will result in
total cancellation of
the bandpass
component at
the
out-
put of
IC2C9B,
leaving
only the flat
signal.
Details of the
bandpass
amplifier
and
voltage-control-
led
attenuator
(IC21 1 and
associated components) is
sub-
stantially
identical
to the
broadband
VCA, IC205
and
associated
components. The
reader should refer to the
previous
section for
a
detailed
description.
The high-frequency
limiter section
has approximately
12
dB gain at low
frequencies. Therefore, in
order to
use the same
comparator reference voltage
as
the
broad-
band
limiter,
the output of
the
high-frequency
limiter is
attenuated approximately 12
dB by voltage divider
R263,
265 before being applied to dual
comparator
IC213.
1C
2
1 3
and associated
circuitry is functionally
identi-
cal to
the
control circuitry in the broadband
AGC
with
one exception.
Instead of
a
complex
release time cir-
cuit, the high-frequency limiter utilizes
a simple resistor,
R283,
to determine
the release time.
For
other
details,
the
reader is referred to the
previous
section.
De-emphasis and Output
Stage:
In
order to restore flat response at low levels,
a com-
plementary de-emphasis must be applied to the pre-em-
phasized signal. This is done by
IC215A
and associated
circuitry.
IC215A is
augmented by
a
discrete comple-
mentary-symmetry output stage, which is inside
IC215A's
feedback loop. This
output stage
makes
it
possible
for
IC215A
to drive 600 ohm loads
with
low distortion.
In order
to permit the
de-emphasis to
be determined
by
means
of
a
single resistor,
a
differential de-emphasis
scheme is used.
C253
forms a
6 dB/octave highpass
fil-
ter
when loaded by
R267, 269
plus
a
parallel resistor
(R
1
17,
119,
121, 123).
One
of these resistors is selected
by the front-panel TIME
CONSTANT
switch from
terminal
7
to
ground.
R267,
269 form
a
voltage divider which
divides
the output
of
said highpass filter by 2. The pre-em-
phasized signal is applied to the inverting input
of
IC215A
through
R271
.
The same signal is
applied to the non-
inverting
input of
IC215A through
the highpass filter.
1C
2
1 5 A
thus subtracts the highpassed signal from the flat
signal,
yielding a
lowpassed (de-emphasized) signal
as de-
sired.
The
time constant
of
this de-emphasis is
accurately
determined by
a
single precision resistor
between terminal
7
and ground.
The output stage
is largely conventional.
Q201
,
205
are
diode-connected and thermally
connected
to
their
associated
output
transistors,
Q203, 207,
to
stabilize
bias through
the
output stage and operate
the
output
stage class AB.
R275
,
277
provide local feedback
to
improve
DC
stability. They also work with
CR217,
219
to short-circuit-protect the output stage. If
the
current
through
Q203
or Q207 forces the voltage across R275 or
R277 to exceed the turn-on
of
CR217, 219
(approximately
0,6 volt), then
CR217,
219 will turn on, shunting drive
current
away
from
the output transistors
G203, 207,
and
protecting
them from
burnout.
R279 loads IC215A and provides
current drive
for
Q207
on
negative
half-cycles.
6
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