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The
output
may
be
used
to
drive either
balanced
or
unbalanced-and-floating
inputs of
external
equipment.
A
three
conductor
shielded cable should
be
used, with
the
inner
conductors
connected
to the
418
output
and
signal
ground,
and
the
shield
terminated
to
the
418
chassis
ground,
and
unterminated
at
the other
end.
The comments
concerning ground
loops
in
INSTALLATION:
MECHANICAL
(above), should be noted.
In
most
cases,
the
balanced
input
will
provide
ground-loop
protection
if
grounding
is
effected
as
described
above.
The
418's
VU
meter,
in
OUTPUT
positions,
is
connect-
ed
to the
output
of
the 418's
line
amplifier before the
OUTPUT ATTEN
control
in
order
to
avoid
reflecting
meter
rectifier distortion into
the output.
Therefore,
this
metering
position
will
not
show any
adjustments
of
the
OUTPUT ATTEN
control
First,
the attack time of the
418
is
moderate
(in
the
order
of
2
or 3
milliseconds)
.
Therefore,
fast
-rising
waveforms
will
overshoot
at
the
418
output.
The
over-
shoots are sufficiently
short so
that
they can
be
clipped
by
the recording
or
transmission
medium
following the
418
without audible
ill
effects,
provided
that said
media
can
themselves
clip.
Examples
of suitable
media
are
magnetic
tape (whose
saturation characteristic provides
an
ideal "soft"
clipping
characteristic),
and
optical
film,
which
has a
much
more sudden
clip point,
but
which
can be
clipped without
damage
to
the
medium.
On
the other
hand,
disc
recording
requires
absolute
protection
in
order
to
avoid overcutting and/or
cutter
lift.
Therefore,
in
the case
of disc transfer,
the
418
should
be followed by
some
sort
of
absolute protection,
such
as
a clipper
or
a very
fast
limiter.
Power:
The power
transformer
can be wired
for
105-125
VAC
or
210-250
VAC
operation,
50
or
60
Hz. The
nominal
voltage
for
which
the
unit
is
wired
is
marked
on
the
carton.
If
the
unit
is
wired
for
230
volts,
a
warning
tag
is
also affixed to the
line
cord.
To change
line
voltage,
remove
the top
cover
to
re-
veal the
power
transformer.
For
115
volt
operation,
connect
terminal
1
to terminal 2,
and
terminal
3 to
terminal
4
(
See
figure
1
)
.
For
230
volt
operation,
connect
terminal 2 to terminal 3.
When
altering the
position of the
jumpers, take great care
not to
overheat
or
bend
terminals
or
the
power
transformer
may
be
damaged.
Do
not
rearrange the
insulated
wiring.
a
.
Power
transformer
wired
for
115
volts
b
.
Power
transformer
wired
for
230
volts
Figure
1:
Power
transformer wiring
If
the
418
is
to
be
used
in
a
situation
where ground
loops
may
be
introduced through
the
power
line,
an
al-
ternative to
fitting
a
three-prong
to
two-prong
adapter
on the
AC
line
plug
is
to
disconnect the green wire
emerging
from the
line
cord
inside
the
chassis.
However,
be
sure that the chassis
is
grounded
through
one
(and
only
one) path
when
the
418
is
installed.
The second
important consideration
is
the
fact that
the
gain reduction capability
of the
broadband
AGC
section
of the
418
has
been
purposely limited to
1
5
d
B
.
This
was done
to obtain a
maximally smooth
characteristic
from the FET attenuator,
and
also to assure that
excessive
compression
(with
accompanying
unpleasant
side-effects)
cannot be
used.
It
is
therefore
essential to
choose
the
input
operating
level
with a certain
degree
of care,
com-
pared with operation
of
compressors
capable
of,
say,
30
dB
compression.
This care
can only
result
in
better-
sounding product,
and
we
therefore consider
it
a
distinct
advantage.
If
attempts are
made
to
exceed
the permissible
AGC
gain reduction range,
the
front
-panel overload
lamp
will
light.
Simultaneously,
a brief
dropout
of
high-frequency
content
will
be noted, because
the
AGC
output
will
overshoot dramatically
and
force the
high-frequency
limi-
ter into
maximum
de-emphasis.
These
conditions are
easily
avoided by
use of
the
GAIN
REDUCTION
metering
position,
which
will
clearly indicate
when
a potential
overload
situation exists.
Third, both
INPUT
and
OUTPUT
attenuators are stereo-
ganged
controls.
They were
configured
this
way
in
order
to
maximize
the
speed with
which
adjustments could be
made
without concern over
unbalancing
the stereo channels.
However,
this
means
that
initial
channel
balance
must
be
done
on the
equipment
driving the
418.
Lastly,
the
418
is
not
two
limiters,
but rather
a
stereo
device
utilizing
a
single control
loop
for
both channels.
The
gains of the
two
channels are forced
to
track
each
other,
and
the
amount
of
gain reduction
is
determined by
the louder channel
at
any
given
moment.
This
means
that
the
two
channels
of the
418 cannot be
used
for
two
inde-
pendent program
sources;
if
the
418
is
to process
mono
material, either
channel
may
be
used,
with the other
channel
carrying
no
signal.
OPERATING CONSIDERATIONS
In
order to use the
418
most successfully,
some
under-
standing
of
its
particular
operating
characteristics
is
required.
There
is
at least
one
specialized
use
for this
facility:
if
a
disc
jockey wishes
to
have voice-actuated "ducking
1
of
music while doing production,
one channel
of
the
418
can
carry
his
voice, while the other channel
can
carry
music
at
a lower level.
Automatic ducking
of
the music
2
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