One of the most earliest forms of analogue compressors.
Optical compressors, or “opto” compressors, rely on a light-dependent resistor and a light source to dictate the way in which compression is applied. An optical compressor’s input signal illuminates a light source—the more powerful the input signal, the brighter the light will shine. As the light source illuminates, the resistor causes the compressor to apply compression, and when the light source dims, the amount of compression applied is reduced.
The thing to keep in mind about optical compressors is that the materials used in their construction greatly affect how they behave. One type of light source may illuminate faster than an other, while something as simple as the material the resistor is made of can alter the way compression is applied as well.
As Messitte states, “…the harder you hit an optical compressor, the quicker its initial release time can be—but the slope back to a normal, uncompressed sound will not fall in a linear fashion. It will “curve.” So if the circuit gives you 10 dB of gain-reduction, the first five decibels might release much more quickly than the following five.”
The result of this is that optical compressors tend to sound musical and “smooth.” To control sharp transients, an optical compressor is usually not the first choice for most audio engineers. Although, optical compressors tend to compliment ballads, strings and folk vocals quite well.
The Universal Audio Teletronix LA-2A is a famous electro-optical compressor/limiter with tube circuitry that applies gain reduction with zero increase in harmonic distortion. When you take a look at an LA-2A, you’ll notice that the primary controls consist of just a Gain knob, Peak Reduction knob, and Compress(3:1 ratio)/Limit(100:1 ratio) switch. There are no attack, release, or threshold controls.
The way in which the LA-2A applies compression is highly program-dependent. When strong signals are run through the compressor, you can expect long release times, and when weak signals are run through the compressor, you can expect short release times. The “uncalculated” nature of the LA-2A is exactly what makes it desirable.
LA-2A for vocals is a great option. I’ll set it to reduce 2 dB or so during tracking. It gives you transparent dynamic control that never feels overdone, while giving the vocalist an extra dose of confidence on the mic. The LA-2A is also high on my list of favourite bass compressors, though not if I need to be precise with attack and release times. I love it on string instruments, pads and any kind of legato source material.
The LA-3A is my go-to choice for any type of rhythm guitar — electric or acoustic. Similarly, I’ll dial in just a couple dB of reduction and then compensate the output gain. More often than not, the amount of volume I get back feels like much more than what I lost. The beauty of these compressors is that if you don’t overdo the gain reduction, the limited controls make it pretty hard to miss with them. Just make sure an opto is really the right tool for the job.