Fuse Audio Labs VCL-4


VCL-4 is the Debut release by Fuse Audio Labs
Founded by legendary Reimund Dratwa, the man behind many plugins from Plugin Alliance and Black Rooster Audio
The press release says:

The VCL-4 is a faithful emulation of the silver face revision of the op amp based successor of THE classic late 60s opto leveler. It provides you with unintrusive to extreme program dependent and musical opto compression. The switchable ratio option makes for a flexible solution over a wide range of source material.

During the modeling process we rebuilt the entire original circuit on strip board, however replacing the original opto-cells that were made up of an LED and an photoresistor with suitable Vactrols. Every part of the circuit including the receiver stage, the side-chain and Vactrol LED driver circuit, the Vactrol itself as well as the push-pull driver stage and the output transformer has been accurately modeled.

Sounds good, but how does it work in real conditions? Well, it’s even better I must say.
I had no idea this thing can be so versatile. Think of classic LA-2A and LA-3A compressors combined and add some 1176 style features. This sounds strange? You’ll get what I mean once you try it.

It can act gently for mastering or be really aggressive squeezing waveforms into sausage-like shapes without sounding artificial or bringing typical compression artifacts such as pumping or harshness. This is extra useful in extreme genres I work in. Bringing vocals to the front is as easy as turning two knobs. And this is what amazes me, with its few controls it can cover a lot of ground. It spoiled me in some way, now instead of reaching for other compressors with many adjustments, I go for VCL-4 instead. A couple of mouse moves and I’m getting exactly what I had in mind for the taken task.

It’s a cliche to talk about GLUE effect on drums, but I can’t avoid mentioning it here. On the drums bus, it can tame transients making them as consistent as you need, bring up room and background sounds without sucking away low end. My favorite settings, in this case, is Ratio set on maximum to 20 or 12 slowly turning Threshold dial until the sweet spot is reached, which is around 3-5 db of gain reduction in my case


Kinda the same technique I use on vocals and bass. I set the ratio at 20 or 12 to  bring the loud spikes down while maintaining the original vibe and warmness.

before afterafter 2

I wish it was built into Console 1

Recently I’ve been working with female vocals and the session end up with me testing several third-party plugins against VCL-4, none of them (except FET compressor) managed to do what VCL did for the task. FET did, it as I said, but not quite exactly as I felt it should be. It doesn’t mean that VCL-4 is best of them all, it means that VCL-4 is a valuable addition to the arsenal, it does bring new stuff to the table.

I was lucky to have a chat with its creator Reimund Dratwa, and my main question was “why is it so good?” “wtf, man!?” and he generously revealed some information about it:


The technical challenges in modeling the LA-4 were mainly two things:

1. The LA-4 is a mixed feed-forward / feedback design, that means that the side chain circuit is fed by a weighted sum of the input signal buffered by the receiver stage (feed-forward) and the already compressed signal buffered after the GR network (feedback) which is basically a voltage divider.
2. As always with optical dynamic circuits it’s the gain cell that’s most crucial to the compression action. In the LA-4 it was made up of an LED – whereas the earlier units (2A and 3A) used a so called electroluminescent panel (ELOP) – and a photo resistor that controls the GR in the voltage divider mentioned above. In the prototype the VCL-4 was modeled after the original gain cell was replaced by a VTL vactrol that integrates LED and photo resistor. The static transfer function R(I) (R: photo resistance, I: LED current) can easily be matched using a polynomial in the log-log domain. It’s the transient behavior, i.e. how the photo resistor reacts to sudden changes in the LED’s illumination, which is directly controlled by the current, however, that’s responsible for program dependent release or the attack and release behavior per se. In order to solve the problem of simulating the photo resistor’s transient response, we have to change into the G=1/R (conductance) domain and resort to methods of solid-state physics, eventually yielding a differential equation that describes the desired behavior well enough for an accurate emulation. Since is that what makes the compressor’s sound after all.

As mentioned, the Vactrol (gain-cell) can be viewed as current controlled resistor. The objective is to find an accurate model to simulate the change of resistance over time in case of a change in the control factor (current). The following figures depict VCL-4 simulation results.
The first plot shows the transient repsonse after an excitation step of 0->1mA and 0->40mA (attack), whereas the second plot shows the response after a step 1->0mA and 40->0mA (release), respectively.
You can see how especially the release time is affected by the swing of the step. It becomes shorter in case of higher steps, or “after bigger transients” so to speak, which is the main contribution to the compressor’s program dependence.
Even if the release plot only shows a time span of 700ms, it’s also visible that it takes the photo resistor a very long time (several seconds) to fully recover its neutral value or “dark resistance”, which is multiple Megaohms. Another property is what some refer to as a dual-release behavior, although it’s actually a continuous process: Quick release right after the transient followed by the long recovery described before.

attack release



  • easy to use
  • versatile – good for mastering and for mixing
  • full circuit model including the output transformer
  • can be very transparent or add color if pushed harder to overdrive
  • addictive
  • very light on CPU
  • good price


  • Lack of Dry\Wet control and independent output level knob.