Orban 418A - Operating Instructions

by voice can
occur
to whatever
degree
desired by adjust-
ing how hard the voice drives
the
418 (and thus,
the
re-
sulting degree of gain reduction
obtained)
.
OPERATING INSTRUCTIONS
Connect the 418 to a source
of AC
power correspond-
ing to the voltage for which
the power
transformer
is
strapped
(see INSTALLATION:
ELECTRICAL
above).
Turn the AC
POWER switch
ON. The neon
pilot lamp
should
glow.
All
metering
positions should read
zero,
except
for
GAIN
REDUCTION
(100%
with
no signal),
and +15
and
-15
monitor the
positive and
negative
regulated
power
supplies respectively, and
verify that
these are
working
properly.
The
L IN
and
R IN
positions
monitor
the input
levels
after
the input
"active transformer"
differential
amplifier,
but
before the
input attenuator.
This
position is
princi-
pally useful in verifying
whether signal
exists
on the in-
put
line,
and if
so
the
approximate
signal
level.
"O" VU
on the
meter
corresponds to
approximately
+4
dBm
(
into
600
ohms
)
equivalent
input level.
The
L OUT and
R
CUT positions
monitor the
output
before
the
output
attenuator.
The
G/R
(GAIN
REDUCTION)
position indicates
the
approximate
amount of
broadband
gain
reduction
in
dB.
The amount of
gain
reduction
is determined
by the
input level
being
presented
to the limiter;
this can
be
affected
by the
drive level to the
418,
and
also by
the
418's
INPUT
ATTEN
control.
As this control
is
turned
clockwise, the
amount of
gain
reduction
will
increase.
If the
operator attempts to
exceed
approximately
15
dB
gain reduction, the
red
OVERLOAD lamp will
light,
and
distortion
and
severe
high-frequency
loss may be
per-
ceived
.
The
RELEASE
TIME control
is not
a
release time
con-
trol in
the
classic
sense,
because automatic circuits in-
side the 418
are
constantly
analyzing the
program
mate-
rial
and continuously
varying
the
release time
to
mini-
mize audible
limiting
"action".
The function
of
the
RE-
LEASE
TIME control
is
to
adjust the
speed of this
entire
process.
As
the
release
time
is speeded
up
(control
to-
wards "fast"),
the 418 will
increase
the
average level
("density") of the
program
to
a greater and
greater
ex-
tent. As the
RELEASE
TIME control
is moved into
the
last quarter of
its
range
towards
"fast", there is
greater
and greater danger
that the limiting
process
will
produce
objectionable
results.
In general, it
is desirable to
use
less
than
6 dB
gain reduction
when
operating with
such
fast
release times.
As
the release
time is slowed
down,
more
and more
gain reduction
can be
used
without
signi-
ficant
audible
side-effects.
Slower release
times are
particularly
useful for
"gain
riding" functions,
such
as
mixing through
the limiter,
where no
change
in the
qual
-
ity
of
the sound is
desired,
but rather
protection
from
excessive level
which
would
tend to
overload the follow-
ing
recording
medium
is wanted.
The
automatic release
time
adjustments
assure that
heavy transients
(like kick
drum)
do not knock
"holes"
in the
audio, while simulta-
neously
instruments like
French
horn or
strings
do
not
"pump". Slower
release times
are
also useful
for prepar-
ing
masters for
cassette
duplication,
where
acceptable
signal-to-noise
ratios must be
obtained
by
a certain
amount
of
compression of the
master.
Most recording
media (with the
exception
of 15 and
30
ips
tape) are significantly
more subject
to overload
at
high
frequencies than
at low frequencies,
due to the
application of record
pre-emphasis.
Cassettes are
par-
ticularly
problematical.
The high-frequency
limiter
in
the
418
can
be
switched
in to
automatically
control the
high-frequency
content
of the audio
so as
to
avoid
audi-
ble
high-frequency
overload
and
accompanying distor-
tion.
The four
"time
constants" do not refer
to attack or
release times (which
are not adjustable),
but
rather to
the
frequencies
at which the control
threshold
is
3
dB
below the
broadband
(low
frequency) threshold.
The
following
table
gives these
frequencies,
as
well
as sug-
gested
uses
for
each time
constant:
25 uS 6.37 kHz 15 ips
tape
37.5
uS
4.24 kHz 7-1/2
ips tape
(modern
oxide);
RIAA
disc (allowing
6 dB
headroom)
50 uS
3.18
kHz
3-3/4 ips tape; 7-1/2
ips
broadcast
tape cartridge
75
uS 2.12 kHz
1-7/8
ips tape; cassette;
optical
film;
RIAA disc (al-
lowing
no high-frequency
headroom)
It
should
be noted that
more high
frequency
content
may always
be
recorded on
a
given medium
by reducing
the low
frequency level,
thus allowing more
headroom
for high-frequency
pre-emphasis.
In
some
cases, this
may be
a
desirable
expedient to
achieve
a
brighter
sound.
In other
cases, the
loss
of
signal-to-noise ratio
may be
intolerable.
If the former
expedient
is adopted,
the
high-frequency
limiter
may be
s'Bt on
a
higher fre-
quency
time constant
than normal,
thus
reducing the
amount of
high-frequency
limiting.
Because
of
the com-
plex nature of
high-frequency
overload distortion,
the
ear
should
always be the final
arbiter
of the proper set-
ting
of the
TIME CONSTANT control
.
Further details
of
418
operation are
largely dependent
upon the
sound and effect
desired from
the
device. Ex-
perimentation in
actual
use situations
is the
best way to
develop
a
knowledge of
the
418's usefulness
as
well
as
its
limitations
.
MAINTENANCE
Preventive
Maintenance:
The 418
is
an entirely
solid-state device. The
only
preventive
maintenance
required
is
keeping the unit
clean.
Dust on the
circuit
board can absorb moisture.
3