Universal Audio LA-3A User manual

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lliJ@D
INSTRUCTION
MANUAL
LEVELING
AMPLIFIER
MODEL
lA-3A
products
of
UNITED
RECORDING
ELECTRONICS
INDUSTRIES
11922 VALERIO STREET,
NO
.
HOLLYWOOD
,
CALIFORNIA
91605
TEL. [phone]
••

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INSTRUCTION
MANUAL
LEVELING
AMPLIFIER
MODEL
l.A-3A
Manufactured
in
USA
by
UNITED
Recording
Electronics
INDUSTRIES
11922
Valerio
Street
North
Hollywood,
California
91605

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TELETRONIX
MODEL
LA-3A
LEVELING
AMPLIFIER
INTROIXJCTION
The
Teletronix
Leveling
Amplifier
will
automatically
reduce
audio
peaks
which
might
otherwise
over
drive
broadcast
or
re~ording
e~ip-
men
t.
Automatic
gain
reduction
is
accomplished
by
the
use
of
an
electro-
optical
voriabl~
attenuator,
which
is
placed
ahead
of
the
first
ampli-
fier
stage.
The
attenuation
is
controlled
by
the
amplitude
of
the
lA-3A
input
signal
0
This
~ystem
permits
up
to
40
dB
of
instantaneous
gain
red~ction,
yet
causes
no
significant
wave
form
or
harmoni~
distortion,
TJ'le
amplifier
provides
suffi~ient
gain
and
output
level
(+24
dBm
nominal)
to
be
used
qs
a
line
or
program
amplifier,
or
for
direct
connection
to
the
transmitter
in
the
case
of
radio
or
TV
operation.
Provi~ions
are
made
for
interconnection
of
two
lA-3A
Limiters
to
provide
equal
gain
reduction
in
both
channels.
1.
Gain
Reduction
2.
Distortion
3.
Respon~e
4.
Noise
SPECIFICATIONS
Up
to
40
dB
Less
than
0.35
%
tqtal
harmonics
at
+24
dBm
+
0oS
dB,
20
Hz
to
20
kHz
80
dB
below
program
level
at
threshold
of
limiting
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5.
Gain
6.
Output
Level
7.
Input
Level
a.
Attac~
Time
9.
Release
Time
10.
Input
Impedance
11.
Output
Impedance
12.
Output
Source
Impedance
13.
·
Panel
Size
14.
Depth
Behind
Panel
15.
Panel
Controls
16.
Meter
17.
Power
Requirements
18.
fuse
30
+ 1
dB,
or
50
!
1 dB
selected
by
switch
on
bac¾
of
Limiter
+20
dBm
nominal;
+27
dBm
max.
peaks
0
dBm
max.
~ith
sq
dBm
gain
setting
+20
dBm
max.
with
30
dB
gain
~etting
1.5
ms
or
less
depending
on
de~ree
of
limiting
Approximately
0.06
seconds
f0r
50
%
release;
0.5
to
5
seconds
for
complete
release
dep,nding
~pon
amount
of
previous
r~quction
600
ohms
floating
600
oh~s
float\ng
Appro~imqtely
50
ohms
One-hqlf
rack
9.25
·
inche!:i
Gain
(input
ley~l),
fea~
Reductio~,
Meter
Selector
Switch,
and
Power
S"1itch
dB
Gain
R~duction
and
dB
Output
read
O
VU
at
+4
dBm
output
115/230
v9lts
50-60
cycle
35
W?tts
3AG,
1/8
AMP
Slow
Blow
for
115V;
3AG
1/16
AMP
Slow
Blow
for
230V
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INSTALLATION
Line
Voltage
SWitch
The
LA-3a
is
set
for
a
power
line
voltage
of
115V
~hen
~hipped.
In
order
to
change
this
for
230
volts,
50
/
60
Hz,
slide
the
power
source
sel,ctor
switch
on
the
rear
of
the
unit
to
the
230
volts
~os~lipn.
~
s~all
screw
driver
should
be
used
to
operate
th+s
switch~
19"
RACK
MOUNTING
(Single
Unit)
I ,
The
following
parts
are
furnished
in
the
~R-3A
kit
for
mounting
one
LA-3A
in
a
standard
19"
USASI
rack:
2
Rack
Adapter
Angles
UREI
Part
#2fJ-lJ179
2
Rack
Extensions
UREI
Part
#25-11176
4
6-32
Oval
Head
Screws
(black)
4 No.
6
Lock
Wasners
4
6-32
Nuts
4
8-32
Pan
Head
Screws
4 No.
8
Lock
Washers
Installation
Procedure
1.
Mount
angles
to
back
side
of
rack
extensions
using
black
6-~2
screws,
lock
washers,
and
nuts.
Do
not
tighten
screws
at
thip
ti.me.
2.
Mount
angle
and
rack
extension
assemblies
to
the
tapped
holes
in
the
sides
of
the
1A-3A,
using
8-32
screws
with
lock
washers.
Do
not
tighten
the
screws
at
this
ti~e~
3.
The
LA-3A
package
is
adjustable
to
allow
optimum
C!llignment
of
the
front
panel
with
the
rack
extensions.
To
effect
precise
alignment,
loosen
the
front
panel
mounting
screws
and
the
top
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and
bottom
cover
mounting
screws
about
1/2
turn.
Adjust
rela-
tive
positions
of
brackets
and
panels.
Tighten
all
hardware.
19
11
RACK
K::>UNTING
(
Two
Uni
ts)
The
following
parts
are
furnished
in
the
DR-3A
kit
for
mounting
two
I.A-3A
units
side
by
side
in
a
standard
19
11
USASI
rack:
2
Joining
Plates
UREI
Part
#25-11168
2
Rack
Mounting
Angles
UREI
Part
#25-11162
2
Spacer
Plates
UREI
Part
#25-1116
.3
4.
8-32
Pan
Head
Screws
4
No.
8
Lock
Washers
Installation
Procedure
1.
Place
units
side
by
side,
with
front
panels
touching.
Remove
proper
screws
in
top
of
each
unit
to
allow
joining
plate
to
be
laid
in
place.
Replace
screws
through
plate
and
covers.
Do
not
tighten
screws
completely
at
this
time.
2.
Turn
assembly
upside
down
and
install
other
joining
plate
as
in
(1)
above.
3.
Mount
one
angle
and
one
spacer
plate
to
each
side
of
the
l.A-3A,
using
the
8-32
screws
and
lock
washers.
The
mounting
holes
in
the
LA-3A
are
tapped,
and
no
nuts
are
required.
The
spacer
plates
should
be
located
between
the
angles
and
the
l.A-3A
chassis.
Do
not
tighten
the
screws
at
this
time.
4.
The
I.A-3A
package
is
adjustable
to
allow
optimum
qlignment
of
the
front
panels
and
rack
mounting
angles.
To
effect
precise
alignment,
loosen
the
front
panel
mounting
s~rews
and
the
top
and
bottom
cover
mounting
screws
about
1/2
turn.
Adjust
rela-
tive
positions
of
brackets
and
panels.
Tighten
all
hardware.
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CIRCUIT DESCRIPTION
The
I.A-3A
Leveling
Amplifier
will
produce
essentially
instantaneous
gain
reduction
of
over
40
dB
with
no
increase
in
harmonic
distortion.
~
typical
gain
reduction
curve
for
this
system
is
illustrated
on
Fig~re
1.
Compressor
action
occurs
from
the
breakaway
point
at
-~O
d~
input
and
up
to
-20
dB,
at
which
point
the
curve
b~comes
ho~izontal
t~
exhibit
limiting
action.
The
input
increases
an
additional
20
dB,
but
the
output
increases
less
than
1
dB.
The
leveling
amplifier
thus
com-
bines
the
characteristics
of
a
compressor
and
limiter.
A
reasonable
amount
of
care
in
gain
riding
will
restrict
normal
operation
to
the
compression
region,
but
uncontrolled
output
levels
will
be
prevented
by
the
limited
action.
The
heart
of
the
leveling
amplifier
is
the
electro-opticql
attenu-
ator
which
is
placed
ahead
of
the
first
amplifier
stage.
The
actual
stage
gains
and
operating
parameters
are
not
varied,
permitting
the
amplifier
to
operate
at
optimum
conditions
regardless
of
the
amount
of
gain
reduction.
The
optical
attenuator
consists
of
photo-conductive
cells,
opti-
cally
coupled
to
an
electro-luminescent
light
source.
The
electro-
luminescent
device
provides
a
light
intensity
which
is
proportional
to
the
audio
voltage
applied
to
its
terminals.
Not
unlike
a
capacitor
in
construction,
the
electro-luminescent
lamp
consists
of
a
plate
of
glass
or
plastic
coated
with
a
clear
conducting
material
on
one
side
and
a
thin
layer
of
phosphor
on
the
other
side.
A
metallic
plate
contact$
the
phosphor
coating.
As
alternating
current
is
applied
to
the
con-
ducting
plates
the
phosphors
are
excited
by
the
voltage
across
the
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dielectric
and
light
is
produced.
The
amount
of
light
depends
upon
the
applied
voltage
and
frequency.
The
gain
or
level
controlling
element
is
the
photo-conductive
cells.
The
resistance
of
the
cells
decreases
with
an
increase
in
the
impinging
light.
Since
the
light
is
produced
directly
from
the
audio
voltage,
the
response
is
instantaneous.
Re~ti~
fication
and
filtering
of
the
audio
to
produce
a
control
signal
are
npt
necessary
as
in
the
case
of
conventional
limiters.
This
system
re~ults
in
automatic
level
control
whose
speed
of
operation
is
limited
only
by
the
response
of
the
variable
resistance
photo
cells
used.
Cells
are
selected
which
provide
minimum
attack
time,
and
a
release
time
which
requires
about
60
milliseconds
for
50%
release,
and
then
a
gradual
release
over
a
period
of
1
to
15
seconds
to
the
point
of
com-
plete
release.
Referring
to
Figure
2,
the
functional
block
diagram,
the
input
signal
is
applied
directly
to
the
optical
attenuator
from
the
high
im~
pedance
winding
of
the
input
transformer.
The
amount
of
attenuation
introduced
by
the
optical
attenuator
is
controlled
by
the
audio
voltage
applied
to
it
by
the
lamp-driver
amplifier.
The
amount
of
signal
applied
to
the
voltage
amplifier
is
also
controlled
by
the
manual
gain
control.
The
voltage
amplifier
stage
provides
a
gain
of
26
dB.
Over-
all
amplifier
feedback
of
approximately
80
dB
provides
low
distortion,
flat
response,
and
gain
stability.
For
stereo
applications,
the
gain
reduction
voltage
amplifier
of
one
unit
may
be
coupled
to
another
unit.
The
output
of
each
voltage
amplifier
stage
is
applied
to
the
stereo
balance
control
and
is
also
brought
out
to
a
terminal
on
the
chassis.
For
stereo
operation,
this
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terminal
is
connected
to
the
same
terminal
on
an
identical
amplifier
and
control
voltage
becomes
common
to
both
units.
A
gain-reduction
control
voltage
generated
in
either
amplifier
will
cause
equal
gain
reduction
in
both
units.
The
control
voltage
is
applied
through
the
stereo
balance
control
to
the
lamp-driver
amplifier.
This
stage
pro-
vides
the
necessary
voltage
to
operate
the
electro-luminescent
light
source.
OPERATION
The
LA-3A
Leveling
Amplifier
is
designed
to
prevent
an
increase
in
output
level
beyond
a
pre-determined
point,
and
due
to
its
unique
design,
functions
as
a
combined
compressor
and
limiter.
The
effect
is
illustrated
in
Figure
1.
The
point
at
which
the
compressed
curve
breaks
away
from
the
straight
"No-Gain
Reduction"
line
is
determined
by
the
setting
of
the
"Peak
Reduction"
control.
It
can
be
seen
from
the
curve
that
compression
occurs
and
gradually
increases
over
the
first
10
dB
of
input
level
rise.
The
slope
of
the
curve
then
becomes
horizontal,
preventing
an
increase
of
output
level
regardless
of
input
increase.
Control
Settings
It
is
recommended
that
the
"Peak
Reduction"
control
be
set
to
prevent
increase
in
output
level
beyond
the
100
%
modulation
point.
This
setting
should
be
made
on
typical
program
material.
Settings
of
the
"Gain"
and
"Peak
Reduction"
controls
are
independ-
ent.
However,
the
"Gain"
control
should
be
set
to
provide
sufficient
output
after
the
"Peak
Reduction"
control
has
been
adjusted.
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The "Peak
Reduction"
control
should
be
set
for
the
desired
amount
of
gain
reduction
as
indicated
by
the
meter.
Continuous
extreme
re-
duction,
such
as
20
or
30
dB,
does
tend
to
reduce
the
dynamic
range
of
music.
Maximum
benefit
is
obtained
by
running
4
to
8 dB
of
compression
continuously.
For
ease
of
control
and
to
prevent
overload
of
the
input
trans-
former,
a
switchable
pad
is
included
ahead
of
the
LA-3A
input
trans-
former.
In
the
high
gain
(50
dB)
position
the
pad
is
switched
out.
When
the
device
is
to
be
used
at
input
levels
greater
than
0
dBm
the
pad
should
be
switched
in
by
using
the
30
dB
position
on
the
rear
panel
mounted
gain
switch.
VU
Meter
The
VU
Meter
serves
two
functions:
it
indicates
output
level
as
well
as
gain
reduction
directly
in
dB.
When
the
meter
selector
switch
is
placed
in
the
"output"
position,
the
meter
will
indicate
output
level
across
the
600
ohm
terminals.
The
meter
is
calibrated
to
read
0
VU
or
100%
when
the
amplifier
output
is
+4 dBm.
The
position
marked
"Gain
Reduction"
permits
the
meter
to
indicate
the
amount
of
gain
reduction
or
peak
limiting
directly
in
dB.
During
periods
of
no
gain
reduction
the
pointer
will
return
to
0
VU
on
the
meter
scale.
The
pointer
is
initially
set
to
this
position
by
means
of
the
screw
driver
adjusted
control
located
near
the
lower
right
hand
corner
of
the
meter.
Stereo
If
two
LA-3A
Leveling
Amplifiers
are
to
be
used
in
tandem
for
stereo,
the
gain
reduction
of
each
amplifier
can
be
made
equal,
regard-
less
of
which
channel
is
instigating
the
limiting.
This
is
accomplished
Page
9