TELETRONIX LA-2A - OPERATION

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construction, the electro-luminescent lamp consists of a plate of glass or plastic coated with a
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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
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the phosphors are excited by the voltage across the dielectric and light is produced . The amount
of light depends upon th e applied voltage and frequency. The gain or level controll ing element is
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the photo ·conductive cell. The resistance of the cell decreases with an increase in the impinging
light. Since the light is produced directly fro111 the audio voltage, the response is instantaneous.
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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
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is limited only by the response of the variable resistance photo cell used .
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cell is selected which provides 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.
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Referring to Figure 2, the functional block diagram, the input signal is applied directly
to the optical attenuator from the high impedance winding of the input transformer. The amount
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of attenuation introduced by the optical attenuator is controlled by the audio voltage applied to it
by the 6AQ5 (V4), which is the luminescent driver amplifier. The amount of signal applied to
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the 12AX7
(Vi) 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
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provides low distortion, flat response, and gain stability.
The output stage is somewhat unconventional in that a totem pole or double cathode follower
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is used. This output stage can tolerate great amounts of output impedance mismatch, but retains
low distortion and flat frequency response.
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For stereo broadcasting applications, a portion of the input signal is fed through the gain
reduction control to the 12AX7 control amplifier (V3). The output at this stage is applied to the
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stereo balance control and is also brought 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 generated in either amplifier
will cause equal gain reduction in both units. The control voltage is applied through the stereo
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balance control R3 to the 6AQ5 driver amplifier. This stage provides the necessary voltage to
operate the electro-luminescent light source.
OPERATION
The LA-2A Leveling Amplifier is designed to prevent an increase in output level beyond
a pre-determined point, and due to its unique deSign, functions as a combined compressor and
limiter. The effect is illustrated in Figure 1. The point at which the compressed curve breaks
LA-2A (3-66)
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