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Retro Instruments Revolver

Dual-channel Valve Compressor By Neil Rogers
Published July 2018

Retro Instruments Revolver

Retro set their sights on modernising a classic design from the golden age of recording — did they hit their target?

I’ve reviewed a number of variable-mu compressors over the years and almost always end up loving them, but considering the conceptual simplicity of the basic vari-mu design I’m often surprised about the learning curve I find myself travelling along as I explore a new variation on the theme. This was very much the case with the new offering from California-based manufacturers Retro Instruments. As I’ll discuss in more detail later, it took a little while for the penny to drop on just how this particular unit could prove itself really useful in both tracking and mixing scenarios, and, potentially, in mastering too.

The Revolver is the latest addition to Retro Instruments’ modest-sized but very highly regarded product line. I’ve previously had the pleasure of reviewing their Doublewide 500-series compressor, and other Retro products have received equally enthusiastic SOS reviews. So it’s fair to say that expectations were high when this dual-channel tube compressor turned up for its audition.

For a product called the ‘Revolver’ and pitched by Retro as “The sound of the British invasion,” it doesn’t take a huge amount of detective work to guess which era and which classic compressors might have inspired this design: the last few years have seen a number of designers doffing their caps to the Altec 436B, and often more specifically to the modifications to the original American design made in some British studios. The Abbey Road engineers are most famously credited with these modifications — those compressors saw a great deal of use on the Beatles recordings — but Joe Meek and others around that era had also discovered that a much more affordable alternative to the prohibitively expensive Fairchild compressor was available with some knowledgeable tinkering under the hood. While the ‘Revolver’ may have its origins in these designs, and while it certainly offers users a generous slice of that style of compression, it is not a clone. In fact, there are some very different features and control options, which I’ll discuss in more detail below...


As I’ve mentioned, this isn’t the first such offering around this particular theme, but it is the first time I’ve personally got my hands on a stereo implementation of it. And that naturally encouraged me to imagine it in certain applications — compressing the drum or mix bus, for instance, or perhaps mastering. But there are actually two almost completely independent vari-mu compressors here that can be linked at the flick of a switch, so it’s also useful on mono sources.

The build quality and general aesthetic standard are very high. You’d rightly expect that of any unit at this price, but this thing really is built like a tank and both looks and feels exceptionally solid and classy. The unit’s valves protrude from the rear panel, and are surrounded by huge transformers that are responsible for the lion’s share of the unit’s considerable weight — at 22lbs, and with most of the weight towards the rear, I’d suggest that accommodating the Revolver into a busy studio outboard rack could be more easily be done by two people than one!

On the rear panel, nestling between some very hefty transformers, you can see the Revolver’s valves.On the rear panel, nestling between some very hefty transformers, you can see the Revolver’s valves.

Looking at the controls, any similarities with other modified Altec 436B-style designs quickly fade from memory, as we’re presented with continuously variable input, output, attack, release and threshold controls. (The original Altec and subsequent EMI RS124 had switched attack and release controls with only a handful of options, and lacked the threshold control.) While switched controls do make recall easier, they can be a slightly blunt tool on occasion compared with fully variable ones.

The input and output level controls should be self-explanatory, but it’s worth pointing out that the threshold is a dual-channel control — in other words, you have only one dial, and it affects both of the compressors. You can still control the amount of compression independently for each channel (when, for example, you’re compressing two different mono sources) by adjusting the input and output level controls. But the threshold is very useful with stereo material — I found it especially useful when using it on a whole mix — and it also allows you to push the signal a bit harder through the electronics to achieve a more ‘saturated’ or ‘coloured’ sound. Another welcome feature is a high-pass side-chain filter, with which you can direct the compressor to ignore anything below either 90 or 250 Hz. Retro state that the Revolver has a ratio of between 1.5:1 and 5:1, and as with all variable-mu designs there’s no user control for this. Rather, the ratio increases the harder the compressor is made to work. This is one reason variable-mu designs can achieve such natural, musical-sounding compression, even when applied very generously!

Pumping On The Stereo

Once you have a feel for how the Revolver introduces gain reduction, you can quite happily use the unit as two separate compressors in one handy box, but it’s definitely worth exploring how the unit behaves when you flick the stereo-link switch and deploy it on stereo sources.

I’ve already mentioned the single control for the threshold and this becomes a key player; the input and output controls always remain independent, and you don’t really want to be continually adjusting four different knobs to achieve just the right amount of gain reduction on a whole mix! The attack and release also remain independent, so (as in many other stereo-linkable compressors) all the stereo-link switch is really doing is summing the unit’s control voltages so that the two compressors react in tandem.

This is an important point to consider when using the unit on stereo sources, because a big part of the way our ears perceive our sense of stereo is from the level differences of sounds received at our left and right ears. The result of linking the two compressors like this could be good or bad, depending on the source you’re processing and just how much gain reduction you’re applying. For example, if a loud floor tom, panned to the left, triggers the compressor, both the left and right channels would be compressed, and if you have, say, a prominent guitar part panned hard-right, that could be an issue.

I made a point of double-checking with designer Phil Moore about how the Revolver behaves when the stereo-link facility is engaged, and he helpfully explained that allowing the input and output controls to remain independent was an important feature — it allows the user to manually balance the input to each compressor when used in the linked stereo mode, so that one side doesn’t dominate and skew the amount of compression applied. (The higher the input signal, the greater the contribution of that channel to the side-chain control voltage.) And with stereo material that’s exactly how I found myself using the Revolver in practice: I’d get the levels roughly where I wanted with all the controls set the same on both channels, and then I’d dial in the amount of compression I wanted using the threshold control, and finally listen and adjust the input/output on either side until everything felt nicely balanced. Of course, you’re also free to use this compressor on stereo sources without the stereo-link facility engaged; it’s great to have both options available here.

In Use

When I got some proper downtime in the studio that allowed me to experiment with the Revolver, I soon started to understand how I could use it to shape the sound and achieve firm control of the dynamics, while still maintaining a sense of energy in an instrument or vocal.

Getting a feel for the unit’s variable attack (2-40ms) and release (200ms to 2 seconds) curves was crucial to my appreciation of the Revolver’s more subtle qualities. Even on the fastest setting, the attack never feels that fast. As with most vari-mus it always lets a little of the transient through, and that’s a good thing, as the result always sounds very ‘musical’. I think this is a big part of why this style of compressor has become so revered over the years. The release control, however, seems to be the real key to the Revolver’s flexibility. There is a large range to play with, and on the slowest setting (two whole seconds!) it behaves as a kind of hold function, by which I mean that the level of any heavily compressed music won’t suddenly leap in volume if the source falls below the threshold for a while.

The Revolver offers a bit more than this, though, due to its thoughtful control features. Many users like to be able to drive a unit to get some extra character, and one of the problems with the older designs is that they often don’t handle louder or low-frequency sources that well at the input stage. I’ve been frustrated when using some such devices on a bass guitar, for example, as in one section it can sound spot on but when the bassist later digs in a bit the sound starts to break up too much before any compression can be applied. Admittedly, this unpredictability is part of the charm of older designs, but it can be really frustrating at times! Happily, then, I had no such issues with the Revolver — and with the availability of input, output and threshold controls, I found I could nearly always get the balance of gain reduction and colour precisely where I wanted it: just back the input off to get a more transparent sound, or crank it up for more analogue ‘thickness’ and general character.

Retro Instruments Revolver

When playing with the variable attack and, especially, the release, I also experienced this lovely sense of control over various sources. On one bass guitar part, I found I could introduce more of the ‘clack’ of the strings by just speeding up the release time, and this had the effect of perking up the feel of the whole part. I was also impressed with the way it tamed some low-frequency plosives I’d discovered on a male vocal, using a slow attack combined with a medium-slow release.

The side-chain high-pass filter controls are a handy feature. Without them engaged, I nearly always liked the way the unit tightened up the low end of individual sources, but I found the filters became really useful on various stereo sources. On a drum bus, for example, I liked the way I was able to let more or less of the kick drum come through by flicking between the 90 and 250 Hz options. And when using the Revolver as a bus compressor, it very quickly revealed just how much the low-energy in a mix can influence any compression on the master bus: on one track, the meter told me I was applying 5-6 dB of gain-reduction but when I engaged the 250Hz side-chain filter the needle stopped moving completely!

While enjoying what the Revolver had to offer in this application, it can be difficult to articulate the subtle effect that running a whole mix through a high-quality valve device like this can have. The natural-sounding, effortless control of dynamics helps of course, but there’s more to it — a little ‘sweetness’, which is difficult to put into words — but what I think my ears particularly enjoyed was the way this compressor tightened up the low end. It’s a subtle effect, but a lovely one, and I can imagine this alone making the Revolver very useful in some mastering projects.

What’s Not To Like?

Like many vintage-inspired designs, while there’s an on/off switch on the front panel, this compressor has no dedicated bypass facility. That’s not a big deal in my own setup, since I have my outboard on a patchbay routed to a mixing console with a dedicated insert switch. With a quick bit of level matching, I could easily audition what the unit is doing to a signal. And, of course, if you’re hooking the device up as an ‘external plug-in’ to your DAW software, then you can bypass it using your software. But if you don’t work with such a setup, this might be something you’d want to consider.

Another potential issue for some is that none of the knobs on the default configuration of the Revolver are fully stepped, and that can make precise recall of settings tricky. Again, I didn’t find that a huge issue in practice — the style and the quality of the nicely weighted controls meant that I was able, without any real difficulty, to dial in very fine settings when required. However, I know some mastering engineers who wouldn’t even entertain the idea of a piece of gear that doesn’t offer stepped controls, so I put this thought to Phil Moore — and he promptly explained that Retro can fit stepped pots to the Revolver if requested!

Final Thoughts

I had great fun using the Revolver, and could easily write more about my experiences with it, but I decided it would be more helpful if I were to provide some audio examples instead so you can hear with your own ears what the unit can do. You can find those examples on the SOS web site at

Wrapping things up, I’ll return to my point about taking the time to learn how to use the tools at our disposal. If I’d bashed out this review after my first week with the unit it would have been slightly different. In the thick of a busy recording period at my studio, my first impressions were of a high-quality but somewhat polite and very transparent-sounding compressor that needed to be used in conjunction with other outboard gear to make a real impression. Yet, over the course of a few more sessions I began to learn not to be so shy with how much compression the meter told me I was applying, and to simply trust the Revolver to do its thing. Whatever genre you’re working on, the ability to control the dynamics of your audio without ruining its energy or feel, even when used quite heavy-handedly, is pretty darn useful!

The Revolver isn’t exactly inexpensive, but when you consider all the modern features I’ve described, that you’re getting two vari-mu compressors in one box, the outstanding build quality, and the variety of scenarios in which you might use it, then the price starts to look very reasonable indeed.  


Alternatives include the Manley Vari-mu, IGS Audio Tubecore 3U, Thermionic Culture Phoenix and Inward Connections Vac-Rac TSL-4, all of which accommodate two channels in a single box. There are also several mono devices that can be stereo-linked, including the Chandler Limited RS124 and the Grove Hill Audio Liverpool.


  • Classic vari-mu compression with modern control.
  • All-tube signal path, with transformer-balanced inputs and outputs.
  • Outstanding build quality.
  • Two mono compressors, with useful stereo-linking facility.
  • Very versatile: potential uses in recording, mixing and mastering.


  • No bypass switch.
  • Some may prefer stepped controls as standard.


This exceptionally well-built, and well thought-out addition to Retro’s product line combines the sound of a classic era of recording with some useful modern control options.


£2928 including VAT.

Funky Junk +44 (0)207 281 4478


Retro Instruments +1 209 810 3344.