Home » Recording Resources » Featured Reviews » October 2021: Slate Digital FG-2A and Custom Opto

Two new optical contenders join the Virtual Analog Bundle


Review by Paul Vnuk Jr.

The Virtual Mix Rack plugin from Slate Digital (reviewed February 2016 and June 2018) is a modular rack enclosure plugin with eight slots that you fill with your choice of unique Slate audio processors to build your own custom channel strips. The latest VMR additions include two takes on the most classic optical compressor of all time: the Teletronix LA-2A—one a meticulous recreation of a great sounding original, and another that blows up the blueprint to add modern features for added flexibility.

The FG-2A

According to the Slate Digital team, the FG-2A took over a year to model. The team dug into the saturation of the analog circuits, the reaction of the T4 optical attenuator, the attack and release times and more. Spoiler alert—it shows!

Classic, simple level control

With just three controls, the LA-2A was and is one of the easiest compressors to use. A peak reduction (threshold) knob clamps down on incoming transients, reducing their level. A gain knob then allows you to seat the results in your mix—that’s all there is to it. A large meter lets you see how many dB of gain reduction you are applying to your signal, and the reaction time. The final control is a compress / limit switch that alters the unit’s ratio and reactive compression curves. Compression sits at around a 4:1 ratio while limiting is inf:1. The compression knee gets harder and faster the more you hit the threshold. An optical compressor by nature is relatively slow compared to its faster FET and VCA cousins. I find the compression setting to offer slower, less obvious compression, while limit mode is snappier and can induce pumping.

Modern extras

The FG-2A adds a highpass filter so low-frequency content can pass through the threshold without getting clamped down, and a mix knob provides parallel compression. Finally, a stereo switch lets you decide if the threshold is independent (wide and open) or equally reactive (gel and glue) on the left and right signals.

No two are the same

My 15-year-old Universal Audio LA-2A hardware reissue is, without exaggeration, the most used compressor I own, finding its way onto almost every vocal and bass tracking session and mix. Does it sound anything like an original 60s / 70s vintage unit? My answer would be, I don’t know, and I don’t care. The sound, settings and ballistics can vary from unit to unit, era to era. Over the years, I have found these sonic differences to be subtle unless a unit is very old or in rough shape. Most of the time, there is a discernible familial sound among them all.

For its model, Slate Digital chose the unit it found to have the warmth, speed, and sound it liked best. To test it out, I started with test tones to dial in the correct gain reduction on the meter and the output level. Once a vocal or bass track was playing, I adjusted the controls for a matched ballistic response and sound. With the hardware at 30 gain and 30 reduction, the FG-2A was similar at 32 gain and 25 reduction on vocals. On a Fender Jazz bass, I found that gain reduction of 25 on the hardware equated to 22 on the plugin. It is important to remember that these numbers are not decibels on the LA-2A and FG-2A, and you need to use your ears and the meters. Slate Digital excels at accurate and detailed ballistic metering response on its compressor models, often more accurate than some hardware units.


I want to be one of those reviewers who retain their street cred by telling you that the hardware is better with more depth and analog air, but in this case, I cannot empirically do so. While there will be a difference in sound and reaction if you match knob for knob, when matched for reaction and proper levels (knob settings be damned!), the sound is as close as any two top-notch hardware units will be. When I compared the vocals and bass soloed, I barely heard any sonic differences, from the deep reach of the low-end to the warm smoothness of the grab, hold and release. Once I un-soloed the tracks in the mix, any subtle differences were nil.

Custom Opto

Just as Team Slate did by offering multiple 1176 FET flavors spread across the FG-116, FG-116 Modern, and Monster offerings, Custom Opto takes the painstaking FG-2A model and crafts a whole new optical compressor experience that beautifully retains its sound and feel.

Pimp my opto

Added to the standard FG-2A controls of Peak Reduction and Output Gain, the Comp/Limit control has been replaced by a variable Ratio knob (1:1 to 10:1). Next up is a Speed control that allows you to tailor the combined attack and release characteristics to suit the music. The default setting is 5, making me guess this is the standard classic knee / speed setting. The mix knob and stereo option are also present in the Custom Opto.

Pimp my tone

The final control is a five-position tone control with a choice of flat, smooth, warm, aggro and airy. A second knob allows you to exaggerate its effect from 100 to 200%. Each setting is well named. Smooth adds a mellow, even sheen to the sound. Warm is the dustiest and most vintage-sounding of the group. Aggro is the most obvious, mid-forward and punchy (almost FET-like), and airy adds a top-end air to the extreme upper register that can nicely open up a compressed source. These tonal variances go from subtle to more pronounced when set to 200% and depend on how hard you squeeze the sound.


For engineers who find old-school opto compression (ahem) limiting, this is the opto compressor for you. It takes the traditional LA-2A and pushes it into realms similar to a Tube-Tech CL1B, Millennia Media TCL-2, and beyond. This makes it versatile and quite handy on a drum mix, modern keyboards, and especially the master bus, which is where the Custom Opto has found the most use in my mixes since it arrived, and I do not expect that to change anytime soon.


The best part is that if you are a Slate Digital All Access Pass Subscriber, you have had the FG-2A for a while now, and the new Custom Opto should be there by the time you are reading this. You can purchase the Virtual Mix Rack ala-carte, but do look into the subscriptions—the All Access Pass includes over 65 plugins, soft synths, online classes, sample libraries, and more.


Price: All Access Pass: $9.99 month for 6 months, then $14.99 monthly (annual); $24.99 month-by-month; $149 annual upfront.

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