Universal Audio LA-2A User manual

Model LA-2A
Leveling Amplifier
Universal Audio, Inc.
www.uaudio.com
PO Box 3818
Santa Cruz, CA 95063-3818

ii
Teletronix LA-2A by Universal Audio
Thank you for purchasing this reproduction of the Teletronix LA-2A. The LA-2A was
originally produced in the early 1960s by Teletronix, which was later acquired by
Babcock Electronics Corporation. My father purchased the product rights and the name
Teletronix from Babcock Engineering in 1967, folding it into his Studio Electronics
Corporation, shortly before he changed the name to UREI. There were three different
variations of the LA-2A during this period before production was discontinued around
1969.
Straightforward in its design, and initially intended for broadcast applications, the LA-2A
quickly became standard equipment in studios worldwide. Many have been painstakingly
maintained for the 30 years that they have remained in use since production stopped. A
tube-based compressor, the LA-2A features hand-wired components and two simple
controls. At the time, its electro-luminescent optical gain reduction was quite revolutionary:
applying the audio signal to an electro-luminescent light panel which shines on a photo-
electric cell which in turn controls the gain. In contrast to the electro-optical devices which
preceded it, the electro-luminescent light source provided the fast attack necessary for
broadcast applications. Additionally, the cadmium-sulfide photo-cells provided a very
natural “two-stage” release which resulted in a compression characteristic more
transparent than the other compressors of its day. To this day the LA-2A delivers a
trademark sound treasured by engineers worldwide.
Here at Universal Audio, we have gone to great lengths to recreate the LA-2A with
complete authenticity. It’s been no small feat to locate obscure parts, put quality control
programs in place that didn’t exist 30 years ago and find personnel capable of the
handwork that goes into each unit. Every step was necessary to ensure that each LA-2A
delivers the sound you expect.
As for the future, we are working on additional reproductions beyond the LA-2A and
1176LN. To that end, we are interested in your opinions about your favorite pieces of
vintage gear. What can’t you live without? What would you love to own, but can’t find
any longer? In addition, Universal Audio is working on new products designed to meet
the demands of the modern digital recording studio, yet retaining the character of classic
vintage equipment.
Developing these products has been quite an adventure, enjoyed by all involved. We
thank you for your support and we thank my father, Bill Putnam.
Thank you,
Bill Putnam

iii
IMPORTANT SAFETY INSTRUCTIONS
Before using this unit, be sure to carefully read the applicable items of these operating instructions and the
safety suggestions. Afterwards keep them handy for future reference. Take special care to follow the
warnings indicated on the unit itself, as well as in the operating instructions.
1. Water and Moisture – Do not use the unit
near any source of water or in excessively moist
environments.
2. Object and Liquid Entry – Care should be
taken so that objects do not fall, and liquids are
not spilled, into the enclosure through openings.
3. Ventilation – When installing the unit in a rack
or any other location, be sure there is adequate
ventilation. Improper ventilation will cause
overheating, and can damage the unit.
4. Heat – The unit should be situated away from
heat sources, or other equipment that produces
heat.
5. Power Sources – The unit should be connected
to a power supply only of the type described in
the operating instructions, or as marked on the
unit.
6. Power Cord Protection – AC power supply
cords should be routed so that they are not likely
to be walked on or pinched by items placed upon
or against them. Pay particular attention to cords
at plugs, convenience receptacles, and the point
where they exit from the unit. Never take hold of
the plug or cord if your hand is wet. Always grasp
the plug body when connecting or disconnecting
it.
7. Grounding of the Plug – This unit is
equipped with a 3-wire grounding type plug, a
plug having a third (grounding) pin. This plug will
only fit into a grounding-type power outlet. This is
a safety feature. If you are unable to insert the
plug into the outlet, contact your electrician to
replace your obsolete outlet. Do not defeat the
purpose of the grounding-type plug.
8. Carts and Stands – The unit should be used
only with a cart or stand that is recommended by
the manufacturer. The unit and cart combination
should be moved with care. Quick stops,
excessive force and uneven surfaces may cause
the unit and cart combination to overturn.
9. Wall Or Ceiling Mount – The unit should be
mounted to a wall or ceiling only as
recommended by the manufacturer.
10.
Cleaning – The unit should be cleaned only as
recommended by the manufacturer.
11.
Nonuse Periods – The AC power supply cord
of the unit should be unplugged from the AC
outlet when left unused for a long period of time.
12.
Damage Requiring Service – The unit
should be serviced by qualified service personnel
when:
a) The AC power supply cord or the plug has
been damaged; or
b) Objects have fallen or liquid has been spilled
into the unit; or
c) The unit has been exposed to rain; or
d) The unit does not operate normally or
exhibits a marked change in performance; or
e) The unit has been dropped, or the enclosure
damaged.
13.
Servicing – The user should not attempt to
service the unit beyond that described in the
operating instructions. All other servicing should
be referred to qualified service personnel.

iv
LA-2A

User’s Guide
Universal Audio, Inc.
PO Box 3818
Santa Cruz, CA 95063-3818
[phone] voice
[phone] fax
www.uaudio.com
Universal Audio Part Number LA2A-M01.
Revision 1.3
Notice
This manual provides general information, preparation for use, installation and operating instructions for the
Universal Audio LA-2A Leveling Amplifier.
The information contained in this manual is subject to change without notice. Universal Audio, Inc. makes no
warranties of any kind with regard to this manual, including, but not limited to, the implied warranties of
merchantability and fitness for a particular purpose. Universal Audio, Inc. shall not be liable for errors
contained herein or direct, indirect, special, incidental, or consequential damages in connection with the
furnishing, performance, or use of this material.
Copyright
2000 Universal Audio, Inc. All rights reserved.
This manual and any associated software, artwork, product designs, and design concepts are subject to
copyright protection. No part of this document may be reproduced, in any form, without prior written
permission of Universal Audio, Inc.
Trademarks
LA-2A, 1176 and the Universal Audio, Inc. logo are trademarks of Universal Audio, Inc. Other company
and product names mentioned herein are trademarks of their respective companies.

v
Table of Contents
Teletronix LA-2A by Universal Audio...................................................................................ii
Specifications.......................................................................................................................1
Operation of the LA-2A .......................................................................................................2
Input and Output Connections............................................................................... 2
Peak Reduction Control........................................................................................ 2
Gain Control....................................................................................................... 2
VU Meter Operation............................................................................................ 3
Limit / Compress Switch........................................................................................ 3
Calibration...........................................................................................................................4
Meter Zero Adjust ............................................................................................... 4
Side-Chain Pre-Emphasis (R37).............................................................................. 4
Stereo Balance Adjust (R3)................................................................................... 4
Theory of Operation............................................................................................................5
Compressor Basics............................................................................................... 5
Gain Reduction Circuit.......................................................................................... 8
Side-Chain Circuit.............................................................................................. 11
Output Circuit.................................................................................................... 11
Metering Circuit................................................................................................. 11
Appendix.......................................................................................................................... 12
Creative Classics: The 1176 Solid State Limiting Amplifier and the LA-2A
Leveling Amplifier.............................................................................................. 12
The LA-2A..................................................................................................... 12
Developing the 1176 ..................................................................................... 13
More Than a Vintage Fad: Classic Sound.......................................................... 13

1
Specifications
!Gain Reduction: up to 40 dB
!Distortion: less than 0.35% total harmonics at +10 dBm, and less than 0.75% total
harmonics at +16 dBm output
!Response: +/- 0.1 dB, 30 cycles to 15 kilocycles
!Noise: 75 dB below +10 dBm output level
!Gain: 40 +/- 1dB
!Output Level: +10dBm nominal, +16dBm peaks
!Input Level: +16 dBm maximum
!Very fast attack time
!Release Time: approximately 0.06 seconds for 50% release, 0.5 to 5 seconds for
complete release depending upon the amount of previous reduction
!Input Impedance: 600 ohms balanced
!Output Impedance: 600 ohms balanced
!Panel Size: 19” x 5 ¼ ”
!Depth Behind Panel: 7 ¼”
!Panel Controls: Gain (Output level), Peak Reduction, and Meter Selector Switch
!Meter: dB Gain Reduction and dB Output
!Tube Complement: (2) 12AX7A, (1) 12BH7A, (1) 6AQ5

2
Operation of the LA-2A
Input and Output Connections
Input and Output connections are made using standard XLR style connectors. Terminal
strips are also provided.
Output connections Input connections
6) Stereo Link 1) + 600 ohms
7) Ground 2) + 250 ohms
8) + 600 ohms 3) Center tap
9) Center tap 4) - 250 ohms
10) - 600 ohms 5) - 600 ohms
Barrier Strip Connections
If you wish to use the barrier strip instead of the XLR connectors, connect as follows:
INPUT: Connect + 600 (1) to HOT and –600 (5) to NEUTRAL.
OUTPUT: Connect + 600 (8) to HOT and –600 (10) to NEUTRAL.
Connect Ground (7) to the cable shield.
Peak Reduction Control
Operation of the LA-2A is very straightforward. There are only two controls that one must
deal with. The first is the Peak Reduction control. This control should be set so that the
compressor exhibits the desired amount of compression. This control should be set
independently of the Gain Control.
Gain Control
This control does not affect the compression. The gain control should be set after the
desired amount of compression is determined using the Peak Reduction control. Once the
Peak Reduction control is set, adjust the Gain Control to achieve the desired output level.
Power
Transformer
12AX7
6AQ5
12AX7
12BH7
R3
R37 HA-100X
T4
A-24
IEC
10 98654321
7Output Input

3
VU Meter Operation
The VU Meter is used for both output level monitoring and gain reduction monitoring.
When used to monitor the output level, the user may choose either +4 dB or +10 dB
settings. The middle setting is for gain reduction, and the meter reads the amount of
compression in dB.
Limit / Compress Switch
The Limit/Compress Switch changes the characteristics of the compressor IO curve. When
in the Compress position, the curve is more gentle, and presents a low compression ratio.
A higher compression ratio results when the switch is set to the Limit position.

4
Calibration
Meter Zero Adjust
The zero-adjust of the meter is set by the screw-adjust potentiometer located next to the
Limit / Compress Switch. To adjust this, put the meter into Gain Reduction mode. Make
sure that there is no signal present. Loosen the locking nut and use a screwdriver to adjust
this potentiometer until the meter reads zero.
Side-Chain Pre-Emphasis (R37)
The LA-2A was designed for use in broadcast applications. The audio signal in FM
broadcasting undergoes pre-emphasis and results in a 17 dB boost at 15 KHz. Due to this
increase in signal level, transmitters are subject to over-modulation. The LA-2A provides a
control (R37) which controls the amount of high-frequency compression.
This potentiometer is factory set for a “flat” side-chain response (clockwise). Increasing
the resistance of this potentiometer by turning it counter clockwise will result in
compression which is increasingly more sensitive to the higher frequencies.
Stereo Balance Adjust (R3)
For stereo operation, two LA-2A units are interconnected in order to achieve the same
amount of gain reduction from each of the two units to maintain stereo imaging. In order
to accomplish this, terminals 6 from each unit should be connected together. Additionally,
their grounds (terminal 7) should be connected together.
The interconnecting wire should be less than 2 feet in length and should be shielded. The
shield should be used to connect terminals 7 from each of the units together.
To calibrate the units for stereo operation:
!Connect the units together as described previously.
!Turn the Peak Reduction knob counterclockwise (no compression).
!Set R3 on each unit to a clockwise position.
!Set each meter to read Gain Reduction.
!Adjust the Peak Reduction control on the left channel until approximately 5dB of
gain reduction is achieved.
!Adjust R3 on the unit that shows the greatest amount of gain reduction until the
gain reduction indications are equal.
!When operating, set the Peak Reduction controls to the same setting on both
channels.

5
Theory of Operation
Compressor Basics
Before we dig in to a description of the LA-2A circuit, it is useful to examine the general
characteristics of compressors and review some terminology. Figure 1 depicts the
input/output characteristics of a compressor, an expander and a perfect amplifier. When
operated within its specified range, an amplifier provides a constant amount of gain
regardless of the level of the input signal. In Figure 1, the middle line depicts a perfect
amplifier with a gain of 10 dB. To see this, notice that a signal with an input level of –30
dB will result in an output level of –20 dB, which is an increase of 10 dB. Similarly, an
input level of 0 dB will result in an output level of 10 dB, hence the gain stays fixed at 10
dB regardless of the input level.
-30
-20
-10
0
+10
-30 -20
-10 0 +10
Input Level (dB)
Output
Level (dB)
Compression
Expansion Perfect
amplifier
Figure 1 - Input/output characteristics of a compressor, an expander and a perfect
amplifier.
In contrast to an amplifier, whose job is to present a constant gain, a compressor varies its
gain in response to the level of the input signal. Large input signals result in less gain, thus
reducing or “compressing” the dynamic range of the signal. Referring again to the line
marked “compression” in Figure 1, we see that an input level of –30 dB results in an
output level of –20 dB, indicating a gain of 10 dB. Repeating this for input levels of –20
dB and –10 dB, we see that the compressor exhibits gains of 5 dB and 0 dB respectively.
From this, it is clear that the gain decreases as the input signal increases.
Referring to the diagram, we see that the compressor will increase its output level by 5 dB
for every 10 dB that we increase the input level. The compression ratio is defined as the
ratio of these two numbers. In this case the compression ratio would be 10:5, which can
be reduced to 2:1.

6
As an aside, an expander is a device which increases the dynamic range of a signal. For
example, a 10dB change in the input signal might result in a 20 dB change in the output
signal, thus “expanding” the dynamic range.
There are several other terms related to compression that can be demonstrated by
referring to Figure 2. The amount of compression or gain reduction is typically given in dB
and is defined as the amount by which the signal level is reduced by the compressor.
Graphically, this can be understood by looking at the difference in levels between what
would have been the uncompressed (the output from an amplifier) output level and the
compressed output level. This value is what is displayed by the LA-2A meter when it is
switched to gain-reduction mode.
As mentioned previously, the compression ratio is defined as the ratio of the increase of
the level of the input signal to the increase in the level of the output signal. In this
example, the input level is increased by 10 dB while the output level only increases 5 dB.
This would be a compression ratio of 2:1. Lower ratios such as 2:1 result in more gentle
compression. (Note that a compression ratio of 1:1 is no compression at all).
Typically, compressors let you choose a threshold. This is the point at which gain
reduction starts to take place. When an audio signal is below this threshold the
compressor acts like an amplifier and there is no gain reduction. Above the threshold the
slope becomes less than 45 degrees, indicating gain reduction and hence compression.
The point at which a compressor transitions into compression is commonly called the knee.
In practical compressors, this transition is more gentle than what is depicted in the
diagram.
Many modern compressors provide a control which adjusts the threshold directly. In the
case of the LA-2A, the Peak Reduction knob controls both the threshold and the amount of
compression.
-30
-20
-10
0
+10
-30 -20
-10 0 +10
Input Level (dB)
Output
Level (dB)
Compression region
knee
2:1 Compression
10 dB of
compression
Figure 2 - Input/output curve of a compressor with a ratio of 2:1 and a threshold of -20
dB.

7
TRIM
IN P U T IN P U T
TRA N SFO RM ER O PTIC AL
ATTEN U ATO R MODULE T4
VO LTAG E A M P L IF IE R (1 2 A X 7 A )
CATHO DE FO LLO W ER (1 2 B H 7 )
PRE-EM PH A SIS
VO LTAG E A M P L IF IE R (1 2 A X 7 A )
ELEC TRO LU M IN ESC EN T D R IV E R (6 A Q 5 )
O U T P U T TRA N SFO RM ER OUTPUT
N EG ATIVE FEED BAC K
G A IN CO NTRO L
PEA K RED U C TIO N CO NTRO L
STEREO IN T E R C O N N E C T IO N
Figure 3 - Block diagram of the LA-2A compressor.

8
LA-2A Block Diagram
A functional block diagram of the LA-2A is provided in Figure 3. A brief overview of the
operation will be provided here. The input transformer provides isolation and impedance
matching. After this the signal is fed into both the side-chain circuit and the gain reduction
circuit. The side-chain is comprised of a voltage amplifier, a pre-emphasis filter, and a
driver stage which provides the voltage necessary to drive the electro-luminescent panel.
This signal controls the gain of the compressor. After the gain reduction circuit, the signal
is sent through an Output Gain control and a two-stage output amplifier, followed by the
output transformer.
Electro-luminescent
Panel Photo-Electric Cell
Figure 4 - Diagram of the T4 electro-optical cell.
Gain Reduction Circuit
As mentioned previously, compressors are devices that vary their gain in a manner which
is dependent upon the level of the input signal. In order to do this, the compressor must
first have some method of determining the level of the signal, and must then be able to use
this to control the gain. There are many different schemes to accomplish these tasks. In
the case of the LA-2A, both of these functions are performed by the T4, which is an electro-
optical element.
A T4 is comprised of an electro-luminescent (EL) panel and a photo-electric cell. The EL
panel is essentially a night-light. As you would expect, the larger the signal that is applied
to it, the brighter the light that is generated. This light shines upon the photo-electric cell.
A photo-electric cell is a light sensitive device whose resistance changes depending upon
the intensity of light to which it is subjected; the brighter the light, the less resistance the
photo-cell will have.
As depicted in Figure 5, the photo-cell is used to control the gain of the circuit. Essentially,
the photo-cell acts as the bottom leg in a voltage divider circuit. The lower the resistance
of the photo-cell, the lower the signal voltage will be at the output of the gain reduction

9
stage. To see why this is true, we can look at the extreme cases. If the resistance is
extremely high (this is the case when there is a small input signal and the light is off) then
the photo-cell does not affect the circuit and there is no gain reduction. The second case
we can look at is when there is a large signal present. In this condition, the light shines
brightly and the photo-cell exhibits very low resistance. If the resistance of the photo-cell
becomes zero (a dead short), then the signal would be grounded and there would be no
output. In reality, the photo-cell resistance can not go completely to zero and hence there
will always be some signal present.
7
R5 68K
R6
68K
R7
2.7K
8
9
10
TO VO LTA G E
A M P L IF IE R
TO G A IN
REDUCTIO N
CO NTRO L
TO ATTEN U ATO R
D R IV E A M P L IF IE R
GAIN R1
100K
PHOTOCONDUCTIVE C ELL
ELEC TRO LU M IN ESC EN T
ELEM EN T
H A-100X
.0047 uF
Figure 5 - Schematic of the LA-2A input and gain reduction circuit.
The T4 electro-optical device is the heart of the compressor and its gain reduction
characteristics. Its unique characteristics affect the overall sound and character of the LA-
2A.
In addition to the compression curve, the combination of the EL panel and the photo-cell
determine the attack and release characteristics of the LA-2A. This is one of the most
important contributors to the sound of the LA-2A. Unlike other compressors which allow
the user to adjust these parameters, the attack and release of the LA-2A are completely
determined by the T4.
There are several important characteristics of the T4 which play crucial roles in the sound
of the LA-2A. The first is the attack. The LA-2A was the first electro-optical compressor to
use an electro-luminescent panel for the light source. Previous attempts at electro-optical
compression employed either neon or incandescent lights. Both of these took time to light
up, and this delay resulted in slow attacks. The electro-luminescent panel resulted in a
faster attack than exhibited by other contemporary devices.

10
The next important aspect is that of the release of the compressor. This is determined
almost entirely by the characteristics of the photo-cell. The LA-2A uses cadmium-sulfide
photo-cells. The first important aspect of the cell is its “two-stage decay”. After the light is
removed from the cell, it releases quickly (40-80 milliseconds) to approximately half of its
off resistance. The remainder of its release can take place over as much as several
seconds.
The next aspect is the “memory” of the cell. This results in two important aspects of the
character of the LA-2A. The amount of time it takes for the cell to recover after the light is
removed depends on how long light had been shining on it and how bright the light. In
the case of the LA-2A this results in behavior where the release time is slower if the unit has
either been in compression for a while, or the amount of compression is large. This signal
dependent release characteristic is critical to the sound of the unit.
The amount of compression, as well as the compression threshold, is controlled by the
“Peak Reduction” potentiometer. This potentiometer controls the gain of the side-chain
circuit. The greater the gain of this circuit, the lower the threshold and the greater the
amount of compression will be. Many modern limiters and compressors allow for the
direct adjustment of the threshold. Other units such as the 1176LN use a fixed threshold
and provide an input level control, which adjusts the signal level before it is applied to the
compression circuit. In contrast, the LA-2A, while also having a fixed threshold,
does not control the input level, but rather controls the amount
of side chain gain applied to the input signal.
C8
.0 3
R31
1K
R2
100K
C9
.0 2
R33
220K
1 Meg
LIM
RESP
R37
R3
1M STEREO ADJ
OUTPUT
R36
1K
C10
50 uF
C7D
30 uF
R35
220K
R 3 4
22K 2W
V4
6AQ 5A
IN P U T
R30
47K
.0 1
ATTEN U A TO R
D R IV E -A M P L IF IE R
C12
.001
STEREO
PA RA LLEL
PO W ER
SU PPLY
R32
1K
C6
12AX7A
V3B V3A
C11
.1
Figure 6 - Schematic diagram of the LA-2A side-chain circuit.

11
Side-Chain Circuit
The previously described gain reduction circuit is controlled by the control voltage which is
supplied by the side-chain circuit. The LA-2A is a feed-back style compressor. This is due
to the fact that the signal that is used to drive the side-chain circuit is affected by the gain
reduced signal. This signal is first fed into the “Peak Reduction” potentiometer (R2), which
controls the amount of side-chain drive and in turn controls the compression threshold and
amount of gain reduction. A 12AX7 is then used as a voltage amplifier to increase the
signal level. A pre-emphasis circuit is provided on the output of the 12AX7. Originally
designed for broadcast, the LA-2A allowed for side-chain equalization, which allowed the
operator to make the compression more or less sensitive to the voice frequency bands.
For musical applications, this equalization is usually set to a flat frequency response.
Subsequent to the filter, a 6AQ5 provides the signal necessary to drive the electro-
luminescent panel. EL panels were often used for night-lights and hence are usually
designed to be driven with 120 volts, 60 Hz AC. They were not designed for audio, and
applying the wide-bandwidth signals that arise in audio applications results in a shortened
lifetime of the part.
Output Circuit
Figure 7 - Schematic diagram of the LA-2A output circuit.
The output circuit is comprised of a 12AX7 which operates as a voltage amplifier followed
by a 12BH7A which operates as a cathode-follower. This is followed by the output
transformer, which provides impedance matching and a balanced output.
Metering Circuit
The metering circuit in the LA-2A has 3 modes selected by a front-panel switch, allowing
for output level monitoring at +4 and +10 dB as well as gain reduction. As mentioned
R9
220K
R10
1.5K R12
470K R14
2.7K
R11
68K
C1
.02
V1A
C7C
30uF
R13
220K
C2
.1
R16
68K
R17 10K
R15
470K R18
1K
R20
1K R19
470K
C3
.1
R21
100K
C2
.1
R16
68K
C5
10 uF
1
34
8
A-24
2
5
V1B
12AX7
12BH7
V2B
V2A
CATHODE
FOLLOWER
VOLTAGE
AMPLIFIER
FROM
R1

12
previously, the gain reduction is controlled by the photo-cell in the T4 el-op. In order to
track the operation of this cell and determine the gain reduction, a second photo-cell is
also illuminated by the same EL panel. This photo-cell is hand-selected to match the gain
reduction photocell and hence gives an accurate indication of the amount of compression.
Appendix
Creative Classics: The 1176 Solid State Limiting Amplifier and
the LA-2A Leveling Amplifier
The LA-2A and 1176 compressor/limiters long ago achieved classic status. They're a
given in almost any studio in the world — relied upon daily by engineers whose styles
range from rock to rap, classical to country and everything in between. With so many
newer products on the market to choose from, it's worth looking at the reasons why these
classics remain a necessary part of any professional studio's outboard equipment
collection.
The basic concept of a compressor/limiter, is of course, relatively simple. It's a device in
which the gain of a circuit is automatically adjusted using a predetermined ratio that acts
in response to the input signal level. A compressor/limiter "rides gain" like a recording
engineer does by hand with the fader of a console: it keeps the volume up during softer
sections and brings it down when the signal gets louder. The dynamic processing that
occurs at ratios below 10 or 12 to one is generally referred to as compression; above that
it's known as limiting.
Modern day compressors offer a great degree of programmability and flexibility while
older devices such as the 1176 and the LA-2A are more straightforward in their design.
Perhaps it is this fact that has contributed to their appealing sound and the longevity of
their popularity.
The LA-2A
The LA-2A leveling amplifier, a tube unit with hand wired components and three simple
controls, was introduced in the mid 1960s. It utilized a system of electro-luminescent
optical gain control that was quite revolutionary; gain reduction was controlled by
applying the audio voltage to a luminescent driver amplifier, with a second matched
photoconductive cell used to control the metering section. With its 0 to 40 dB of gain
limiting, a balanced stereo interconnection, flat frequency response of 0.1 dB from 30-
15,000 hz and a low noise level (better than 70 dB below plus 10 dBm output,) the LA-
2A quickly became a studio standard. Originally patented by Jim Lawrence, it was
produced by Teletronix in Pasadena, California, which became a division of Babcock
Electronics Corp. in 1965. In 1967 Babcock's broadcast division was acquired by the

13
legendary Bill Putnam's company, Studio Electronics Corp shortly before he changed the
company’s name to UREI®. Three different versions of the LA-2A were produced under the
auspices of these different companies before production was discontinued around 1969.
Developing the 1176
It was Bill Putnam himself who, in 1966, was responsible for the initial design of the 1176.
Its circuit was rooted in the 1108 preamplifier which was also designed by Putnam. As is
evident from entries and schematics in his design notebook, he experimented with the
recently developed Field Effect Transistor (F.E.T.) in various configurations to control the
gain reduction in the circuit. He began using F.E.T.s as voltage variable resistors, in which
the resistance between the drain and the source terminals is controlled by a voltage
applied to the gate. His greatest challenge was to ensure that distortion was minimized by
operating the F.E.T.s within a linear region of operation.
After several unsuccessful attempts at using F.E.T.s in gain reduction circuits, Putnam settled
upon the straightforward approach of using the F.E.T. as the bottom leg in a voltage
divider circuit, which is placed ahead of a preamp stage.
The output stage of the 1176 is a carefully crafted class A line level amplifier, designed to
work with the (then) standard load of 600 ohms. The heart of this stage is the output
transformer, whose design and performance is critical. Its primary function is to convert the
unbalanced nature of the 1176 circuit to a balanced line output, and to provide the
proper impedance matching to drive the line impedance of 600 ohms. These two jobs are
accomplished by the primary and secondary windings whose turns' ratio defines the
impedance ratio.
This transformer is critical due to the fact that it uses several additional sets of windings to
provide feedback, which makes it an integral component in the operation of the output
amplifier. Putnam spent a great deal of time perfecting the design of this tricky transformer
and carefully qualified the few vendors capable of producing it.
The first major modification to the 1176 circuit was designed by Brad Plunkett in an effort
to reduce noise--hence the birth of the 1176LN, whose LN stands for low noise. Numerous
design improvements followed, resulting in at least 13 revisions of the 1176.
Legend has it that the D and E blackface revisions sound the most "authentic".
More Than a Vintage Fad: Classic Sound
Both the 1176 and the LA-2A remain in daily use. Busy engineers’ and producers’
comments about both the 1176 and the LA-2A demonstrate their impact on the industry:
Mike Shipley
Mike Shipley (Def Leppard, Shania Twain, Blondie): "I grew up using 1176s --- in England
they were the compressor of choice. They're especially good for vocals, which is also what
I primarily use the LA-2 for. Most anything else I can do without, but I can't be without at
least a pair of 1176s and an LA-2A. For example, on the Enrique Iglesias project I'm

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currently mixing, I'm using both an 1176 and an LA2 on his voice, which is not unusual for
me.
"The 1176 absolutely adds a bright character to a sound, and you can set the attack so
it's got a nice bite to it. I usually use them on four to one, with quite a lot of gain reduction.
I like how variable the attack and release is; there's a sound on the attack and release
which I don't think you can get with any other compressor. I listen for how it affects the
vocal, and depending on the song I set the attack or release--faster attack if I want a bit
more bite. My preference is for the black face model, the 4000 series--I think the top end
is especially clean.
"The LA-2A is not as versatile, but it also has a sound that I really like. On certain voices
you can crank it heavily, to where you almost want to put a piece of tape over the meter
because there's so much gain reduction that you don't want anyone else to see it! I'm not
particularly into over-compression, but when you use it that way there's something about it
that just sounds really great. It does depend how it's set on the back, where there's a flat
control that can be set to roll off certain frequencies when you reduce more gain. If you
have a singer with an intensely piercing voice I find that compressor a good one,
incredibly useful. It makes things warmer, especially when you crank it, and for thinner
voices that can be just the ticket."
Allen Sides
Allen Sides has always been known for having golden ears when it comes to the sound of
equipment. The owner of Ocean Way Studios in Los Angeles and Nashville, he's also a
speaker designer and engineer who is especially respected for his work with live
musicians, including orchestra and string dates. Among his recent credits are work with the
Goo Goo Dolls, Alanis Morissette and Green Day. Sides brings his different perspectives
into play when he talks about using the 1176.
"The 1176 is standard equipment for my sessions. I just used them last night, as a matter of
fact, on a project for singer Lisa Bonet that Rob Cavallo was producing at Ocean Way.
We were recording drums and I used them on the left/right overheads as effects limiters.
It's something I learned from (engineer) Don Landy, who worked with Randy Newman a
lot. I mult the left and right overheads and bring them back on the console, then insert a
pair of 1176s into a pair of the mults. Push in 20 to one and four to one simultaneously
and it puts the unit into overdrive creating a very impressive sound."
Murray Allen
Murray Allen is a veteran engineer and Director of Post Production for the San Franciso
Bay Area company Electronic Arts. He has a fascination for gear both old and new and
he explains why he thinks the 1176 has been so popular for so long. "It has a unique
sound to it that people like, it's very easy to operate, and it does a great job. You have just
two controls relative to the ratio of compression. You have input and output and you have
attack and release. That's all there is. It's still my favorite limiter for Fender basses and
string basses, because you don't know it's working. It doesn't change the way the bass
sounds, it just keeps the level at a more controllable place.

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Ken Kessie
Mixer Ken Kessie (En Vogue, Tony! Toni! Tone!, Celine Dion) is known for being
experimental. "Seems like everybody knows the basic tricks for he 1176," he says, "But
here are two that might be lesser known. If you turn the attack knob fully counterclockwise
until it clicks, the 1176 eases to be a compressor and acts only as an amplifier. Sometimes
this is the perfect sound for a vocal. And of course the unit can be overdriven, adding
another flavor of distortion in case your plug-ins are maxed out!
"Then, for that hard-to-tame lead vocalist (the one that backs up from the mic to whisper
and leans in for the big ending chorus), try an 1176 followed by a DBX 165. Use the
1176 as a compressor, and the DBX as a peak limiter...it's guaranteed to be smooth as
silk."
Jim Scott
Jim Scott shared a Grammy for Best Engineered Album for Tom Petty's Wildflowers. He's
also known for his work with Red Hot Chili Peppers, Natalie Merchant and Wilco. "I use
1176s real conservatively and they till do amazing things," he comments. "I'm always on
the four to one button, and the Dr. Pepper--you know, 10 o'clock, 2 o'clock, and it does
everything I need.
"I always use them on vocals. I use them on room mics, on acoustic guitars--sometimes in
mixing I'll sneak a little on a snare drum or a separated channel of a snare drum. I'm not
one of those guys who leaves it on everything, but I'd have to say I've used an1176 on
everything at one time or another.
"They have an equalizer kind of effect, adding a coloration that's bright and clear. Not
only do they give you a little more impact from the compression, they also sort of clear
things up; maybe a little bottom end gets squeezed out or maybe they are just sort of
excitingly solid state or whatever they are. The big thing for me is the clarity, and the
improvement in the top end.
"The 1176 has that same kind of phenomenon, where, when you patch something through
a Neve equalizer and you don't even engage the EQ, it sounds better. It's just a
combination of the amps.
"I also use LA-2As all the time. I use them on bass, and it's one of the compressors I use for
reverb. Often I'll send a bunch of things to one LA2A and bring it back into the console
like a return--it's great for drums, great for kick drum especially.
"LA-2As warm things up. They're the opposite of the 1176--they EQ all the warmth and
low mids and bass. When you put bass and drums in them they get fatter and bigger. And
unless you hit them way hard and make the tubes sizzle they don't really distort. Of
course, you can get them to sound like an AC30 if you want to, just turn them all the way
up. They are very loud, powerful amplifiers.
"I've also used both the 1176 and the LA-2A for stereo buss compressors--you just have to
be a little bit careful that your mixes don't go one sided. Tom's [Petty’s] records have often
been mixed through 1176s. I've also done that with LA-2As --they are of course, more