IGS Audio ONE LA 500 is a tube optical compressor in 500 Series format — the only unit in this format built around a full-size T4B photocell operating at 250 V DC, generated by an onboard step-up converter. Double-wide (2 slots), mono. The audio path follows the Teletronix '68 LA-2A circuit: JJ 12AX7 on the input, JJ 12AU7 on the output, Sowter input transformer, Edcor output transformer, and an OEP transformer in the sidechain circuit. Components are 1% metal film resistors and Wima capacitors, individually calibrated per unit. Two controls — Peak Reduction and Gain — and a Compress/Limit switch form the complete interface.
250 V DC in a 500 Slot — Full Tube Performance Without Compromise
A standard 500 Series slot delivers ±16 V, which is adequate for transistors and ICs but wholly insufficient for triodes, which produce their characteristic sound at plate voltages of 200–300 V. Starved tubes at low plate voltage don't operate in their linear range and instead produce hard, unpleasant distortion rather than the smooth, creamy character of an LA-2A. IGS Audio solves this with an onboard boost converter that draws from two adjacent slots (combined 250 mA draw) and generates 250 V DC — the 12AX7 and 12AU7 tubes operate under conditions identical to full-size rackmount hardware. The result is optical compression with the natural program-dependent release curve of the T4B photocell, triode creaminess, and the transformer warmth of Sowter and Edcor — all in a double-wide 500 format body.
T4B Photocell and Three Transformers — Signal Path Architecture
The ONE LA 500 is the only optical compressor in 500 format built around a full-size T4B photocell — not a miniaturized T4Bx substitute, but the same element used in full-size LA-2A units. Three transformers across the path: Sowter on the audio input, Edcor on the audio output, and OEP in the sidechain control circuit — transformer coloration at every stage of the chain. The VU meter is calibrated from the front panel. Power draw of up to 250 mA total from a standard 500-series chassis with no requirement for a specialized power supply beyond two available slots.
KEY FEATURES
- Tube optical compressor in API 500 Series format, double-wide (2 slots), mono
- First high-voltage tube 500 Series module from IGS Audio
- Plate voltage: 250 V DC (onboard step-up converter from 2 slots)
- Audio path: Teletronix '68 LA-2A circuit topology
- Tubes: JJ 12AX7 (audio input), JJ 12AU7 (audio output)
- Photocell: full-size T4B (field-replaceable on special order)
- Transformers: Sowter (audio input), Edcor (audio output), OEP (sidechain)
- Compress and Limit modes (switchable)
- Controls: Peak Reduction (continuous, 41-position), Gain (continuous, 41-position)
- Max gain: 36 dB; max gain reduction: 36 dB
- Hardwire bypass
- VU meter calibrated from front panel
- Input impedance: 10 kΩ (balanced)
- Max output: +21 dBu; dynamic range: 100 dB; noise: –99 dB(A)
- Frequency response: 20 Hz – 20 kHz, ±0.1 dB
- Power draw: max 250 mA total (from 2 standard 500-series slots)
- 1% metal film resistors, Wima capacitors throughout
- Handmade in Poland
TECHNICAL SPECIFICATIONS
- Format: API 500 Series, double-wide (2 slots), mono
- Type: tube optical compressor
- Plate voltage: 250 V DC (onboard converter)
- Tubes: JJ 12AX7 (input), JJ 12AU7 (output)
- Photocell: T4B (full-size)
- Transformers: Sowter (audio input), Edcor (audio output), OEP (sidechain)
- Input impedance: 10 kΩ (balanced)
- Operating level: +4 dBu
- Max input level: +20 dBu
- Max output level: +21 dBu
- Dynamic range: 100 dB
- Noise level: –99 dB(A)
- Max gain: 36 dB
- Max gain reduction: 36 dB
- Frequency response: 20 Hz – 20 kHz, ±0.1 dB
- Power consumption: max 250 mA total (2 slots)
- Made in: Poland
