Inova Stereo Amplifier LA 2A User Manual

Mode l LA-2 A  
Le ve ling Amplifie r  
Universal Audio, Inc.  
PO Box 3818  
Santa Cruz, CA 95063-3818  
 
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 . Wa te r a nd Moisture – Do not use the unit  
near any source of water or in excessively moist  
environments.  
8 . Ca rts a nd Sta nds 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.  
2 . Obje ct a nd Liquid Entry – Care should be  
taken so that objects do not fall, and liquids are  
not spilled, into the enclosure through openings.  
9 . Wa ll Or Ce iling Mount The unit should be  
3 . Ve ntila tion – 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.  
mounted to  
a
wall or ceiling only as  
recommended by the manufacturer.  
1 0 .Cle a ning The unit should be cleaned only as  
recommended by the manufacturer.  
4 . He a t The unit should be situated away from  
heat sources, or other equipment that produces  
heat.  
1 1 .Nonuse Pe riods The AC power supply cord  
of the unit should be unplugged from the AC  
outlet when left unused for a long period of time.  
5 . Powe r Source s The unit should be connected  
to a power supply only of the type described in  
the operating instructions, or as marked on the  
unit.  
1 2 .Da ma ge Re quiring Se rvice The unit  
should be serviced by qualified service personnel  
when:  
6 . Powe r Cord Prote ction – 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.  
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  
7 . Grounding of the Plug  
This unit is  
e) The unit has been dropped, or the enclosure  
damaged.  
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.  
1 3 . Se rvicing 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.  
iii  
 
LA-2 A Use r’s Guide  
Universal Audio, Inc.  
PO Box 3818  
Santa Cruz, CA 95063-3818  
(831) 454-0630 voice  
(831) 454-0689 fax  
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.  
Tra de ma rks  
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.  
iv  
 
Ta ble of Conte nts  
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  
v
 
Spe cifica tions  
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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  
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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  
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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  
1
 
Ope ra tion of the LA-2 A  
Input a nd Output Conne ctions  
Input and Output connections are made using standard XLR style connectors. Terminal  
strips are also provided.  
Output  
Input  
10 9 8 7 6  
5
4
3 2  
1
6AQ5  
IEC  
R3  
A-24  
T4  
Power  
Transformer  
HA-100X  
R37  
12AX7  
12BH7  
12AX7  
Output connections  
Input connections  
6)  
7)  
8)  
9)  
10)  
Stereo Link  
Ground  
+ 600 ohms  
Center tap  
- 600 ohms  
1)  
2)  
3)  
4)  
5)  
+ 600 ohms  
+ 250 ohms  
Center tap  
- 250 ohms  
- 600 ohms  
Ba rrie r Strip Conne ctions  
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.  
Pe a k Re duction 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.  
Ga in 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.  
2
 
VU Me te r Ope ra tion  
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 / Compre ss 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.  
3
 
Ca libra tion  
Me te r Ze ro 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 -Cha in Pre -Empha sis (R3 7 )  
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.  
Ste re o Ba la nce 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:  
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!
!
!
!
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.  
4
 
The ory of Ope ra tion  
Compre ssor Ba sics  
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.  
Expansion  
Perfect  
amplifier  
+10  
0
Compression  
Output  
-10  
Level (dB)  
-20  
-30  
-30  
-20  
0
+10  
-10  
Input Level (dB)  
Figure 1 - Input/ outp ut cha ra cte ristics of a compre ssor, a n e x pa nde r a nd a pe rfe ct  
a mplifie r.  
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.  
5
 
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.  
Compression region  
10 dB of  
compression  
+10  
0
Output  
Level (dB)  
2:1 Compression  
-10  
knee  
-20  
-30  
-30  
-20  
0
+10  
-10  
Input Level (dB)  
Figure 2 - Input/ output curve of a compressor with a ratio of 2:1 and a threshold of -20  
dB.  
6
 
Figure 3 - Block dia gra m of the LA-2 A compre ssor.  
7
 
LA-2 A Block Dia gra m  
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 - Dia gra m of the T4 e le ctro-op tica l ce ll.  
Ga in Re duction 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  
8
 
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.  
R6  
R7  
H A-100X  
G AIN R1  
100K  
68K  
2.7K  
TO VO LTAG E  
A M PLIFIER  
7
9
PH O TO C O N D U C TIVE C ELL  
8
10  
R5  
ELEC TRO LU M IN ESC EN T  
ELEM EN T  
68K  
TO G AIN  
RED U C TIO N  
C O N TRO L  
TO ATTEN UATO R  
D R IV E A M PLIFIER  
Figure 5 - Sche ma tic of the LA-2 A input a nd ga in re d uction 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.  
9
 
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.  
STEREO  
PARALLEL  
PO W ER  
SU PPLY  
IN PU T  
O U TPU T  
ATTEN U ATO R  
D R IV E -A M PLIFIER  
1 Meg  
LIM  
RESP  
R37  
R3 4  
22K 2W  
C 11  
C12  
.001  
R33  
220K  
C 9  
C 6  
R35  
220K  
.1  
.0 2  
.0 1  
R30  
47K  
V4  
V3B  
V3A  
R32  
1K  
R3 STEREO  
1M AD J  
R2  
100K  
6AQ 5A  
12AX7A  
C 8  
.0 3  
C 10  
C 7D  
R36  
1K  
R31  
1K  
50 uF  
30 uF  
Figure 6 - Sche ma tic dia gra m of the LA-2 A side -cha in circuit.  
10  
 
Side -Cha in 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  
R16  
68K  
C2  
VOLTAGE  
CATHODE  
AMPLIFIER  
FOLLOWER  
R16  
.1  
68K  
C7C  
R13  
220K  
30uF  
R17  
V2A  
R21  
100K  
10K  
R9  
220K  
C1  
.02  
C2  
C5  
A-24  
V1A  
1
.1  
V1B  
10 uF  
FROM  
R1  
C3  
.1  
R11  
68K  
12AX7  
R15  
470K  
R18  
1K  
2
R10  
1.5K  
R14  
2.7K  
R12  
470K  
4
5
3
V2B  
12BH7  
R20  
1K  
R19  
470K  
8
Figure 7 - Sche ma tic dia gra m of the LA-2 A 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.  
Me te ring 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  
11  
 
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.  
Appe ndix  
Cre a tive Cla ssics: The 1 1 7 6 Solid Sta te Limiting Amplifie r a nd  
the LA-2 A Le ve ling Amplifie r  
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-2 A  
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  
12  
 
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.  
De ve loping the 1 1 7 6  
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 Tha n a Vinta ge Fa d: Cla ssic 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 Shiple y  
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  
13  
 
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."  
Alle n Side s  
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."  
Murra y Alle n  
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.  
14  
 
Ke n Ke ssie  
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  
15  
 
inconsistent piece to piece than the 1176s, because of the tubes and the difference in  
fatigue of the tubes.  
"My big mentors were Andy Johns and Lee DeCarlo and Ron Nevision because they were  
all Record Plant guys. I learned how to make a rock and roll record from them. Although  
over the years it's become my own thing, my style still tends to be that Record Plant style,  
U87s, 1176s, LA-2As, 47 F.E.T.s...it's what I like."  
Mike Clink  
Producer/ Engineer Mike Clink (Guns N' Roses, Sammy Hagar, Pushmonkey) also comes  
from the Record Plant school of recording. "I find that I actually use 1176s more now than  
I ever did," he comments. "I like them because they bring out the brightness and presence  
of a sound--they give it an energy. It seems like when I'm mixing I end up using an 1176  
on the vocals every time. And if I want to compress a room sound I'll take a mono room  
mic, put an 1176 across it and push in all the buttons."  
Bruce Swe de in  
Bruce Swedien is a master engineer who needs no introduction. He also is a die-hard  
1176 fan. "I have two silverface 1176LNs in my rack that Bill Putnam personally picked  
out for me," he says. "I remember sitting at Bill's place in the Channel Islands, and talking  
about the 1176 and how I wanted a pair . The next time we went over he'd picked this  
pair out and they were sitting in his garage waiting for me. I love them on vocals. All of  
the Michael Jackson and James Ingram vocals that everyone has heard so much were  
done with at least one of those 1176s. I couldn't part with them for anything. They sound  
fabulous."  
16  
 

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