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10
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 near ly 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 16
1
have respective actua
l
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 h igh.
The dbx inputs wil
l
br idge
150-ohm
or
600-ohm
(low-Z)
lines, and will match
10,000-ohm
or greater impedance
(high-Z)
l
ines, 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
term inating resistor may be
required
is when the manu-
facturer of the source device specifica
ll
y 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.
T
erminating 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."
OPERATION
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
l.
high settings for limiting (10:
1
to infinity).
PROCEDURE
App ly normal-level program material to the input. The
BELOW THRESHOLD
LED
wi
ll
remain
on, except when
input levels exceed the threshold setting. The ABOVE
THRESHOLD
LED
indicates when compression is taking
place.
Starting with the TH RESH OLD 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 counterc lockwise rotation of the THRESHOLD
control wi
ll
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 norma
l
ly at any settings of front panel controls.
Output Gain
Control
Adjustment
When the 160 or 161 is used as a com press or, OUTPUT
GAi N 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, whi le decreasing its dynamic
range. To
increase
the gain, rotate the OUTPUT GA IN
control clockwise past the
"OdB"
position; to decrease the
gain, rotate the control counterclockwise.
THRESHOLD
fully clockwise past the 3V position, and
set COMPRESSION RATIO comp letely counterclockw ise,
(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 identica
l
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.
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 stil
l
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
(+21dB 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 cl i P
program peaks
...
even when maximum
in
put 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
substantia lly above the
input level
may cause
cli
pping.
It is suggested that,
for
normal operation,
OUT
PUT
GAIN be set at 12 o'clock (OdB) position.
Meter
Calibration
&
Use
The meter
in
•
t he 160 and
161
is factory calibrated to
read
"O"
at +4dB (1.23V) output level. To recalibrate the
meter, depress the
IN
PUT
LEVEL
meter function switch.
Feed a 1 kHz signal, at your selected
nominal
operating
level
(the level desired for a
"O"
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
*The term "factor"
refers to the compression ratio.
The 160
or
161
as
a
Line Amplifier
To use either model as a line amp
l
ifier, adjust COM-
PRESSION RATIO to
its
maximum counterclockw ise
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 Contro
l
Adjustment"
in
preceding paragraphs).
To
add compression, adjust the
COMPRESSION RATIO and the
TH
RESHOLD to the
desired settings.
11