Teletronix LA2A - History and How it Works
With the advent of classic gain reduction units in software/plug-in form, I thought I would start a series of threads explaining how these original classic units worked - kind of a "what makes them tick" series of threads, for those who are using emulations from companies like
I believe that knowing how these original models worked, and what they were/are designed to do, will perhaps help those who are using the plug-in versions of them to better understand them, and hopefully, to use them to their optimum.
Part2 in the series...
The Teletronix LA2A
The LA-2A Leveling Amplifier is an audio gain reduction device, invented by James F. Lawrence II, founder of the Teletronix Engineering Company in Pasadena, California in the early 1960s. Teletronix was sold to Babcock Electronics of Costa Mesa, California in 1965. In 1967, Studio Electronics (eventually renamed UREI), picked up Babcock's broadcast division, including the Teletronix brand. Three versions of the LA-2A were made until 1969. The LA-2A was inducted into the TECnology Hall Of Fame in 2004.
The LA-2A has simple controls: a Peak-Reduction knob controls the gain of the side-chain circuit, and therefore, the gain reduction; a Gain Control for make-up gain; and a Limit/Compress switch which alter the compression ratio. The VU meter may also be switched to show gain reduction or output level.
Compared to other gain reduction devices, the LA2A is considered to be "slow" in reaction time, with the attack set for 10 milliseconds. The LA-2A has a sonically "smooth" character that has made it sought after by many recording engineers over the years. Widely used on vocals, but compatible with all instruments, it is often found as a second stage compressor/limiter located after a "faster" type GR is used first (FET-style such as the Urei 1176).
The Teletronix Leveling Amplifier will automatically reduce audio peaks which might otherwise over-drive broadcast or recording equipment.
How It Works:
The LA-2A is a hand-wired, tube-based, compressor. It uses an electro-luminescent panel, together with a cadmium-sulfide light-dependent resistor, to provide gain reduction, which, in the LA-2A, is called the T4 cell. The properties of the T4 give the LA-2A its unique character by making it an entirely program dependent design. The attack time is 10 milliseconds, and the release time is about 60 ms for 50% release and 0.5 to 5 seconds for full release, depending on the previous program material.
Automatic gain reduction is accomplished by the use of the T4 Cell, which is placed ahead of the first amplifier stage. The attenuation is controlled by the amplitude of the LA-2A input signal.
This system permits up to 40 DB of instantaneous gain reduction, yet causes no wave form or harmonic distortion. The amplifier provides sufficient gain and output level (10 DBM nominal) to be used as a line or program amplifier, or for direct connection to the transmitter in the case of radio and TV operation. Provisions are made for interconnection of the optical attenuators to provide equal gain reduction in both channels when two of the LA-2A Leveling Amplifiers are used for FM stereo broadcasting.
The LA-2A Leveling Amplifier will produce essentially instantaneous gain reduction of over 40 DB with no increase in harmonic distortion.
Compressor action occurs from the breakaway point at -30 DB input and up to -20 DB, at which point the curve becomes horizontal to exhibit limiting action. The input increases an additional 20 DB, but the output increases less than 1 DB. The leveling amplifier thus combines the the characteristics of a compressor and a 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.
Let There Be Light...
The heart of the leveling amplifier is the electro-optical attenuator which is placed ahead of the first amplifier stage. The actual stage gains and and tube operating parameters are not varied, permitting the tubes to operate at optimum conditions regardless of the amount of gain reduction. The optical attenuator consists of a photo-conductive cell, which is optically coupled to an electroluminescent 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 contacts the phosphor coating. As alternating current is applied to the conducting plates, the phosphor particles s are "excited" by the voltage across the 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 cell. The resistance of the cell decreases with an increase in the impinging light. Since the light is produced directly from the audio voltage, the response is instantaneous. Rectification and filtering of the audio to produce a control signal, are not necessary as in the case of conventional limiters. This system results in automatic level control whose speed of operation is limited only by the response of the variable resistance photo cell used.
A cell is selected which provides a 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 complete release.
The input signal is applied directly to the optical attenuator from the high impedance winding of the input transformer. The amount of introduced by the optical attenuator is controlled by the audio voltage applied to the 6AQ5 which is the luminescent driver amplifier. The amount of signal applied to the 12AX7A voltage amplifier is also controlled by the manual gain control. The voltage amplifier stage provides a gain of 40 DB. Overall amplifier feedback of approximately 20 DB provides low distortion, flat response, and gain stability.
The output stage is somewhat unconventional, in that a totem pole or double cathode follower is used. This output stage can tolerate great amounts of output impedance mismatch, but retains low distortion and flat frequency response.
For stereo broadcasting applications, a portion of the input signal is fed through the gain reduction control to the 12AX7A control amplifier. The output at this stage is applied to the stereo balance control and is brought out out to a terminal on the chassis. For stereo operation, this terminal is connected to the same terminal on an identical amplifier and control voltage becomes common to both units. A gain-reduction control voltage in either amplifier will cause equal gain reduction in both units. The control voltage is applied to the stereo balance control to the 6AQ5 driver amplifier. This stage provides the necessary voltage to operate the electro-luminescent light source.
sources: Wiki, Teletronix/UA, Dreamhire