Optical compression

Optical compressors have a slower attack, controlled by light.¹ They have a frequency-dependent attack time² and react based on RMS rather than peaks.³ The LA2A is the most famous optical compressor.² The LA3A is similar, but valveless.⁴

These sound great on vocals and bass, especially when doing just a few decibels of gain reduction,⁵ but they can also compress a lot without it being so obvious.³ They can, however, be susceptible to pumping if a loud spike is fed into them.²

Using RMS compression with lookahead can simulate opto.⁶ Kotelnikov GE’s Inertia mode is similar to optical compression. LALA is an LA2A emulation. I like Pro-C 2’s optical mode. MTurboComp does an LA2A.

Mike Clink, Mark Endert, Tony Maserati, Jim Scott, and Rick Beato⁷ all like an LA2A on bass.⁴ Beato likes an LA2A and acoustic guitar and an LA3A clean guitars.⁷ Serge Tsai and Glen Ballard like the LA2A for how warm and gentle it is on vocals. When pushed, the LA2A can distort nicely, say Joe Chiccarelli and Joe Barresi.⁴