Circuit Description - TELETRONIX LA-2A Instruction Manual

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10.
Input Impedance:
50, 150, 250, and 600 ohms, balanced or
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unbalanced
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Out~ut Impedance:
50, 150, 250, and 600 ohms, balanced or
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unbalanced
12. Output Source Impedance:
approximately 150 ohms, at 800 CPS
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13. Panel Size:
standard 19" x 5-1/4"
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14. Depth Behind Panel:
7-1 / 4"
15. Panel Controls:
Gain (input level), Peak Reduction and Meter
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Selector Switch
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Meter:
DB Gain Reduction and DB Output
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17. Power Requirements:
115/ 230 volts 50-60 cycle 35 watts
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18. Tube Complement:
( 2 ) 12AX7A
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( 1 ) 12BH7A
( 1 ) 6AQ5
19. Fuse:
3AG, 3/ 8 AMP Slow Blow for 115V;
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3AG 15/ 100 AMP Slow Blow for 230V
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CIRCUIT DESCRIPTION
The LA - 2A Leveling Amplifier will produce essentially instantaneous gain reduction of
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over 40 DB with no increase in harmonic distortion.
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A typical gain reduction curve for this system is illustrated on Figure 1. 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,
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but the output increases less than 1 DB. The leveling amplifier thus combines the characteristics
of a compressor and limiter. A reasonable amount of care in gain riding will restrict normal
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operation to the compression region, but uncontrolled output levels will be prevented by the
limited action .
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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 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
electro-luminescent light source. The electro-luminescent device provides a light intensity
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which is proportional to the audio voltage applied to its terminals. Not unlike a capacitor in
LA-2A (3-66)
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