IGS Audio One Leveling Amplifier is a mono tube optical compressor inspired by the Teletronix LA-2A — a fully tube signal path with transformer-balanced input and output and a T4Bx photocell as the gain reduction element. Input transformer is IGS Audio 2510, output transformer IGS 15k/600; two 6N1P-EW dual triodes in the audio circuit and a 6N2P-EW + 6AQ5 pair in the photocell control circuit. Maximum gain reduction of 36 dB, dynamic range above 110 dB(A), 40 W internal power supply. Stereo Link via rear RCA connector.
T4Bx Photocell and Tube Topology — LA-2A Character in a New Design
The LA-2A is defined by optical compression: an electroluminescent panel drives a photoresistor, which attenuates the signal proportionally to its illumination. Attack and release timing derive from the photocell's physics rather than potentiometer settings — attack at 1 ms, release from 0.5 to 5 s with the 50% point at 60 ms. This means the compressor breathes with the material: fast attack catches transients, but the long release tail ensures the return to unity is always gradual. IGS Audio uses its own T4Bx photocell as a modern equivalent of the T4B, hand-matched for unit-to-unit consistency. Compress mode produces a soft knee and musical compression — the standard setting for vocals, acoustic guitar, and brass; Limit mode sharpens the knee and responds more aggressively to transients. Three controls complete the interface: Peak Reduction (threshold and depth of gain reduction), Gain (makeup gain to 30 dB), and the Compress/Limit switch. The VU meter toggles between output level and gain reduction display.
Full Tube Circuit and Transformer Color
Four tubes across the signal path and control circuit mean that every stage — amplification, sidechain, photocell drive — contributes tube harmonics to the output signal. THD+N of 85 dB(A) is typical for a studio-grade tube optical compressor: higher distortion than a transistor design, but harmonically musical and part of the character. The IGS Audio 2510 input transformer and 15k/600 output transformer add transformer coloration — midrange density and warmth that complements the photocell's organic response. Two One LA units linked via the rear RCA Stereo Link connector operate as a stereo pair with independent per-channel adjustment.
KEY FEATURES
- Mono tube optical compressor — inspired by the Teletronix LA-2A
- T4Bx photocell (IGS Audio Custom, equivalent of T4B) — hand-matched per unit
- Full tube circuit: 2 × 6N1P-EW (audio) + 6N2P-EW + 6AQ5 (sidechain control)
- Transformers: IGS Audio 2510 (input) + IGS 15k/600 (output)
- Compress and Limit modes (switchable)
- Controls: Peak Reduction, Gain, Compress/Limit switch
- Max gain: 30 dB; max gain reduction: 36 dB
- Attack: 1 ms; release: 0.5–5 s (50% at 60 ms)
- Input and output impedance: 600 Ω (balanced)
- Max input: +16 dBu; max output: +18 dBu
- Dynamic range: >110 dB(A) at 1 kHz, +24 dBu
- VU meter: output level or gain reduction (switchable)
- Stereo Link: RCA connector (rear panel)
- Internal power supply, 115/230 V AC, 40 W
- Channel configuration: mono (stereo pair via Stereo Link)
- Made in: Poland
TECHNICAL SPECIFICATIONS
- Type: mono tube optical compressor/leveling amplifier
- Compression element: T4Bx photocell (IGS Audio Custom)
- Tubes: 2 × 6N1P-EW (audio), 1 × 6N2P-EW (sidechain), 1 × 6AQ5 (sidechain)
- Input transformer: IGS Audio 2510
- Output transformer: IGS 15k/600
- Input impedance: 600 Ω (balanced)
- Output impedance: 600 Ω (balanced)
- Operating level: +4 dBu
- Max input level: +16 dBu
- Max output level: +18 dBu
- Dynamic range: >110 dB(A) at 1 kHz, +24 dBu
- Max gain: 30 dB
- Max gain reduction: 36 dB
- Attack: 1 ms
- Release: 0.5–5 s (50% at 60 ms)
- THD+N: 85 dB(A)
- Stereo Link: RCA (rear panel)
- Power: internal, 115/230 V AC, 40 W
- Made in: Poland
