The RMSMC5 Valvop from Roll Music Systems.

A real, transformer-coupled tube optical compressor in the convenient package you never thought possible. At long last, all the warm sweet satisfaction of an optical compressor with a real transformer-coupled, high-voltage vacuum tube audio path in the convenient and cost-effective 500-series modular format. You love the sound of the RMS5A7 Tubule tube mike preamp module, and now you can enjoy that same sonic bliss in a line-level compressor and the same form factor. The compression is all optical for a smooth compression that's ideal for both vocals and electric bass. (But don't let us stop you there!) The audio path uses the same all-tube amplifier circuit found in the Tubule.

The Valcop has been confirmed to be fully compatible, both electrically and mechanically, with any enclosure that complies with the standards for the 500-series format. 3rd-party approval is pending.