Orban 8182A - Page 8

A-2
Orban
Circuit
Description
of
8182
Bargraph
LED
Bargraph
GAIN
REDUCTION
Meters
The
gain
REDUCTION
meters
each consist
of ten
comparators
with
open
collector outputs.
The ten
LEDs
in
the bargraph
are
connected
in series,
and a
current source
forces a
con-
stant
current into
the LED
string. As
the
magnitude
of the
input
voltage
to the
compara-
tors
decreases,
they
progressively
shunt to
the
—15V
rail the
current that
would
ordinarily
flow
through
the LEDs,
turning off all
LEDs
closer to
the
-15
V rail. The
comparators
are
biased
with
equal
voltage
increments,
producing
a
meter
whose scale
is linear.
Because
the
gain
control
voltage is
linear in dB,
this
produces a
dB-linear
meter scale.
A
250Hz
“dither”
signal is
injected
into the
reference
voltage
string,
modulating the
turn-
on
threshold
of
each comparator
and
causing
the
LEDs to
appear to
smoothly
fade on and
off instead
of
switching
abruptly.
Component-Level
Description:
All four
meters are
electrically
identical.
However, the
LIMITING
meter
has a
different
front-panel scale
(0
to 5dB
instead
of
+
10
to
-
15dB)
and is
driven
from
a
differential amplifier so
it can
indicate
the
difference
between
the
slow
compo-
nent and the
peak-held
TOTAL
component
of
the master
gain reduction.
We
will
describe
Meter
1
(the
TOTAL meter).
Current source
Q
1 ,
R7,
R5, R6,
CR5
applies a
constant
current
to the series-con-
nected diodes
in
LED
assembly CR1.
A
string
of
ten comparators
IC1,
IC2,
IC4-A,
IC4-B,
receives the
negative-going
dB-linear
gain reduction
voltage.
A
voltage divider
string biases
the
comparators
with
DC
reference
voltages
from
-0.0746
to
-0.746VDC in
0.0746V
steps.
In
meters
1,
2,
and
4 each
0.0746V
step
corresponds to
2.5dB
additional gain
reduction. In
meter
2,
each
0.0746V
step
corresponds
to
0.5dB
additional
gain
reduction.
When
the
gain
reduction is
less
than
the first step
(2.5dB or
0.5dB,
depending
on
meter),
the gain
reduction
voltage is
less than
0.0746V and the
outputs
of
all
comparators
are active
negative,
shunting
current from
the
series-connected
LED
string. As the
gain
reduction
increases, the output
of
each
comparator
in
turn becomes
open
collector
and
current is
permitted to
flow in
the LED
string
until it
encounters a
comparator
whose output
is still
active
negative, at
which
point
the active
comparator
shunts
the
current away from
the rest
of the
LEDs.
A250Hz
triangle-wave
“dither” signal
is
injected into
the
reference
voltage
string
through
Rl. This AC
signal
modulates
the
turn-on
threshold
of each
comparator,
causing the
LEDs to
appear to
smoothly
fade
on and
off instead
of
switching
abruptly.