Adjusting The 424A To Provide; Protection - Orban 424A Operating Manual

SOME
SPECIFIC
APPLICATIONS
Adjusting the
424A To
Provide Peak Overload
Protection
The
typical
"peak operating
level" of
an
analog tape recorder
is
that level
producing
3%
total
harmonic
distortion; of a
power
amplifier, that
level
which
drives
the amplifier to
its
rated output voltage
(power
will
vary according
to load
impedance); and
of
an
STL
transmitter, that
level
which drives the transmitter to
100%
modulation.
The
following procedure
matches
the
424A
to
the driven device.
It
assumes that
the
input
attenuator
of
the driven
device
(if it
exists)
is
fixed,
and
adjusted
to
produce
approximately correct operating
levels
when
driven
by
a
+4dBm
or
+8dBm
line.
(If
the
driven device
is
significantly
more
sensitive,
you
may
find that the
424A
OUTPUT
ATTENuator
must be operated very
close to
its
counterclockwise
end
stop,
and
may
be touchy to
adjust.
In
this case,
it
may
be
desirable to insert a
loss
pad between
the
424A
and
the input of the driven device.)
First, you
must decide
if
you
will
leave the control
loop
in
its
normal
"bass
rolloff"
mode, or defeat the bass
rolloff so
that the control
loop
can
sense the
true
peak
level of
the output.
We recommend
leaving the control
loop
in
"bass
rolloff"
mode
to
achieve
most
natural
sound, and
to control
any
transient
peak
overshoots which
may
occur by permitting the
VCA
to clip
them. However,
if
you
wish
to use the
424A
as
a fast-attack true-peak
limiter,
then you should
restrap
the control
loop rolloff
according to instructions provided
in
the Control
Loop
Bass
Rolloff paragraph
in
Part
B
above.
After
you have restrapped the control
loop as
necessary, apply
a
400Hz
sinewave
oscillator
to
the input of the
424A. Switch both 424A OPerate/DEFEAT
switches
to
DEFEAT,
and
adjust the
424A INPUT ATTENuator
and/or the
oscillator
output
attenuator
as
necessary
to
produce
a reading
of
"+2dB" on
the
VCA LEVEL
meter.
This
level
is
within IdB
of the clipping level of the
VCA
output. Adjust the
424A
OUTPUT
ATTENuator
until
the driven device
is
driven
to
the following
level:
Analog Tape Recorder: 5dB above
"Operating Level"
(i.e.,
5dB above the
nominal 1%
total
harmonic
distortion level).
Audio Power
Amplifier: IdB
below
clipping.
Studio-Transmitter Link Transmitter:
90%
modulation
if
flat;
40%
modulation
if
pre-emphasi zed
at
50 or 75us.
(NOTE: The
40%
modulation figure
may
be
increased
if
the
de-esser
is
used
to control occasional
high frequency
peaks.
However, beware
of
possible audible side-effects
if
this
is
attempted!)
U.S.A. Standard Telephone
Line: +
l6dBm
into
the
line
(through
a
Telco-
supplied repeat
coil).
(NOTE: Place a 240
ohm +5%
1/4
carbon
film resistor in
series
with
each
leg of the
424A balanced
output
to
assure that the
phone
line
is
driven by a true 600
ohm
source impedance.
The +l6dBm must
be
measured
on the phone
line side of the resistors.)
Other Devices: IdB below
the
nominal overload
point
if
flat;
if
pre-
emphasized, on the
basis of
experience.
19