OPTO Compressors

 In TSM Studio Blog

Today’s topic of discussion will center around OPTO (sometimes referred to as Optical) Compressors. 

  • OPTO Compressors function by using the incoming audio signal to feed a light element (such as an LED or a lightbulb) which shines upon a light-sensitive resistor. The resistance of this light-sensitive element informs the compression circuit how much and how quickly the signal should be attenuated. This form of gain-reduction is essentially controlled by a photo or light cell. The way the photocell reacts to the light source causes some inherent delay in the response time that triggers the attack and release of the compressor. This delay attributes to the OPTO compressor’s reputation for having a response time deemed, “smooth” and “musical”. Although the attack is relatively fast and the release initially begins quickly, the release rate gradually slows as the signal’s voltage decays.  
  • Different OPTO compressors are known to have characterizing sounds that are dependent on the light sources and resistors used in construction.
  • One famous OPTO compressor is the tube-based LA-2A. A defining part of this compressor lays in its T4 photoresistor. The T4 cell in a LA-2A was originally constructed with an attack time of 10 milliseconds paired with a gradually sloping release that can last up to five seconds.
  • With an attack and release time that are almost always non-linear, the slope from “compressed signal” to “uncompressed signal” will not fall in a consistently predictable fashion. This means that if you were to set the gain-reduction to 20 dB (a very high amount), the first 10 dB would release much quicker than the following 10 dB.
  • Because of the slow, smooth nature of OPTO compressors, they’re primarily useful when tending to macro-dynamics. OPTO compressors often perform well when applied to vocals, bass, and guitars, but due to their slower attack and release times don’t tame transients as well as VCA or FET compressors.
  • Most OPTO compressors have two parameter knobs; gain and threshold. The threshold knob, often labelled, “Peak Reduction”, determines at what level the compressor will begin to engage. The gain knob functions post-compression and IS NOT the input gain to the compressor. This parameter is the output gain to a DAW or tape machine.
  • Examples of OPTO Compressors include:
    • LA-2A
    • LA-3A
    • LA-4A
    • TubeTech CL1B
    • Manley ELOP