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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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