The Compressor For The People

by Fritz Fey
Studio Magazin (Germany) – October 2011

These days, there are a dime and a dozen compressors available in the market. There are analog and digital versions, available as hardware or software – and most of them do a quite proper job. If, however, an excellent product development stands out of the main stream, the reason for this can usually be found in the creativity and imagination of its maker. The requirements on a compressor can be very diverse, as they reach from achieving consistency for program material our a subgroup to drastic changes on the envelope progress of single signals. 

These are all more or less creative applications, but the original task of a compressor is to reduce the dynamic range in order to produce 'audible' results on the playback side of the story – for example in the living room, in cinemas, in your car or on headphones. And of course, this process should happen without generating noticeable artifacts. 

Many product developments and circuits are focused on a special discipline, which can be anything from an effects compressor, a bus or summing compressor, a mastering processor or a vintage compressor up to special species like a multiband broadcasting compressor. Not many of the market players appear with a universal approach in order to generate their sales points from that. 

But we think that especially having todays small budgets in mind, the universality of a unit can be a very good reason to buy it. The more often and flexible I will be able to use the product, the more frequently my production chain will benefit from it, which will make spending the money worth while after all. And this is especially true for analog devices, as they do not come with the big benefit of multiple instances. 

According to its manufacturer, the idea behind the xpressor is a product which really earns the "Made in Germany" title and which is able to cover a broad range of possible uses. Fortunately, a lot of special feature have been added to this unit, or in other words, it is a 'Best of elysia' for an affordable price. The concept of the xpressor is similar to its bigger brother, the alpha compressor, in certain aspects. Many of the alpha's basics have been implemented in the xpressor with different components, for example the amplifier design, but also the general processing characteristics. 

The xpressor is a discrete class-A design without any ICs in its audio path. This sounds expensive, but it does not necessarily have to be, as really good integrated circuits can be more expensive than a discrete design based on standard components. But what else enabled the company to come up with a price that is so much lower than what we have seen from them so far? It starts on the outside, where the aluminum knobs are now being ordered from a far east manufacturer – we could not notice any change in quality, however. 

Another important means in keeping the price down is the use of more affordable potentiometers without degrading signal quality by doing so. This is done by a certain electronic 'trick' which we will tell you more about later during this view. The developer, Ruben Tilgner, has also managed to employ a transformerless input and output balancing technology on a very high level. This also means substantial savings, which the manufacturer has passed on to their customers. 

SMD groups of components have become extremely reliable with extremely low failure rates. And although this miniaturized parts is still likely to produce risen eyebrows with certain audiophile minds, this technology also breeds devices with an excellent audio quality, if the choice of components has been done right. 

The slide ways of the potentiometers do not carry any audio signals at all, but only control voltages for the amplifier stages. These VCAs are not the usual suspects, but discrete arrays of sixteen transistors which have been specifically developed for this task, optimized for transparency in sound and for an excellent stereo accuracy. 

Overview
The xpressor lives in a 19" 1U housing. Apart from a power supply of its own, it is identical with the 500 series version. The stereo unit has single set of controllers for both channels. A dual mono use is not possible, but of course you can process a single mono signal and just leave the other channel unused. Stereo coupling is done by a summed mono signal generated from both channels. This way, strong amplitudes in the sides will not lead to pumping effects in the other channel or in a shift of the phantom center. 

The blue aluminum front panel features eight controllers and for push buttons with status LEDs. The illuminated elysia logo in the middle serves as pilot light, and on its right side there is the gain reduction meter and the indicator for the Gain Reduction Limiter. 

The controllers on the left side set the threshold, the time constants for attack and release and the compression ratio. The attack time reaches from 0.01 to 120 milliseconds, and the release can be set from 5 to 1,300 milliseconds. The ratio range goes from 1:1 to 1:unlimited (limiter), followed by negative ratios for special compression characteristics. 

Four push buttons in the middle of the front panel provide the following functions: Bypass (the LED meter remaining active), Warm Mode (a slew rate limiter for slowing down the rise time of the output amplifiers combined with a fixed EQ curve), Log Release (changing the release curve from a linear to a logarithmic type) and Auto Fast (a program adaptive control of the attack time). 

The right side starts with the controller for a continuous high pass filter up to 1 kHz for the control path of the compressor, which can be used to make it less sensitive towards low frequency energy. Then follow the controller for limiting the control voltage (the so called Gain Reduction Limiter), the make up gain, and the mix knob for blending the direct with the compressed signal. 

On the rear panel we find the input connectors for the audio inputs and outputs both in XLR and jack format. Also, there are additional jacks for the external sidechain of the compressor marked EXT 1 for the return and EXT 2 for the send. You can use this option for many things, e.g. for inserting an external equalizer in order to make the compressor more or less sensitive towards boosted or cut frequency areas. Just as one example, a peak in the upper mid range combined with an infinite ratio makes a de-esser in no time at all. 

Measurements
During our measuring tests the xpressor impressively proved that analog technology can deliver outstanding performance even with limited budgets. The maximum input and output headroom is 21 dBu; while in unity gain position there is a very small boost of 0.2 dB. Our diagrams 1 and 2 show the phase frequency range in normal and Warm mode, and it becomes obvious how neutral the phase progress performs even into the lowest frequency range in normal mode. In Warm mode there is a very broad boost starting around 2 kHz with a little bit more than 1 dB. 

The effective unweighted output noise measured at -89,2 dBu RMS (22 Hz to 22 kHz), and this results in a comfortable total dynamic of 110 dB. The quasi-peak measurement with CCIR filter resulted in -78,7 dBu. The FFT noise spectrum shown in diagram 3 is completely trouble-free, and the common mode rejection shown in diagram 4 is OK as well. Diagram 5 shows the broad range of possible input to output settings including negative ratios, and diagram 6 revels the way the Gain Reduction Limiter works nicely. Diagram 7 shows the harmonic distortion of the compressor at an input level of 10 dBu. The symmetry between both channels is perfect with almost no measurable differences at all. 

In action
Everybody who is interested in a compressor first needs to answer themselves the question which specific problems one wants to address with it. Is it a certain sonic structure or character of a unit which already becomes obvious when processing no dynamics at all? Or is a very subtle and inaudible way of processing the ultimate goal? Are special effects on the agenda? Or is achieving good results quickly of the highest importance? What about more special features like de-essing and frequency-dependent compression? Or simple leveling scenarios? 

The xpressor makes the decision easy for you, because it can do all of this – at a price point which is really hard to believe. If we put the many special features of this unit aside for a moment, we look at a very precise and transparent compressor, handling the basic task of reducing dynamics. In practical use, you instantly notice how well the xpressor reacts to any setting made. It behaves like a didactic play in terms of dynamics processing. 

If one wants to round out the transients, choosing a fast attack time and a release which fits the envelope of the signal is the way to go. This works equally well with single signals as with more complex program. If remaining the transients and achieving a higher signal desnsity is the goal, a longer attack time combined with a shorter release will get you there. And I have not even used a single of the special features yet... In terms of sound, the xpressor is very neutral and transparent. The processed signal keeps its sonic structure without any degradation at all. 

So, let's have a look at the time-based processing parameters. The Auto Fast function makes a program-adaptive control of the attack time possible, and this is very nice for working fast and efficiently. On fast and strong signal impulses, the attack time will automatically be reduced to its fastest value, and after this it will automatically return to the value which has been set with the controller. 

Basically this means working with two attack parameters which take it in turns, based on the structure of the processed audio signals. This makes the compressor fast enough in any situation, but without generating any artifacts. All signal peaks will be caught when the Auto Fast mode is active, therefore you obviously cannot 'shape' transients in this mode. 

The counterpart – the release parameter – offers the choice between a linear and a logarithmic progression curve. In logarithmic mode, the release time becomes shorter on high amounts of gain reduction. This means that on signals with a lot of transients, like drums or percussion, the release time will be shorter, while it will be longer on signals with a more even structure. This behavior is nice when subtle compression results are wanted, making it the mode to choose for sum and complex single signals. 

A test with an acoustic guitar played in a pretty percussive way revealed how 'intelligent' the control process can be. Obvious string noises got caught by the Auto Fast function reliably and got reduced by the logarithmic curve quickly, while the tonality itself remained its consistency and focus. The result is a very dense sounding guitar with a strong presence. The principle works pretty much the same way when processing sum signals or final mixes. Maybe one would approach this without Auto Fast activated in order to keep the existing transient structure intact. 

Activating the Warm function generates some kind of vintage impression. It rounds or slows down the transients in the output stages, and this shifts the focus to the low mids and low frequencies with a moderate raise of total harmonic distortion as an addition in the high frequency spectrum. As this feature is placed after the stages which are responsible for dynamics processing, the user can use any of the compressor's features for further signal shaping. 

But the xpressor also has an ace in its sleeve for the friends of more extreme compression effects, and this 'toy' is called negative ratios. The ratio parameter determines the proportion between the input and the output signal. When a 10:1 ratio is applied, a rise of 10 dB at the input will be reduced to a rise of a single dB at the output. 

At a negative ratio, for example 2:1, the level will be reduced below the threshold of the compressor, this means the characteristic curve bends down instead of remaining horizontal (which it would in a limiter at an infinite ratio). You can find a diagram of what exactly is happening here in the measurement section of this review. It's actually not really easy to describe how it sounds … you can imagine it like some kind of ducking or backwards effect. The envelope of the signal is extremely deformed and turned upside down. 

A special goodie of the xpressor 's control options is the gain reduction limiter. This is not a limiter working in the audio path, but it limits the control voltage to the value set with its controller. Is is basically a second threshold which tells the compressor not when to start, but when to stop working. Signals with high levels above this second threshold keep their original form, while everything below is processed according to the set rules. 

The advantage of this is that high compression rates will generate a high signal density, but the dynamics of the transients are preserved. It's a very interesting effect, as it gives the user the possibility to keep even very extreme settings (like negative ratios) under reasonable control. The combination with the mix controller for parallel compression opens further perspectives for creative signal processing. 

Just as one example, I compressed a drum set in such a brutal way that it sounded like room mics, as the relation between direct and ambient sound had been changed dramatically. Because the mix controller provides the possibility to blend the direct and the processed signal, it is possible to just ad a part of this ambience compression to the original. This is a very interesting option to rise the density of a signal without destroying the original dynamics. 

In my example, I also used the integrated highpass filter for the sidechain in order to focus the effect on to the high frequency range. In many cases, sidechaining with a compressor is equated with inserting an EQ. However, the sidechain is not limited to spectral affairs, but it can also generate a dynamic interdependence. This can be achieved by feeding an audio signal into the sidechain which results in a dynamic movement of the complete program material, or it can be used to generate 'free spaces' for specific instruments. Your own imagination is the limit of what is possible with it. 

And of course you can use all the other special features already mentioned in combination with it. In other words, the xpressor is an incredibly versatile machine with many extras, and everything is happening on a extremely high level of sound and technical quality. In no way this reduces to the extreme effects the xpressor is capable of. This device is a first class dynamics processor that in a neutral, transparent and subtle way, if you want it to do so. For this reason and because of its excellent technical measurements it is also a good choice for mastering applications.

No matter if it is used as a bus compressor or on single signals, the xpressor covers any aspect imaginable in dynamics processing – and its truly creative concept shows the user many things which even reach further than that. 

Result
The title of this review, "The compressor for the people", is in no way meant to be interpreted as the description for a standard medium range unit doing its daily business. On the contrary, it describes a very broad range of possible uses, ranging from standard situations to the most brutal and absurd effects, always on a high level in terms of sound and technology, and with a remarkable transparency and neutrality. If I now go ahead and tell you that this magic machine comes at 990 Euro excl. VAT, you might not believe my enthusiastic comments any longer. I could not believe it myself when I heard the price for the first time. 

The xpressor meets the requirements for being used in professional environments in every single aspect, and it outperforms considerably more expensive competitors easily, also in terms of its excellent technical specifications. With such an attractive price, the range of interested parties will reach even into the home recording domain, enabling users to lay their hands on professional technology for an affordable price. 

And to crown it all, there is the 500 series variant which is even a couple of hundreds less. Never before has such a high level of technology in combination with such a creative concept been offered for so little money. "Best of elysia"? Absolutely! I roll out my virtual prayer rug and bow towards Nettetal, the new Mecca of analog dynamics processing. And this is something the elysia boys really deserve. You know me and therefore you also know that I am not using this platform to publish any remotely-driven bullshit. The xpressor defines a new value for money class which stands unequaled today...