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Retro Instruments Sta-Level

Variable-mu Compressor By Hannes Bieger
Published December 2014

RetroInstruments Sta-Level

Is Retro’s recreation of the legendary Gates Sta-Level more than just a reissue?

Trawl through old interviews and you’ll find that plenty of big-name producers offer nothing but praise for the Gates Sta-Level compressor. It was one of the most successful and enduring compressors of the post-WWII era, and its unique sonic qualities mean that it remains in demand today, although for a rather different application. Second-hand units change hands only rarely, and for a high price. Retro Instruments, though, offer a version of the Sta-Level, which remains true to all the important aspects of the original design, while also incorporating some very useful additions.

Bettering The Sta-Level

As with Retro’s take on the Urei 176 (, the build quality is flawless. It combines the heavy-duty nature of studio kit from the ’50s with a few key qualities of a well laid-out modern design. In fact, Retro took everything that was good on the original unit and enhanced it, to make the compressor more suitable and more convenient for daily use today. XLR connectors, a standard IEC outlet, the possibility of coupling two or more units, facilities on the rear panel for easy balancing of the 6V6 output valves... all of this represents a welcome improvement.

In contrast to the original Gates unit, which only offered a two-position toggle switch for the release, Retro’s Sta-Level boasts a vastly expanded range of controls. The compressor can be operated in Single, Double or Triple mode, and there’s also a six-position rotary switch for the release, allowing the user to fine-tune the results. With program-dependent time constants, the Double mode comes closest to the behaviour of the vintage original, whereas the Single mode expands both time constants for a very slow response, yielding maximum transient punch. The Triple mode, however, combines faster attack values with a more responsive, yet still quite slow, release, making this option particularly suitable for vocal compression. This combination of pretty fast attack and rather slow release characteristics (which is a typical quality of another legendary compressor, the Fairchild 660) means that even the fastest of vocal transients can be kept at bay reliably, while at the same time the moderate release prevents the signal from pumping and exhibiting other unnatural artifacts. Despite the program-dependent nature of the compression, the results can still be adjusted to some degree by finding the most appropriate setting on the release switch.

That’s it as far as the controls are concerned; it’s delightfully uncomplicated, and that’s part of this device’s appeal. The front panel also features a power switch, accompanied by a beautiful red light, and the two screws which hold the hinged front panel in place. Once released, this can be tilted downwards, providing access to the circuitry inside, just as on the original. However, Retro have, very sensibly, added another cover inside the enclosure that puts the high-voltage sections of the circuitry out of direct reach. There’s also a potentiometer for VU-meter calibration on the rear of the front panel.

Circuit Training

Unusually these days, most of the circuitry which makes this bit of kit special can be found on the back panel, rather than inside the case. On this are mounted the large power transformer, the equally chunky shielding enclosure of the output transformer, the input transformer, and six thermionic valves.

Quite typically for an early  post-WWII design, all transformers and most valves are mounted outside the rear panel of the enclosure. For those seeking true authenticity, an optional and rare 6386 compression/input valve can be placed in the socket on the right-hand side.Quite typically for an early post-WWII design, all transformers and most valves are mounted outside the rear panel of the enclosure. For those seeking true authenticity, an optional and rare 6386 compression/input valve can be placed in the socket on the right-hand side.Two further valves operate at the heart of the gain-reduction circuit, and are located inside the unit. The 6386 dual-triodes, which acted in this role in the Gates version, have become scarce and expensive, so Retro decided to replace them as standard with a pair of 6BJ6 pentodes, but wired as triodes. However, for authenticity hunters, there’s a socket for the 6386 on the back of the unit, allowing the original valve complement to be restored at any time, with no need for modifications. The signal from the input/control valve(s) is fed through the push-pull output amplifier, which employs a 12AT7 dual-triode as the intermediary stage and a pair of 6V6 pentodes.

In essence, then, five of the eight valves of the stock configuration play a role in the signal path. The others are a 5Y3, employed as a rectifier in the power supply, and a 6AL5 dual-diode, used as the side-chain rectifier. The presence of all these glowing glass cylinders means that the Sta-Level generates a substantial amount of heat, so it’s a good idea to mount the Sta-Level in a well-ventilated position.

As is typical of a variable-mu feedback design, the side-chain signal is tapped at the plates of the 6V6 output valves, then sent through the OB2 regulator valve, which sets the threshold level for control. Finally, the combined DC from the 6AL5 is applied as a variable negative bias to the grids of the 6BJ6/6386. This yields a typical compression ratio of around 3:1, but the compression is not rigid and aggressive, as, for example, it would be with a VCA design; its behaviour is program-dependent and quite variable.

The hinged front panel allows convenient access inside the unit. In the stock version, two more readily available 6BJ6 valves replace the original 6386.The hinged front panel allows convenient access inside the unit. In the stock version, two more readily available 6BJ6 valves replace the original 6386.As with most variable-mu compressors, this one works with a pretty soft knee. Much of the original gain reduction is restored very quickly after the signal falls below the threshold again, yet for the last few decibels the VU needle takes longer to return fully to zero, especially after heavy gain reduction. The result is a quite ‘musical’ behaviour that allows the Sta-Level to ‘breathe’ with the input signal.

The Sta-Level is capable of up to 40dB of gain reduction, a staggering statistic for an early post-war design, but its idiosyncrasies help to iron out the artifacts normally associated with heavy compression. As a result, you find yourself compressing signals much more assertively than you’d normally choose to, because the results sound so pleasant.

In Use

Getting your head around the compression characteristic of the Sta-Level can take a little work. Most VCA compressors, for instance, sound very precise and rigid, and put their stamp on the signal, regardless of its character. The Retro Sta-Level, on the other hand, seems to adapt to the input signal very sympathetically. But it is not inaudible; it imposes a strong character, with some of this ‘attitude’ shining through only when you overdo it, but other aspects being audible right from the start.

No matter how you use the Sta-Level, the sweet spot where it sounds the best is usually large, and very forgiving. The well-tempered nature of the Sta-Level is one of the main — if not the most important — sonic reasons why this unit once became so popular. It also has simplicity of operation in its favour, as with any two-knob compressor, and, of course, it’s so effective at controlling the signal level.

The time constants may offer less variable and less precise control than a modern VCA unit, but the different modes and the additional release settings mean a whole lot more ground can be covered than you might expect of an early post-WWII tube device. The Sta-Level can slap out bass lines with a heavy, thumping attack in Single mode, and it can squeeze vocals quite brutally in Triple mode. The Double setting occupies a sort of ‘safe ground’ in between these extremes.

The Retro Instruments version offers far more versatile attack/release controls than the original Gates unit from the ‘50s.The Retro Instruments version offers far more versatile attack/release controls than the original Gates unit from the ‘50s.The colour imparted by the transformers and the various valve stages adds further detail to the sonic picture. In many cases, the Sta-Level proves an invaluable tool for ‘slowing down’ and smoothing unruly signals, because it does so in a way that never sounds flat or dull. In fact, the line stages add a very vibrant texture to the sound that’s difficult to describe, but which sounds absolutely beautiful. Words like ‘thick’, ‘velvety’, and ‘gooey’ come to mind — the Sta-Level does, if you’ll forgive the crude analogy, the same thing for audio that a spoon of creamy honey does for a cup of tea!

Of course, for all its gooey goodness, the Sta-Level cannot hide the fact that it has at its heart somewhat ancient technology, and in use there are some things to watch out for. Great care must be taken when feeding the compressor with bass-heavy signals, for instance. Especially on transients, it breaks up much faster and more easily than someone more familiar with contemporary designs would expect. That’s entirely understandable, as the amount of sub bass energy we cherish today was simply unheard of when the original Sta-Level was conceived — but you must listen out for this. Another issue that mustn’t go unnoticed is that the Retro generates a sub-sonic punch on the transients, especially when applying stronger amounts of compression. To some degree this ‘thumping’ is typical of variable-mu compressors more generally. It’s caused by slight, and inevitable, imbalances of the circuitry and it’s usually more apparent when employing faster settings and applying more compression. I had access to two different Retro Sta-Level units during the review period, and both caused stronger ‘thumps’ than other vari-mu compressors I’m familiar with. Again, while this is something to listen out for, it shouldn’t be viewed as a problem. In fact, on sparsely arranged acoustic music, it may even restore some of the punch and immediacy of a signal that was lost during recording! But employing the Sta-Level when mixing more dense arrangements, especially in the field of contemporary genres, where headroom and loudness are always a concern, I found myself reaching more often that not for some kind of low-cut filter to use in the signal path after the Sta-Level.


This is not just a 1:1 reissue of the vintage original. It’s more like a slightly enhanced and expanded recreation, and considered as such the Retro Instruments Sta-Level proves a worthy successor to the legendary Gates design. With its impeccable build quality, the beautiful design and, most importantly of all, the great-sounding and relatively controllable results, the Retro Sta-Level has plenty to offer.

Granted, these qualities do not come cheap, particularly if you need two for stereo use, but Gates units are still in service today and the Sta-Level promises years and years of reliable operation. Perhaps this will be a future vintage classic in its own right?


There aren’t many mono all-valve variable-mu compressors in current production, at least not in this price bracket. Retro Instruments’ own 176 would be another option. However, just like the ADL 1700, this isn’t modelled after the Gates unit, but a different vintage classic. Units like the ADL 660 or the EAR 660 are based on yet another classic, the Fairchild 660, and carry a much heftier price tag. Although its gain control is based on an entirely different topology, the Teletronix LA-2A may achieve similar results in certain applications, and the same can be said for a few other optical compressors.

The Gates Sta-Level

With its simple and, for the time (it was released in 1956), small and lightweight layout, the Gates Sta-Level paired a powerful signal path with great sound quality. It could also be serviced and modified quite easily. Consequently, it became a real ‘industry standard’ broadcast compressor in the USA, and could be found controlling the program level in numerous radio stations for many years. Even after it was replaced in its original role by more versatile, elaborate devices, a great number of the Sta-Levels remained in service in other applications, for instance during pre-production or as a gain controller for incoming phone calls.

As with other influential dynamics processors of the post-war era, from the likes of General Electric, RCA, Collins, Fairchild, and CBS, the Gates design employed a 6386 dual-triode valve in a variable-mu configuration, allowing it to act as an automatic gain-control device. To achieve a good signal-to-noise ratio at the output, even under heavy gain-reduction, the amp was designed to sound as clean as possible, and a large maximum amplification was important. Both aspects were accomplished courtesy of the way the three active stages were laid out.

The Sta-Level amplifier boasts a standard system gain of 35dB, yet it’s capable of a maximum amplification of over 60dB with the output pad removed — and this means it can be employed, just like the Teletronix LA-2A, not just as a dynamics processor, but as a microphone preamplifier, too. Although the LA-2A was designed a few years later, and employed an optical element for gain control, both units share a few conceptual similarities, not least the basic control concept, with input and output potentiometers and a fixed threshold, which always seems a very intuitive way to set up a gain-control device.

Most original valve designs were swept away following the transistor revolution (they were often literally tossed out of the control rooms!), but a few savvy producers and engineers had the foresight to rescue some of them, and today an increasing number of manufacturers are working to bring back to life the qualities of the all-valve designs of this era. These units may be more chunky, and offer less versatile controls than more modern designs, but they present a timbre that, at this time, just cannot be fully reproduced without employing the transformer-balanced high-voltage circuitry that was so common back in the day.

Audio Examples

During the course of this review, I created a number of audio examples, which you can find on the SOS web site.

Retro Instruments Sta-Level: Audio Examples

The following audio examples accompany the review of the Retro Instruments Sta-Level vari-mu compressor, in SOS December 2014.


The vocal example shows how effortlessly the Sta-Level can handle rather hefty amounts of gain reduction. The Triple mode controls the transients in a very effective yet quite natural-sounding manner. The Double and Single mode open up the signal, and a slow release also adds something to the end result. However, at times the “thump” is clearly audible, and visible on a meter. A low cut in the chain after the Sta-Level would probably be a good idea.

01: Original

02: Triple mode, fast release, 15dB gain reduction

03: Double mode, fast release, 15dB gain reduction

04: Single mode, fast release, 15dB gain reduction

05: Triple mode, slow release, 15dB gain reduction

Moog Bass

Some additional punch and some euphonic coloration — these were the goals in the Moog bass example. The faster the setting, the more distortion comes into play, for a grittier and more characterful end result.

01: Original

02: Single mode, fast release, 10dB gain reduction

03: Double mode, fast release, 10dB gain reduction

04: Triple mode, fast release, 10dB gain reduction

Fender Bass

This example shows a rare quality of the Sta-Level: it can exaggerate the transients and still increase the average volume of a signal. Not too many compressors can do this. And here’s the reason why: the Sta-Level saturates on the transients, which is more or less audible depending on the setting. It must not be overlooked that 10dB is already pretty strong compression. Should fewer artifacts be desired, a maximum gain reduction of around 5dB would be more appropriate. Please note how the rather extreme last round pins the signal in the front. It gets louder, more punchy, and some exciting distortion has been added. The Sta-Level does not only let signals sit better in the mix, it also imparts character in spades.

01: Original

02: Single mode, medium fast release, 10dB gain reduction

03: Double mode, medium fast release, 10dB gain reduction

04: Triple mode, medium fast release, 10dB gain reduction

05: Triple mode, fast release, 20dB gain reduction


  • All-valve variable-mu design, with no semiconductors in the circuitry.
  • Greater time-constant control options than on the original.
  • Superb sonic and build quality.
  • Allows ‘authentic’ valve to be used with no modification.
  • Multiple devices can be linked.


  • Stronger compression generates ‘thumping’ artifacts that may be undesirable.


Retro’s Sta-Level oozes quality. It proves a worthy successor to the vintage Gates original and actually improves on it in several ways. The results that can be achieved with the Sta-Level definitely justify the substantial investment.


£2796 including VAT.

Funky Junk +44(0)207 281 4478


Retro Instruments +1 209 810 3344