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01-24-2025, 06:39 PM
Hi Guys
Below is a schematic of the Tektronix LA-2A.
It looks pretty complicated until you follow it through from the input at the left. Really, it is just the left half of the circuit that comprises the compressor, the right half is simply a line amplifier and meter.
The signal from the input feeds a voltage divider comprised of R7 (68k), R6 (1k-2k7) and R1 (1M Gain pot). The output side of the electroluminescent device is wired in parallel with the Gain pot. The side-chain signal is tapped after R6 and is fed to a parallel 12AX7 (V3), which in turn feeds a 6AQ5-EL84 (V4) that drives the input of the EL element through a capacitor. The 12AX7 also drives the meter in one of the selectable monitoring positions.
In the voltage divider, R7 can be shorted out so there is a harder compression effect. Otherwise, the function is as a softer limiter. In the side chain, the Lim Response control shunts a cap that effects the attack of the compression.
The right-hand side of the circuit is the line amp comprised of cascaded 12AX7 stages (V1) feeding a Whyte cascode follower (V4, 12BH7). There is feedback from the follower output back to the cathode of the input stage, setting a fixed gain of about 66x, or 36dB. No doubt the input and output transformers and this line amp add character to the sound which many sound engineers and artists enjoy.
The power supply is a simple CTed plate winding with solid-state diodes feeding a CRC filter. There is tap off of this to a tube zener which provides a reference voltage for the EL to work against in one of the meter switch positions that would indicate the amount of compression rather than the output signal level.
Right click on the image to see it larger in a separate tab.
Below is a schematic of the Tektronix LA-2A.
It looks pretty complicated until you follow it through from the input at the left. Really, it is just the left half of the circuit that comprises the compressor, the right half is simply a line amplifier and meter.
The signal from the input feeds a voltage divider comprised of R7 (68k), R6 (1k-2k7) and R1 (1M Gain pot). The output side of the electroluminescent device is wired in parallel with the Gain pot. The side-chain signal is tapped after R6 and is fed to a parallel 12AX7 (V3), which in turn feeds a 6AQ5-EL84 (V4) that drives the input of the EL element through a capacitor. The 12AX7 also drives the meter in one of the selectable monitoring positions.
In the voltage divider, R7 can be shorted out so there is a harder compression effect. Otherwise, the function is as a softer limiter. In the side chain, the Lim Response control shunts a cap that effects the attack of the compression.
The right-hand side of the circuit is the line amp comprised of cascaded 12AX7 stages (V1) feeding a Whyte cascode follower (V4, 12BH7). There is feedback from the follower output back to the cathode of the input stage, setting a fixed gain of about 66x, or 36dB. No doubt the input and output transformers and this line amp add character to the sound which many sound engineers and artists enjoy.
The power supply is a simple CTed plate winding with solid-state diodes feeding a CRC filter. There is tap off of this to a tube zener which provides a reference voltage for the EL to work against in one of the meter switch positions that would indicate the amount of compression rather than the output signal level.
Right click on the image to see it larger in a separate tab.
