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INTRODUCTION
The
principal use of
your
new
418
Limiter
is
to
condi-
tion arbitrary
complexaudio
signals (not
just
single
tracks,
although
the
418
may
be
used with these,
too)
in
such a
way
that
they can be recorded on tape
or
opti-
cal film
without overload, excessive
noise, or distortion
due
to
excessive high frequencies.
In
order
to
do
this
while
introducing minimal
artifacts into
the signal, the
418
incorporates a highly refined
program-controlled re-
lease
time
circuit, as
well
as
a separate
high-frequency
limiter
which
can
control
the high frequencies present
in
the
program
on
an
instantaneous
basis
without
distortion
and
without
disturbing the basic
program
loudness.
The
release
time
and high-frequency
limiter
are both
adjustable by the
user
in
order
to
obtain the desired
amount
of
high-frequency
control, level control,
com-
pression,
and
density
augmentation
.
The
controls
have
been
arranged
to
minimize
the probability
of
audible
side-effects
of
an
undesirable nature regardless
of their
settings
Some
uses of
the
418
include:
(1)
mixing through
while doing
multitrack reductions
in
order to
save time
while doing
radio
commercials,
demo
sessions,
and
the
like;
(2)
conditioning recorded
program
material
so that
it
can be copied onto
cassette
or
low-speed tape without
excessive
hiss
and/or high-frequency
distortion;
(3)
trans-
fering
from
magnetic
to
optical
film;
(4)
transferring
from
disc to
7.5
ips
tape
cartridge
in
broadcast
stations;
(5)
re-
cording production
work
through
in
broadcast production
studios;
and
(6)
transferring
from tape to
disc (with certain
limitations
—
see
OPERATING
INSTRUCTIONS).
Other
applications
will
doubtless suggest
themselves
to
the
creative
engineer
The 418
is
not rigorously
RFI/EMI
shielded,
and
its
output contains
substantial
amounts
of
fast
overshoots.
It
is
therefore specifically not
recommended
for
driving
broadcast
transmitters.
Orban
Associates Division
manu-
factures a
line
of
devices, the
OPTIMOD-FM
and
the
OPTIMOD-AM,
specifically
designed
for this
purpose.
INSTALLATION:
MECHANICAL
The 418
Limiter
is
designed
to
mount
in
a
standard 19"
(48.26
cm) rack,
and
requires
3.5"
(8.89 cm)
of vertical
space.
All
operating
controls are
accessible from the
front.
The
fuse,
AC
line
cord,
and audio
connections
are
made
from the
rear.
The audio
connects
to
a
Jones
140-type
barrier
strip
(^5
screw),
and
connections can
be
made
by
means
of
spade
lugs or
a
fanning
strip, in
those
cases
where quick connects/disconnects
are required.
In
a
rack
mount
installation,
the
418
will
pick up
its
chassis
ground from
the rack.
Be
sure to
measure
the re-
sistance
from
chassis to
rack
after installation
and
correct
any
high-resistance
situations
before proceeding.
It
may
be
necessary
to
scrape the paint from the rack
and/or
the
rear of the
panel
in
order
to effect
an adequate ground.
It is
advisable
to
make
sure that
the rack
is
grounded
to
some
earth
ground
simultaneously.
Grounding
of racks
and
other
equipment
to
power
line
conduit grounds
as
a
sole
means
of
grounding
often creates
troublesome prob-
lems.
In
rack
-mounting
the
418, very
strong
AC
magnetic
fields
should
be avoided because
these
can introduce
hum
into
the input transformers.
In
addition, the
418
should
not
be
mounted
directly
over
equipment producing
large
amounts
of
heat,
like
vacuumtube
power
amplifiers.
Ambient
temperature should
not
exceed 45°C
(11 3°F
when
the
418
is
powered.
If
the
418
is
rack-mounted,
the
jumper on the
rear
barrier
strip
connecting
terminal
7
(signal
ground)
to
terminal
8
(chassis
ground) should be
removed
to
avoid ground
loops.
If
the
418
is
used
in
a
portable
situ-
ation,
this
jumper should be retained
in
order to assure
that the chassis
is
grounded.
When
the
jumper
is
retained,
it
is
advisable
to
terminate
the
line
cord with a three-
prong
to
two-prong
AC
adapter
in
order
to
avoid
intro-
ducing ground
loops
through the
AC
power
line
grounding
system.
The
center (ground) prong on the
418
line
cord
(green
wire)
is
connected
directly to the chassis.
INSTALLATION:
ELECTRICAL
Input;
The
input of
the
41
8A
is
a
100K
ohm
balanced
bridging
input.
It
is
synthesized
by means
of
an
electronic
differ-
ential amplifier;
no
transformer
is
used.
Absolute
input
clipping occurs
at
+21
dBm;
higher
levels
require use of
an
external
balanced
pad.
With
the
INPUT
ATTEN
full
clock-
wise,
-10
dBm
will
produce
10
dB gain reduction.
It
is
important
that
both
(+)
and
(-)
inputs
be
driven
by
a source
impedance
of
600 ohms
or
less
in
order
to
assure best
"common mode
rejection" (i.e.,
ground
loop
hum
rejection).
If
the
device
driving the 41
8A
has a
balanced
output, the
two
output
leads
should
be
driven
directly
into
the
(+)
and
(-)
inputs
of the 41
8A.
If
the
device
driving the 41
8A
has
an unbalanced
output,
it
snould
also
be
connected
to
the 41
8A
with a
two
conductor
shielded cable.
The
black wire should be connected bet-
ween
the driving device's
signal
ground
and
the 418A's
(-)
input.
The second
wire should
be connected between
the driving device's output
and
the
418A's
(+)
input.
This
arrangement
takes
maximum
advantage
of the
hum-
reducing
ability
of the
418
A's
balanced
input.
In
either
case,
the
shield of
the interconnecting cable should
be
connected
to chassis
ground
at
one end
only.
No
special
RF
suppression
techniques
have been
used
in
the
418A.
If
RF
interference
is
experienced
in
high RF
fields,
we
suggest
bypassing audio
inputs,
outputs,
and
the
power
line to chassis
ground through 0.001 mfd ceramic
capacitors with
short as possible leads.
Be sure
that the
voltage
rating of
the
power
line
bypass capacitor
is
at least
1
.6
kV, and
that
bypassing occurs
after
the
fuse to
avoid
a potential
fire
should the capacitor
short
.
Because
the
chassis
metalwork
has not
been designed
to
be
rigorously
RF-tight,
it is
unlikely
that
the
418A
could
be
operated
immediately adjacent
to
a transmitter.
In
a broadcast
production
studio, sufficient
RF
suppression usually
exists
so that successful
operation
can be
easily
obtained.
Output:
The
output
of the
418
is
unbalanced, and
follows the
OUTPUT ATTEN
control,
which
is
configured
as
a standard
potentiometer.
Maximum
output resistance occurs with
this
control
at 12
o'clock;
in
this
case, the output
resis-
tance
is
approximately 400 ohms.
Maximum
available
level
is
+1
3 .4
dBm
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