dbx 160 Buku Petunjuk - Halaman 6
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OPERATION
Input
Impedance
&
Terminations
There
is
sometimes
a
misunderstanding
regarding the
nature
of
matching and
bridging
inputs,
the use of termi-
nating
resistors,
and
the
relationship
between
actual
input
impedance and nominal
source impedance.
Most
electronic
outputs
work
well
when
"terminated" by an input
(con-
nected
to
an
input)
having the
same
or
a
higher
actual
impedance. Outputs
are usually
overloaded
when
terminated
by
an
impedance
that
is
lower than the source
impedance.
When
the input
impedance
is
nearly the
same
impedance
as
the source,
it
is
known
as a
"matching" input
When
an input
is
10-times the source
impedance,
or
more,
the
input
is
considered to be
a
"bridging"
input.
The dbx 160 and
161 have
respective actual
input
impedances
of
50,000
ohms
and 25,000
ohms
(they are
high-Z
#
inputs).
This
makes
the
dbx
inputs
suitable for
use
with
virtually
any nominal
source impedance,
low
or
high.
The dbx
inputs
will
bridge
150-ohm
or
600-ohm
(low-Z)
lines,
and
will
match 10,000-ohm
or greater
impedance
(high-Z)
lines.
It
seldom
is
necessary
to place
a
600-ohm
"terminating
resistor"
across the
input
of the
dbx
unit.
In
fact,
most
600-ohm
outputs operate normally
when
bridged
by
a
high
impedance;
it is
as
though
no
load
were
connected
to the
source
device.
The
only
instance
where
a
terminating
resistor
may
be required
is
when
the
manu-
facturer of the
source device
specifically states that
a
terminating
resistor
is
necessary.
In
such
cases,
there
is
usually a
special
type
of
output
transformer
in
the source
device,
and
the terminating
resistor
assures
optimum
frequency response
in
that device.
Terminating
resistors
are
not
needed
for
the
dbx
unit to
operate
correctly.
If
a
150-ohm
or
600-ohm
resistor
is
specified for the
source
device,
it
should
be
installed at
the
end
of the
cable nearest
the
dbx
unit
in
order
to
minimize
possible
hum,
noise or
signal losses in
the
cable.
•"Z"
is
an accepted
abbreviation
for
"
impedance
.
"
Power
Switch
Depress the
"Power"
switch
for
the
160
or 161.
The
"BELOW THRESHOLD"
LED
and
the
meter lamps should
illuminate.
It
is
normal
for
the
"ABOVE THRESHOLD"
LED
to
flicker
with
no
input
signal
applied during the time
when
the
power
is
turned
on
or
off.
Threshold
&
Compression
Ratio
Adjustments
INITIAL
CONTROL
SETTINGS
THRESHOLD
fully
clockwise
(3V),
OUTPUT
GAIN
at
"12
o'clock" (OdB),
COMPRESSION RATIO
at
the
appropriate
ratio,
low
settings for
compression
(1:1 to 4:
1
),
high
settings for limiting
(10:1 to
infinity).
PROCEDURE
Apply
normal-level
program
material to the input.
The
BELOW THRESHOLD
LED
will
remain
on,
except
when
input
levels
exceed
the threshold
setting.
The
ABOVE
THRESHOLD
LED
indicates
when
compression
is
taking
place. Starting
with the
THRESHOLD
fully
clockwise,
rotate
it
counterclockwise
until
the
ABOVE THRESHOLD
LED
begins the
flicker.
At
this setting,
compression
will
begin
whenever
the input
level
exceeds
the threshold
setting.
Further counterclockwise
rotation of
the
THRESHOLD
control
will
cause
compression
to begin
at
a
lower point
relative
to
the
maximum
input
level.
For
a further discussion
of the use of the
COM-
PRESSION
RATIO,
and
THRESHOLD
controls,
refer
to
the
final
section of
this
manual,
"COMPRESSION
RATIO,
A
REVIEW."
NOTE:
The
160's
ground-loop compensation
circuitry
and
power
turn-on
turn-off transient
protection
circuitry
operate
normally
at
any
settings
of front panel
controls.
Output Gain
Control
Adjustment
When
the
160
or
161
is
used
as
a
compressor,
OUTPUT
GAIN
can be used
to increase
overall
level
that
is
partially
decreased
by
compression.
The
effect
is
to
raise
the average
level
of the
program
material,
while decreasing
its
dynamic
range.
To
increase the
gain,
rotate the
OUTPUT
GAIN
control
clockwise
past the
"OdB"
position; to
decrease the
gain,
rotate the control
counterclockwise.
Audio
signals
often have peaks
that are
20dB
above
VU
meter
readings
(VU
meters
indicate
average
levels).
Even
when
compressed
at a
2:1
ratio,
such peaks can
still
reach
10dB
above VU-indicated
levels.
To
avoid
clipping,
use
an
average input
level,
such
as
-10
to
+8dB,
that
is
below
the
maximum
specified
input
levels
(+21
dB
for
the 160,
+17
dB
for
the 161
).
When
the
COMPRESSION RATIO
is
set
at a
low
factor',
extreme
clockwise
rotation of
OUTPUT
GAIN
may
cause
the
160
or
161 output
stage to
clip
program peaks
.
.
.
even
when
maximum
input
levels
are
not exceeded.
Due
to the
fact
that
20dB
of gain
can
be
added
in
the
160
or 161's
output
stage, raising
the
output
level
substantially
above
the input
level
may
cause
clipping.
It
is
suggested
that,
for
normal
operation,
OUTPUT
GAIN
be
set at
12 o'clock (OdB)
position.
Meter
Calibration
&
Use
The
meter
in
the
160 and
161
is
factory calibrated to
read
"0"
at
+4dB
(1.23V)
output
level.
To
recalibrate
the
meter, depress the
INPUT
LEVEL
meter
function
switch.
Feed
a
1kHz
signal, at
your
selected
nominal
operating
level
(the
level
desired for
a
"0"
meter
reading) to
the
com-
pressor/limiter input.
Then
adjust the
160
or
161
meter
calibration control
(on the
rear
panel)
until
the
meter
indicates
"OdB".
To
check
the
meter
calibration, rotate
THRESHOLD
fully
clockwise
past
the 3
V
position,
and
set
COMPRESSION RATIO
completely counterclockwise,
(to
the
"1:1"
position).
Connect
an
accurate,
VU-reading
voltmeter to
the
160
or
161
output
terminals,
and
adjust
the
OUTPUT
GAIN
control
to
produce
a
reading
on
the
outboard meter
that
is
identical
to the input
level.
Then,
depress
the
meter
OUTPUT
button
on
the front
panel.
If
the
160 meter
still
reads
"OdB",
the unit
is
properly
calibrated.
The 160
or
161
as
a
Line Amplifier
To
use either
model
as
a
line
amplifier, adjust
COM-
PRESSION RATIO
to
its
maximum
counterclockwise
position ("1:1"),
THRESHOLD
to
its
maximum
clockwise
position
("3V"),
and
OUTPUT
GAIN
to
whatever
setting
is
needed
for
the application.
Remember
that, as
with
any
amplifier, excessive gain
may
cause
output
clipping of
high-
level
signals (see
"Output
Gain Control
Adjustment"
in
preceding
paragraphs).
To
add
compression,
adjust the
COMPRESSION RATIO
and
the
THRESHOLD
to the
desired
settings.
*The term "
factor
"
refers
to the
compression
ratio.