Model 2-LA-2 Twin T4 Leveling Amplifier Universal Audio Manual Number 65-1601 Revision 2.
-LA-2 Twin T4 Leveling Amplifier Thank you for purchasing the 2-LA-2 Twin T4 Leveling Amplifier. The 2-LA-2 Takes the best our UA Classics LA-2A reproduction and updates it with modern features, modern components, and a spacesaving design. The original LA-2A was designed by Jim Lawrence and produced in the early 1960s by Teletronix, which was later acquired by Babcock Electronics Corporation.
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, 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.
Notice This manual provides general information, preparation for use, installation and operating instructions for Universal Audio’s 2-LA-2. 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.
Table Of Contents Performance and Audio Specifications.................................................................................................... 5 2-LA-2 Operating Instructions................................................................................................................. 2 2-LA-2 Front Panel Channel Controls ...................................................................................................... 2 Peak Reduction ......................................................
Gain Reduction Up to 40 dB Maximum Gain 35 +/- 1dB Attack Time 10 ms Release Time 0.06 for 50% release; .5 to 5 seconds for complete release Frequency Response 30 Hz to 15 kHz +/- .1 dB Signal to Noise -70 dBu Distortion Less than 0.35% at +10 dBm, and less than 0.
2-LA-2 Operating Instructions Figure 1: Full view, front panel of the 2-LA-2 The 2-LA-2 is a two channel, vacuum tube amplified, optical compressor utilizing T4 cell gain reduction elements. The 2-LA-2 features linkable sidechains allowing the user to stereo couple the two channels at the flip of a switch.
Gain Once the desired Peak Reduction has been set, use Gain to make up any lost signal. The Gain knob controls the final output signal of the 2-LA-2. Limit / Compress Switch The Limit/Compress Switch changes the characteristics of the compressor’s ratio curve. When in the Compress position, the knee is gentler, and presents a low compression ratio, roughly 3:1. A higher limiting ratio with a harder knee results when the switch is set to the Limit position.
2-LA-2 Front Panel Global Controls Figure 3: Front panel global controls Link / Dual Use the Link/Dual switch to easily go between dual mono or stereo linked operation. The Dual position allows the two channels of the 2-LA-2 complete independence from each other. In this position, the 2LA-2 operates as two separate compressors.
Rear Panel Connections The rear panel has two identical channels each with Line Input and Line Output connectors. The rear panel also has an AC Power input with a fuse holder. These connectors and controls are discussed in the following sections. Figure 4: Full view, rear panel of the 2-LA-2 Input / Output Standard XLR input and output connectors are provided on the rear panel. Pin 2 is wired positive (hot) on the Line Input, and on the Line Output.
Figure 5: Operating Voltage Selector AC Power The 2-LA-2 uses a standard, detachable IEC power cable. Fuse / Mains / Bulbs The AC power fuse is located in the AC power connector block. Remove the power cord before checking or changing the fuse. A 1.6 A time delay (slow blow) fuse is required for operation at 115 V. A 800 mA time delay (slow blow) fuse is required for operation at 230 V. A 6.3 V bulb (1847) is used for the power indicator light.
Appendixes Compressor Basics Before we dig in to a description of the LA-2A/2-LA-2 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.
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.
Figure 3: Block diagram of the LA-2A compressor 9
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.
output. In reality, the photocell resistance cannot go completely to zero, hence there will always be some signal present. HA-100X 7 R6 R7 68K 2.7K GAIN R1 TO VOLTAGE 100K AMPLIFIER 9 8 R5 68K TO GAIN REDUCTION CONTROL ELECTROLUMINESCENT ELEMENT .0047 uF 10 PHOTOCONDUCTIVE CEL TO ATTENUATOR DRIVE AMPLIFIER 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.
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.
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. R16 68K VOLTAGE AMPLIFIER CATHODE FOLLOWER R16 C7C R9 220K V1A 30uF .1 68K R13 220K C1 R17 10K C2 .02 V1B R21 100K .1 C3 .1 R10 1.5K R12 470K C5 A-24 V2A FROM R1 R11 68K C2 10 uF 1 12AX7 R14 2.
Historical Notes Bill Putnam Sr. was awarded the 2000 Technical Grammy for his multiple contributions to the recording industry. He was highly regarded as a recording engineer, studio designer/operator and inventor. Putnam was considered a favorite of musical icons including Frank Sinatra, Nat King Cole, Ray Charles, Duke Ellington, Ella Fitzgerald and many, many more.
Legendary engineer Wally Heider, manager of remote recording at United, used his 610 console to record many live recordings including Peter, Paul and Mary “In Concert” (1964), Wes Montgomery’s “Full House” (1962), and all of the Smothers Brothers Live albums. Heider’s console was later acquired by Paul McManus in 1987, who spent a decade restoring it. [We thank Paul for his efforts and his contribution to our efforts to trace the history of the 610.] At least one 610 module is still in use at Ocean Way.
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 . 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 .
Notes 17
