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What and Why Opto?<\/h3>\n\n\n\n
Optical compressors date back to the 1950s when Telektronix (which eventually became part of Universal Audio) debuted the LA-1 compressor. By the mid-1960s the LA-1 developed into the current LA-2A compressor design\u2014 which has become a staple both as recording hardware and DAW-based software. Universal Audio manufacturers an extremely popular hardware reissue of the original LA-2A to this day.<\/p>\n\n\n\n
The magic of the LA-2A, and every other optical compressor that followed lies in an optical circuit that acts as the level control, or attenuator. In effect, the audio signal lights a small light panel and a nearby photoresistor senses the light and controls the behavior of the gain reduction. The compressor then adds make-up gain with a tube amplifier (LA-2A) or a solid-state amplifier (LA-3A). <\/p>\n\n\n\n
The user-adjustable parameters of an opto compressor are output gain and gain reduction (threshold). The fixed attack is fast, while the fixed release happens in two stages as described above, and would be generally characterized as slow. The LA-2A provides two modes: compress (3:1) and limit (100:1). Remember that the frequency-dependent nature of the opto blurs these parameters a bit\u2014in a musically helpful way. Most digital models of the LA-2A and LA-3A provide an emphasis control (sometimes labeled pre-emphasis) that often looks like a slotted screw. This screw exists on the original hardware units (sometimes on the rear panel) and was factory set and perhaps adjusted by a studio tech to suit the studio\u2019s preferences. The emphasis control was designed to provide radio stations with better control over the broadcast signal\u2019s high-frequency content.<\/p>\n\n\n\n
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But, the Sound!<\/h3>\n\n\n\n
Optical compressors may contain tubes and transformers, or they may be solid-state and even transformerless. Their compression behavior remains natural and smooth regardless of a tube or solid-state engine, and the distortion and sonic color of a specific opto compressor relate to the amplifier circuitry, not the compression action. Many companies besides Universal Audio produce optical compressors and each design may use a slightly different opto device, so their attack and release times may differ, or even be user-selectable. As optos age, they become weaker, so after only a few years, the performance of an opto compressor may suffer until the opto module is replaced. The LA2A and many other opto compressors use a plugin-in opto module referred to as a T4, which are readily available from a few manufacturers for around USD 100.<\/p>\n\n\n\n
I find opto compressors extremely useful for tracking vocals and acoustic instruments. It\u2019s almost impossible to ruin a take with an opto compressor and between the tube (LA-2A) and solid-state (LA-3A) versions, you can capture a clean or harmonically rich track. When mixing, it really helps to delve deeper than simply adjusting the gain reduction and output knobs, so you can dial in precise variations of compression to fit the track you\u2019re working on. <\/p>\n\n\n\n
Many engineers swear by using an opto and a FET or VCA compressor in series, but before you get into that, spend some time learning to dial in the opto. Let\u2019s get into the gritty details of the Universal LA-2A and LA-3A. Then I\u2019ll provide a list of other optical compressors that provide similar compression with more or less character. Every DAW provides a stock opto compressor, so you won\u2019t even have to purchase anything to get into these comps.<\/p>\n\n\n\n