Compression Study #8 – Opto Compressors

So far this series of blog posts have covered technical aspects of different 1176 style FET compressors. We will be moving away from FET from this point onwards and will be looking briefly into other styles of gain reduction, before applying these compressors for mix purposes. 

This post will look at Opto compression, and how this style of gain reduction is different from FET. One of the most popular Opto compressors is the LA-2A (Silver faceplate) and its previous revisions, the LA-2A (Grey faceplate) and LA-2. The optical gain reduction circuit paired with input/output transformer and tubes along the signal path made for a very colourful audio processor (Universal Audio, 2013).

Opto (Optical Photocell) gain reduction circuits use a light source, controlled by the voltage changes of the audio, and a light detector which imparts gain reduction. In the original designs this system used none-linear components that could produce unpredictable results. The combinations that yielded musical results formed the circuitry of the LA-2A and set the template for the ‘Opto sound’. The LA-2A used the T4 optical attenuator. They typically have a much slower attack than FETs, and a smooth logarithmic release. This release phase changed depending on input material however, drawing out the release based upon light intensity and duration. Modern Opto compressors use more accurate LED light sources for more controllable result (Sweetwater, 2002).

OneLA 5kHz CompThe gain reduction graph above shows the IGS OneLA Opto compressor, undergoing gain reduction on the ‘Comp’ setting. This produces a ratio around 4:1. Its behaviour is evidently that of an Opto, as aforementioned. The attack phase is fixed and closes down over 10ms. The release phase shows a logarithmic curve that smoothly open up. The input dependant release can be seen to be in effect, when comparing the length of the release after the transient burst and sustained burst.

OneLA 5kHz LimitSurprisingly the shape of the gain reduction curve remains very similar when switched to ‘Limit’, although the ratio is changed to 100:1. Unlike the FET ratio changes, the Opto’s light receiver maintains the same characteristic whilst imparting gain reduction.

OneLA Frequency ResponseThe green line represents a clean sine sweep over 10 seconds, the red is the resulting distortion after processing through the IGS OneLA. The frequency response is fairly similar up to 2kHz. At around 2.4kHz the OneLA begins to colour the high frequency content.

“Optical compressors, especially those that don’t use super-well-behaved integrated optical circuits (or those that use them imaginatively) usually impose more of their own character on the material being treated, making it sound larger than life. In this context, the compressor is as much an effect as a gain-control device, and such compressors are popular for treating vocals, drums and basses.” (White, 2003)

Screen Shot 2016-03-29 at 15.30.31This distortion is imparted on all audio that passes through the LA-2A styled Opto compressor, even whilst not under gain reduction. The above signal flow diagram shows the audio passes though two transformers and the tube gain stage, where two dual triode tubes are run in series (IGS Audio, 2015).

After understanding the characteristics of classic FET and Opto compressors, they can be used together to address dynamic pieces of audio that one single compressor would struggle to handle without noticeable pumping and artefacting. For example, on a particularly dynamic vocal, the 1176 can be set to catch peaks at 12:1 and the LA-2A can then be set to round the vocal and add the ‘larger than life’ weight without over-comprerssing (Crane, 2010).

Crane, D. (2010). Chaining the 1176LN and LA-2A Compressors for Maximum Control – Blog – Universal Audio. [Online]. Available at: [Accessed: 29 March 2016].

IGS Audio. (2015). ONE LA 500 – IGS Audio. [Online]. Available at: [Accessed: 29 March 2016].

Sweetwater. (2002). What’s so good about optical compressors?. [Online]. Available at: [Accessed: 28 March 2016].

White, P. (2003). SOS – Q+A – Your Questions Answered. [Online]. Available at: [Accessed: 29 March 2016].



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