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Slate Digital Custom Opto

Analogue-modelling Compressor Plug-in By Matt Houghton
Published December 2021

At heart, it models the same hardware as Slate’s FG‑2A, but Custom Opto’s tweaks make it useful on so many more sources!

Back in SOS June 2021, I reviewed Slate Digital’s FG‑2A compressor plug‑in, and judged it to be amongst the best software emulations of the Urei/Teletronix LA‑2A tube opto hardware currently available.

Slate Digital Custom Opto compressor plug-in.As I explained in that review, there’s an awful lot to love about the LA‑2A, not least the subtle enhancement that it seems to deliver alongside the compression. But, while its operational simplicity and relatively relaxed optical compression can be just the ticket in some scenarios, in others they can render it of more limited use. For instance, it offers very little by way of control over the attack and release timings or ratio. Typically, if when using an LA‑2A you find you want more control, you’d turn to a different sort of compressor altogether or use the LA‑2A along with something else; it’s often partnered with an 1176, for example. But in the software world, developers have more scope to tweak things, and Slate recently asked themselves if it might be possible to augment the FG‑2A with new features that would make it more versatile, without sacrificing its overall tube opto vibe.

In answering that question they’ve developed Custom Opto, which joins, rather than replaces, FG‑2A in Slate’s range. If you simply want a great LA‑2A plug‑in, the latter remains, in my view, one of the most convincing around, but Custom Opto opens up so many more possibilities!

Like FG‑2A, Custom Opto is a module that’s hosted within Slate’s Virtual Mix Rack (VMR), itself a host plug‑in available for Mac and Windows hosts (VST, AU or AAX). As with all Slate’s plug‑ins, you can purchase Custom Opto outright if you prefer, but it is also included in their All Access Pass, a subscription bundle with an impressive and growing range of effects, instruments and sample libraries, which seems to me to offer remarkably good value for money.


Before you start tweaking, it’s worth listening out for the general tonality of Custom Opto, which can be useful even when it’s not compressing: there’s a touch of subtle but pleasing harmonic distortion and a slight EQ curve being applied all the time. In practical terms, this has the effect of making things seem a hint more ‘forward’ in the midrange and it can be a lovely, flattering effect. If you want make A/B comparisons with the unprocessed signal, though, note that quite a bit of make‑up gain is applied in the default preset. There’s nothing wrong with a compressor preset including make‑up gain to offset the envisaged gain reduction, but you’re definitely going to want to dial it back a bit for this purpose.

As on FG‑2A, the main controls for dialling in gain reduction are the Peak Reduction and Output Gain knobs. The former is essentially a threshold control, but the latter does more than simply dictate the output level: as you turn it, the balance of distortion harmonics — and thus the audible character — changes a hint, rather like it does when you drive an output transformer that bit harder. (For clean gain, you can always use VMR’s free Trimmer plug‑in).

Custom Opto also inherits FG‑2A’s wet/dry Mix knob, for parallel compression, and with this you can, for instance, bring up lower‑level details without squeezing the life out of the transients. FG‑2A’s handy Stereo Link control is also retained in this new plug‑in.

But while Custom Opto shares plenty of DNA with FG‑2A, there are several important departures. Its Ratio control, for example, is continuously variable between 1:1 (no gain reduction) and 10:1 (effectively limiting), where before you had access only to the LA‑2A’s binary Limit/Compress options. This change allows you to fine‑tune the gain‑reduction to better suit the individual source, and just this, on its own, makes Custom Opto suitable for a wider range of sources.

So too does the continuously variable Speed control, and this, to me, is a much more important development. In FG‑2A, as on the LA‑2A, the time constants are preset, and depend on the signal level and Compress/Limit setting. Slate seem deliberately to have opted for one‑knob simplicity here: a single control governs both attack and release simultaneously. You might wonder why they didn’t go the whole hog and implement separate attack/release controls, but there are plenty of compressors that do that already, and I reckon you’d probably lose you some of the appealing LA‑2A‑style character and ease of use into the bargain. There’s certainly enough control here to make Custom Opto a more realistic option on, say, percussive sounds or rhythmic guitar parts.

...the Tone label is fitting, since the different presets can have a huge bearing on the sound.

Custom Opto’s biggest headline feature, though, is arguably its Tone control. Being honest, I’d half expected from the name that this would be a saturator of some sort, or perhaps a way to tweak the attack and release curves. But as far as I can tell it is a side‑chain EQ facility. That’s not to belittle it, though, as it’s rather a special one, and the Tone label is fitting, since the different presets can have a huge bearing on the sound.

There are five Tone presets to choose from: Flat, Smooth, Warm, Aggro and Airy. Each changes the compressor’s sensitivity to the source in a different way but, rather than have to think about what you’re carving out of the signal, the names describe the likely tonal effect. The Smooth setting, for example, tends to rein in the midrange, while Aggro does pretty much the opposite.

The point of reducing the options down to these five Tones is to make it quick and easy to get a great result and in practice, once you’ve dialled in broadly the amount of gain reduction you want, it can be as simple as clicking through the Tones until you decide on your favourite setting. But you can take things further too, courtesy of a knob that amplifies the Tone preset’s peaks and troughs. I found that cranking this up all the way to 200 percent was a great way to listen to what each preset was really doing, and fine‑tune other settings accordingly, before backing it off to best suit the material in question.

Custom Opto In Use

Importantly, this is a plug‑in that always remains really easy to use; it’s pretty hard to get a ‘bad’ sound out of it. Just as importantly, it can be deployed to good effect on so many more sources than an LA‑2A. For example, I wouldn’t generally tend to reach for an LA‑2A plug‑in as a drum or stereo bus compressor for a heavy rock mix. I might use it for its tube character, but in terms of compression it tends to be a bit too slow and smoothing for my tastes, and there’s no easy way to prevent it from overreacting to, say, a very prominent snare. Custom Opto can work really well in this role, with the Aggro setting seeming to make it much snappier and punchier than the FG‑2A, and despite the lack of separate attack and release, the Speed knob seemed to allow me to tweak the timings just fine. Similarly, the Aggro Tone and Speed control make Custom Opto a genuine option for processing individual drums, particularly if you ‘leak’ some of the transients back in using the Mix control.

...this is a plug‑in that always remains really easy to use; it’s pretty hard to get a ‘bad’ sound out of it.

But it’s not just about ‘new sources’ or aggression: between the Tone options and Speed control you have the ability to do different things with sources, such as vocals and bass, that I’d more typically think of running through an LA‑2A. Some of the other Tones can serve to beef up the bass end, or to darken/soften things, pushing them back a bit in the mix. That’s something I found could work well to tuck dry‑ish backing vocals in just a touch behind the lead part, without resorting to a separate EQ or reverb.

Do I have any criticisms? Not really. There have inevitably been occasions during the review period when I’ve wanted separate control over the attack and release or a more clinical‑sounding gain‑reduction. But, on the whole, I love both the vibe and versatility of this thing, and I’d more than happily set out to mix a record with this as my first‑choice channel compressor. I’d be happy trying it out on the mix and subgroup busses too, and while I might also want to audition differently charactered compressors in those roles, it’s worth noting that this is kind of the point of Slate’s All Access Pass: it includes various great‑sounding compressor options!

Opting In?

In summary, FG‑2A is a great plug‑in but Custom Opto builds on it to create a much more versatile processor without sacrificing its core tonal identity. It’s every bit as great‑sounding and intuitive and it can sound just as smooth, but it can also be much more assertive when you want that. The Speed control and Tone options extend its usefulness a great deal, particularly when it comes to transient‑rich or rhythmic sources, such as drums, bass and acoustic guitar, but also for drum‑ and mix‑bus duties. I find that I can usually set the release fast enough without the attack squishing the life out of the transients, and all with a twist of a single control. Importantly, you don’t have to live with that slight ‘softening’ effect sometimes associated with the LA‑2A either; an effect which is sometimes a blessing, sometimes a curse.

The world doesn’t lack LA‑2A plug‑ins or clinical compressors that offer in‑depth control over every aspect of their response, so it’s refreshing that Slate have delivered something a little different from either. Better still, it sounds great, it’s easy to use and you can just get on with the job of mixing. Recommended.


An interesting take on the LA‑2A tube opto compressor, Custom Opto adds a range of useful controls and tonal options, without over‑complicating operation.


Custom Opto $149. Also available bundled with FG‑2A, $249 or with the All Access Pass subscription (from $14.99 a month or $149 per year).

Custom Opto $149. Also available bundled with FG‑2A 9$249) or with the All Access Pass subscription (from $14.99 pcm or $149 per year).