538T OPTO Stereo Compressor is a two‑channel analogue optical compressor in 500‑series format. It accepts left/right balanced or unbalanced inputs and balanced, transformer‑coupled outputs, with a hard‑wired bypass for instant comparisons. It offers wide compression ratios from gentle to extreme, UV/compression metering, and a stereo width control. Frequency response is 20 Hz–20 kHz within 1 dB; gain up to 24 dB; power 3 W. Designed for 500‑series racks.