# 24LA technical research summary

## Signal path recreated

The original system places the T4 optical attenuator ahead of a 12AX7 voltage-amplifier stage, followed by a 12BH7 cathode follower and output transformer. A separate 12AX7/6AQ5 chain drives the electroluminescent panel from the input-derived side-chain signal. This means Peak Reduction changes side-chain drive while Gain belongs to the audio amplifier path.

24LA follows that topology at a grey-box level:

1. Transformer-like input conditioning and linked energy detection.
2. Soft-knee nonlinear light-drive curve.
3. Multi-state T4 conductance memory.
4. Optical attenuation ahead of amplifier gain.
5. Asymmetric tube and transformer saturation plus band limiting.

## Timing decision

The 1966 instruction manual calls the basic action essentially instantaneous and lists a 10 microsecond figure, while modern Universal Audio documentation characterizes the average T4 attack around 10 milliseconds. Those describe different scopes: EL excitation versus the observed complete opto system. 24LA uses a 10 ms complete-system attack.

The release model follows the well-documented split response: roughly half the recovery occurs in about 60 ms, and the remainder takes approximately 1–15 seconds depending on the strength and duration of prior reduction. Three optical memory states and a separate illumination-dose state create that dependency without a user-facing release control.

## Primary references

- Teletronix, *Model LA-2A Leveling Amplifier Instruction Manual* (1966), Library of Congress scan: https://tile.loc.gov/storage-services/master/mbrs/recording_preservation/manuals/Teletronix%20Model%20LA-2A%20Leveling%20Amplifier.pdf
- James F. Lawrence, *Variable gain amplifier system utilizing a solid electroluminescent cell*, US Patent 3,258,707: https://patents.google.com/patent/US3258707A/en
- Universal Audio, *LA-2A Tube Compressor Manual*: https://help.uaudio.com/hc/en-us/articles/19378009641748-LA-2A-Tube-Compressor-Manual
- Universal Audio, *A History of the Teletronix LA-2A Leveling Amplifier*: https://help.uaudio.com/hc/en-us/articles/215779663-A-History-of-the-Teletronix-LA-2A-Leveling-Amplifier

## Research literature considered

- Wright et al., *Efficient Neural Networks for Real-time Modeling of Analog Dynamic Range Compression*: https://arxiv.org/abs/2102.06200
- Simionato and Fasciani, DAFx 2022 optical-compression modeling paper: https://www.dafx.de/paper-archive/2022/papers/DAFx20in22_paper_35.pdf
- Parker et al., *Modeling Analog Dynamic Range Compressors using Deep Learning and State-space Models*: https://arxiv.org/abs/2403.16331

The shipped DSP is intentionally a transparent, deterministic grey-box implementation rather than a black-box neural model. Exact unit matching would require calibrated sweeps and program captures from a specific physical LA-2A/T4 cell.
