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IGS Audio Tubecore 3U

Vari–mu Mastering Compressor By Bob Thomas
Published April 2016

IGS Audio Tubecore 3U

Polish company IGS Audio’s Tubecore is back, with more facilities and more options. What’s not to like?

Dynamic–range processors have come a long way since the late 1930s, when the first, relatively crude, valve peak limiters (designed to protect radio transmitters from being damaged by overmodulation) began to appear. Launched in the late 1940s, the Langevin Progar (Programme Guardian) was the first unit to combine intelligent compression and limiting in a single unit, using a 6L7 heptode ‘variable–mu’ tube to provide gain control for radio broadcast applications.

Vari–mu Overview

A valve is usually designed so that its mu — or amplification factor — is constant throughout its entire operating range. Since a valve’s control grid is negatively biased to reduce conduction through it when no input signal is present, the negative swing of a large input signal can push the valve into cut–off. Valves of this type are known as ‘sharp cut–off’ valves, a characteristic that normally causes no problems until the valve is required to amplify input signals above its operating range without producing distortion.

To cope with this situation, the variable–mu tube was developed. This type of valve can reduce its amplification factor as its input signal gets larger, thereby making it less likely that it will be driven into cut–off. Also known as a ‘remote cut–off’ valve (since its cut–off point is further away from the zero signal level), the key to its ability to avoid cut–off under high input-signal conditions lies in the construction of the control grid. In contrast to the uniform spacing of the grid wires in a conventional tube, the grid spacing in a variable–mu tube is a mixture of uniform gaps and some larger–spaced anomalies. This produces a variable mu, as the wider grid spacing requires a higher negative voltage to reduce electron flow. This requirement not only lowers the likelihood of the tube reaching cut–off, but also results in an exponential relationship between the tube’s input and output levels, making a variable–mu tube well–suited to providing gain control of highly dynamic audio signals.

Unfortunately, no variable–mu tube was ever meant for audio compression duties and it took some highly talented designers — most famously, Rein Narma in the US, whose first design (the mono 660) was licensed to Fairchild — to make high–performance compression and limiting a reality. Narma’s search for a high–performance variable–mu tube led to his identification of the GE 6386, a medium–mu, military spec twin–triode tube. Although it was used in the Fairchild 660 and its stereo sibling, the 670, and in other classic variable–mu compressors of the time, the 6386 has been out of production for some years and original NOS versions have become exceedingly rare, commanding considerable prices.

Two popular alternatives (though not substitutes) for the 6386 are the 5670 (still available as NOS, though prices are climbing) and the 6BC8 (used in the Altec 436 and still readily available cheaply). In the Tubecore 3U Stereo Mastering Compressor, reviewed here, Polish manufacturers IGS Audio have taken the unusual, though extremely welcome, step of designing the unit to allow the use in the gain–control stage not only of all three of those tubes (plus the 6BZ8 variant of the 6BC8), but also of the 6N1P–EW, a Russian–made, military grade twin–triode that powers the 3U’s push–pull channel output stages.

Tubecore 3U

IGS Audio manufacture a range of analogue compressors, preamplifiers and equalisers across 19–inch rackmount and 500–series formats. The Tubecore 3U is a development of the Tubecore ME that impressed reviewer Matt Houghton in the June 2011 edition of SOS. Its front panel is dominated by two identical sets of controls and a couple of handsome illuminated VU meters on a 3U 19–inch chassis, with a retro appearance that echoes the tube–based circuitry that sits inside. Although it is initially described in the manual as a stereo compressor, and its channels are labelled as Left and Right, the Tubecore 3U is actually a dual–mono unit and also features a Mid–Sides (M–S) operational mode, in which an incoming stereo signal is encoded into separate Mid and Sides signals which are then processed separately.

The Tubecore 3U’s twin channels are balanced to within ±0.1dB of each other and, to help keep that balance, its rotary controls are either stepped or switched, with Elma 24–pole units taking care of Gain, Output and Mix (wet/dry). Six–position switches set the values for Attack (0.1 to 30 ms), Release (0.1 to 4 seconds) and Threshold (0 to -12 dB). Although no external access to the 3U’s side–chain is available, a high–pass filter at 60Hz or 120Hz is provided to help prevent bass frequencies from triggering unwanted compression. Finally, in a nice touch that I really liked, the Mode switch has four positions so that you can switch the 3U directly into bypass from either stereo or M–S modes, in order to make a quick A/B comparison between the compressed and uncompressed signals.

In addition to the connectors for the seriously chunky external power supply and the balanced XLR inputs and outputs, the 3U’s rear panel carries six recessed tube sockets, three per channel. Two of these are permanently occupied by the 6N1P–EW output tubes and, of the other four, any two positions (one per channel) can be occupied by any combination of 6386, 5670, 6BC8/6BZ8 or 6N1P–EW tubes. Obviously, if you’re going to be mastering in stereo, you will need to use a balanced and matched identical pair across the channels. However, if you’re tracking or working in Mid–Sides mode you could, if you wished, experiment with a different tube type in each channel.

Interestingly, the Tubecore 3U has been designed to allow the user to install different valve types, either for different sonic characteristics, or to improve the ease and cost of replacing out–of–production models.Interestingly, the Tubecore 3U has been designed to allow the user to install different valve types, either for different sonic characteristics, or to improve the ease and cost of replacing out–of–production models.As no alternative tubes were supplied with the review 3U and I don’t have any of the specified types in my own collection, we’ll have to rely on the manual’s description of the various ‘flavours’ that changing tubes would give. The supplied NOS 6N1P–EW is described as delivering a ‘standard’ IGS compression, the 6BC8/6BZ8 as being “tight, transparent, punchy”, the 6386 as providing “legendary compression”, and the 5670 as being “deep and dark”.

Under The Hood

Lifting the lid reveals that PCBs have replaced point–to–point wiring, while the presence of Carnhill transformers at the input of the tube compressor stages and Sowter transformers at their outputs, together with the use of Elma switches, points to IGS’s choice of high–quality components. The 3U runs fully balanced throughout, with the balanced input signals passing through (or bypassing) the op–amp–based M–S encoder before reaching, via the Gain control, their respective tube stage balanced input transformers.

The compressor stage has a circuit topology somewhat similar to that of the Altec 436C, in that the balanced ‘hot’ and ‘cold’ secondary outputs of the input transformer each drive one half of whichever twin–triode you have in there, and those outputs drive both the compressor’s solid–state side–chain and the two halves of the 6N1P–EW that, together, make up the push–pull output stage that drives the channel’s output transformer.

The signal from the output transformer then passes through the Output level control, which can apply up to 12dB of attenuation, before passing (or not) through the M–S decoder. From there, the compressed signal arrives at the Mix control that can combine it with the original uncompressed signal to deliver one–knob parallel compression.

In Use

Since the Tubecore 3U operates at +4dBu, you’ll need to do the same to get the best out of it, especially as its meters display only gain reduction. Other than that, there’s nothing more to do than to connect up its inputs and outputs, leave it to warm up for at least 15 minutes, line everything up and get to work. The only slight operational niggle that I have concerns the rear–panel tube mountings, which leave around 50 percent of the tubes’ envelopes exposed and vulnerable. In a rack, that isn’t going to be much of a problem except that, if you’re going to be a habitual swapper of tubes, you’ll need to find a hassle–free way to get at them.

Once you’ve got your system and the 3U level–matched, the fun can begin — and what fun it is! I first sat it across a stereo acoustic guitar track that I’d recorded with a pair of spaced omnis, where its mere presence, even before introducing any gain reduction, added a subtle sense of warmth arising, no doubt, from the transformers and tubes in the signal path. Adding some gain brought in a bit more of that character and the feeling of relaxed transient response that comes with so much tube–based audio equipment — with the right source material, that’s something I love.

In a variable–mu compressor, the compression ratio increases the more the signal exceeds the threshold, so the real engineering art lies in the balancing act between input–gain settings, threshold level, the resulting compression ratio, attack and release times, and output attenuation — and that’s before you start entering the realms of parallel compression. The Tubecore 3U makes everything so simple, since each channel is precisely matched to the other and every control is detented, making it easy to repeat settings and copy them across accurately from one channel to the other.

In contrast to the all–valve compressors of old, the 3U’s solid–state side–chain endows it with a variable 0.1ms to 30ms attack time, which allows it to grab faster transients or to take a more relaxed approach where appropriate. The release times I mentioned earlier are more leisurely, and the threshold switches between its 0 and –12 dB extremes in five steps, the increments increasing as the level reduces. The side–chain’s final facility is that high–pass filter.

Going back to the acoustic guitar recording, setting the 3U up to give between three and five decibels of fairly gentle compression, with the 120Hz side–chain HPF engaged, brought everything together nicely. Mixing some of the dry original in with the compressed signal produced a sonically rewarding result that felt very natural, sounded larger and had more weight.

Running various stereo drum, guitar and vocal stems and final mixes through the 3U produced consistently impressive results, as its innately smooth sound and somewhat limited (20dB) gain reduction has the ability to pull tracks together in a very cohesive way. That’s not to say that the 3U can’t add character — drive the compressor hard, pull the threshold down and you can get it to pump away quite happily in its own sweet, but not exactly punchy, way.

The external power supply attaches to the rack unit via a  sturdy locking connector.The external power supply attaches to the rack unit via a sturdy locking connector.However, where the Tubecore 3U really scores is with its M–S compression mode. Here, the incoming stereo mix is encoded into Mid and Sides signals, which are then processed separately by what are normally the Left and Right channels. In very simple terms, the Mid signal contains everything that is panned to the centre and the Sides signal contains everything else. The Tubecore 3U allows you to change the balance between the two signals and to compress them differently, enabling you, for example, to change the apparent stereo width by boosting the Sides signal and/or to tame a slightly too prominent vocal, guitar or hi–hat by compressing the Mid signal slightly, without having to go back and remix the entire track.

Having experimented with the 3U, the keys to success in M–S compression seem to be not to be too heavy–handed, to be clear about what you want to achieve, to continuously refer back to the original, uncompressed signal, and to keep careful notes so that you can back–track when required. When I kept within these guidelines, the 3U produced stunning results, and I’m now a convert to this approach to stereo master compression.


The IGS Tubecore 3U is a versatile and sonically excellent unit that delivers everything that you’d expect from a high–quality dual–mono tube compressor. All really good tube equipment carries an inherent cost penalty, but IGS have managed to bring the Tubecore 3U to market at an extremely competitive price for the level of performance on offer, whilst also delivering high component and build quality and a three–year warranty.

When Matt Houghton reviewed its predecessor, the Tubecore ME, he suggested some additional facilities that he wanted to see included, and in this iteration of the design IGS have, with only one exception, implemented them. In fact, I’m left with nothing to criticise — so I’ll just have to gently remind IGS that external side–chain insert points would be very welcome on the next iteration.

Until this review, I hadn’t really spent much time exploring the world of Mid–Sides compression, but now my next major analogue hardware purchase is going to be a high–quality dual–mono tube compressor with an M–S mode. You can probably guess the one that I’m currently considering, and I’d suggest that you also take a serious look at it if you have the budget to float that particular boat.


There are a quite a few tube compressors around these days — new, used and vintage. If you have the budget, a Manley Labs Vari–Mu, with its Mid–Sides option fitted, is a direct but pricier mainstream equivalent. If you don’t require the M–S facility, there’s the Thermionic Culture Phoenix ME. There are also a number of mono variable–mu compressors which can be stereo linked, such as Chandler’s Abbey Road Series RS–124 and Phaedrus Audio’s Phamulus (a take on the Altec 436C).


  • Has that lovely ‘tube and transformer’ sound.
  • Stepped controls make stereo processing simple.
  • High–quality build and components.
  • Competitively priced.


  • No side–chain access.
  • Requires matched and balanced tubes.


The IGS Tubecore 3U delivers everything that you’d expect from a high–quality, dual–mono, variable–mu, tube mastering compressor, in a package that compares very favourably with its more established, better–known (and more expensive) competition.


£2130 including VAT.

IGS Audio +48 601 597 592.


IGS Audio +48 601 597 592.