Dynamic Rollercoaster

by Fritz Fey
Studio Magazin (Germany) – January 2008

How nice – the sixteenth remake of an opto-compressor! These days, our enthusiasm about cloned studio technology from the old days has reached very limited dimensions, as we are virtually flooded with interpretations and copies of audio gear that has been rated good somewhere down the road. But what has been good at its time does not necessarily have to be good today. A simple compressor box from the sixties was quite simply everything that was available at the time, and therefore the question is allowed if a usually quite expensive reflection of such a unit is still able to do a proper (and/or flexible) job in today’s studio environments anymore. Granted, there are some treasures in the market which convince with their sonic charm time and again for a reason! But still not everything carrying the label ‘old’ has to be inevitably good.

In the past, I have addressed constructive criticism into the direction of the developers of digital audio technology to stop trying to copy analog archetypes in favor of concentrating on their own strengths. Thus, the same acclamation has to be sent into the direction of analog developers: Please stop copying yourself! There are convincing examples of modern analog concepts that can inspire us with new ideas, and therefore we are always glad when we are offered such a product for review. And of course it is even nicer if the developer – in this case Ruben Tilgner from the still young analog manufacturer elysia – brings the contestant to the editorial office himself; the right hand balancing a baking dish with a special ‘Mama Angelina’ version of the Philadelphia cake: a truly stunning creation, only to be tasted by a few chosen ones (which we are now pleased to be among). The urge to create originals somehow seems to be essential in this family. After agreeing on the fact that this kind of presents would not have any influence on the outcome of the review at all, we started the inevitable discussion on dynamic processing, but not before performing an intensive collective tasting test on the cake…

Talking about archetypes, I remember the strange and really crazy Omnipressor by Eventide from the seventies. With its extreme control settings and negative ratios, this unit was very inviting for performing experiments, but the price was an amount of noise that just could not be ignored and that strongly damaged the fun created by the crazy sonic events. The mpressor by elysia cultivates the control possibilities of a compressor on the highest technical level beyond every standard dimension and offers the pleasures of dynamic rollercoaster rides that so far no other unit has to offer. Nevertheless, it can also be fairly well-behaved in order to cope with the standard tasks of everyday business; still providing them with a truly individual signature.

Overview
The concept of the mpressor allows the use of the term ‘new’ with a really firm conviction. Some of the functions of the unit exceed the tried and trusted choice of parameters of ‘standard’ compressors and allow for dynamic processes that would not come to the mind of even experienced users. Specifically I am talking about functions like the control voltage limiter, anti-logarithmic release curves, a Niveau Filter as a sound shaping tool and negative ratio values. The technical level of the mpressor is characterized by a completely discrete design, that is to say that the complete class-A signal processing including detector path and power supply has been designed by the use of single transistors. All controllers are conductive plastic types; current feedback amplifiers instead of opamps and the internal temperature stabilization for certain discrete transistors are a proof for the developer not taking the conventional paths.

If you open the unit, you will instantly notice the complete mechanical symmetry of the audio electronic and the high-capacity power supply with its big electrolytic capacitors. Both channels use exactly the same components in order to achieve an ideal symmetry (the boards could even be swapped with each other). This approach is also a decisive factor for reduced channel crosstalk and optimized signal paths. The build quality is first class and contains a great deal of development details of the alpha compressor. The maximized technical quality allows extreme processing results without the remorse of degrading the signal.

User interface and functions
The layout of the control elements is easy and clearly arranged and similar for both channels, of course. The middle holds a disc with the company logo illuminated by white light similar to a luminous advertising, flanked by LED gain reduction meters to the left and right. The buttons for activating the left and/or right channel plus the stereo link function (covering all functions besides the audio filter and the makeup gain) are placed below. With the exception of the specified functions, the mpressor is controlled by the controller set of the left channel in stereo mode. The top row starts with the good old friends: threshold, attack, release and ratio. The space in between the controllers features three push buttons that have a red glow when they are active. They are responsible for switching external signals into the detector path (sidechain), activating the adaptive attack time (Auto Fast) and engaging the anti-logarithmic release curve (Anti Log). The bottom row features another four controllers for boosting and cutting the Niveau Filter, the center frequency of this EQ, the threshold of the control voltage limiter and the makeup gain. The three corresponding push-buttons activate the filter, the x10 frequency shift and the Gain Reduction Limiter.

For your interest, here are some of the controller ranges: The threshold can be adjusted between +16 and -18 dB, the ratio (between input and output level) goes from 1.2:1 to 10:1 and then from -0.3:1 to -4:1. The attack time works with very fast 0.01 to 150 milliseconds and the release covers a range from 5 milliseconds to 1.2 seconds. The scale for boosting and cutting the Niveau Filter goes from -6 to +6 dB and it features additional Lo and Hi functions in the extreme left and right settings of the controller (you can see the corresponding high/low pass effects in the measurement chapter). Taking the x10 switch into consideration, the center frequency of the filter can be set between 26 Hz and 22 kHz. The limiter (not for the audio signal; more about it later) covers a range from 0 to 21 dB and the makeup gain can be between 0 and 20 dB.

Specials
In addition to the classic parameters which do not need any further explanation, the mpressor features a set of special functions to be explained in this chapter. Let's start with the Auto Fast function which our readers already know from our review of the alpha compressor we published one year ago. Auto Fast is a semi-automatic, program-adaptive process that reacts on short signal peaks by accelerating the attack time, after which it returns to the manual setting of the controller. This makes it possible to combine an inertial setting which has been optimized to the musical material with an overlaid faster integration time component, with the result that the compressor can react ‘correct’ to the program content. The compressor only becomes fast when the program actually needs it.

The Gain Reduction Limiter is a function which I have never seen on any other unit before. It reduces the maximum amount of the control voltage (and not the audio signal) to an adjustable value which is completely independent from the input level. With this function (which will receive more detailed honors in the practice chapter), strong signal peaks can keep their dynamics. Conversely, low level passages can be raised without influencing the original dynamics. This is very easy, very clever and very effective, as we will see later. Anti Log is a function that influences the release characteristic of the compressor. There are compressors that work with a linear or logarithmic curve for releasing the compression process. The mpressor offers a linear characteristic plus an antilogarithmic curve as an alternative, which means that the release will be accelerated more and more in its progression. The characteristic of this process is independent from the total amount of gain reduction. Depending on the basic settings, function results in lots of unusual compression effects that could not be achieved with any other compressor. More information on this can be found in the practice chapter, too.

Last but not least, the list of specials includes the Niveau Filter which has also been inspired by the alpha compressor. This EQ makes shaping the sound of the input signal very easy. Because of the high bandwidth of the filter that can be understood as a pair of scales around an adjustable pivot point, the effect is a simultaneous cut and boost of treble, mid and bass in two frequency areas. You cannot use this filter as a de-esser module, though, as this would demand a steep bell and an access to the detector path, but you can insert an external equalizer into the switchable sidechains of the mpressor instead. Of course (but this is not a specialty of the mpressor) you can also send other audio signals into the sidechain in order to create musical and rhythmic compression styles.

Technical measurements
As expected, the results of our technical measurements did not provide any reason for criticism. The fact that the harmonic distortion rises continuously and reaches remarkable dimensions at higher settings is part of the sound shaping concept of the unit. Still, this circumstance makes it a little difficult to state a concrete maximum output level – about +14 dBu produce 1% of THD, at +18 it is 3% and at +22 it is 9% already. Even though threshold and ratio were set as low as possible, higher input levels already caused a control process in this case. Of course you cannot compare the intended harmonic distortion the mpressor generates with the clipping distortion of arbitrary semiconductor circuits, and moreover, it starts much softer and can be controlled much better.

Diagram 1 shows the frequency response of level and phase between the input and the output. Especially interesting is the frequency response of the phase which stays completely flat down to 10 Hz. The output noise with the output controller at unity gain showed unproblematic -80.2 dBu RMS effective unweighted (22 Hz to 22 kHz). The quasi peak measurement with CCIR filter resulted in exactly -70.0 dBu. The overall dynamic range results from the RMS value and in turn depends on the maximum output level taken into consideration; at the classic THD limit of 1 percent it is approximately 94 dB. Diagram 2 shows the FFT noise spectrum of the output; diagram 3 is a typical FFT THD spectrum. The asymmetric divergence of the input lay at -71 dBr at 1 kHz and -55 dBr at 15 kHz which are unproblematic values. The same is true of the crosstalk attenuation that lay over -115 dBr at 1 kHz and -82 or -105 dBr respectively at 15 kHz – depending on the direction of the measurement.

Diagram 4 and 5 show the control characteristics of the compressor at variable threshold and ratio values. Diagram 5 also reveals the declining response curves caused by negative ratio settings. Diagram 6 shows the effects of the Niveau Filters in the extreme left and right positions and some interim positions as well. Diagram 7 shows the effects of the Gain Reduction Limiter: if its threshold is crossed, the response curve will return to a linear progress determined by the ratio. Diagram 8 shows the bellied release progress of the activated Anti Log function.

Listening and practical experience
Equipped with signal sources from a great spectrum of different categories (for example drum sets, bass drums, grand piano and acoustic guitars) I start my tests which become a delight right from the start. This unit lets you start in the classic manner instantly, and it is easy to get used to the parameters. This thing plainly does what you tell it to do – and it does it well. This fact alone would convince some colleagues already, maybe not for the price, but it is worth noticing that the mpressor is a great compressor for single signals and subgroups as well, even if you keep the slightly hidden bag of tricks closed. But once it is opened, you quickly understand what the developer had in mind when coming up with this tool. First of all, it is pure fun to drive a drum set so hard against the wall that it can hardly breathe anymore. Once you had enough fun with this for a start, a phase of creative reflection begins.

It did not take me long to understand how I could use negative ratio settings to bring back something like dynamics and intimacy to an over-compressed voice, or how to play with the fade ins and outs of a mellow grand piano. The envelope of single sources can virtually be turned upside down, and the mpressor does not seem to reveal any limits concerning its processing capacities. Especially interesting is the use of the antilogarithmic release time that often sounds more ‘correct’ than the linear one. If you use an extreme setting of the Gain Reduction Limiter to approach the point where you will notice a grin all over your face, you will understand what this tool is all about. You tailor the desired envelope developing with the time constants, play around with the ratio in the area between 10:1 to -2:1, and then you notice that you can influence the structure of your signals in a very creative way.

Tthe negative ratios not only give you the fun factor of ‘backwards dynamics’, but by the use of careful threshold settings they can also be used to achieve a dynamic progress that could not be achieved with any ‘normal’ compressor. This way of raising loudness is a very unique thing. In other words: The mpressor is not just the ‘big hammer’ you can use to bash your audio material, but it can also be a filigree tool to enhance the dynamics while keeping its ‘natural’ structures. Here is an example: You can use the Gain Reduction Limiter with an ‘overdone’ threshold to make controlled changes that will help to give the attack of instruments a better definition (the actual threshold is the key-sensible value here). The Niveau Filter seems to be a very simple design at first look, but it lets you generate musical timbres that can be the final touch for an already equalized signal after the dynamics processing.

The Auto Fast function works in direct connection with the threshold controller. If you drive your signals into compression too heavily, the transients will virtually be eliminated, and this will not always be the final goal. The semi-automation works much better on vocals, guitars and other non percussive instruments than with drums, for example, where you would usually prefer to emphasize the attack phase. I did not elaborate on the completely wild possibilities that the external sidechain has to offer, as they are no exclusive domain of the mpressor.

Result
It is very obvious that a great part of this unit has been developed with the ears, which you can notice from the ‘floating’ and THD loaded ‘overload zone’, for example, or from the slightly dropping frequency response in the treble area that was implemented because of the general sonic impression caused by fast control processes (even if the optics of the measurement diagram suffer a little bit from it). This is one of the facts that lead to the conclusion that the mpressor is an absolutely unique design that outranges the limits of conventional concepts by far. This unit is a construct with a very individual profile and it does not even try to profit from development achievements from the past. The complete circuitry layout is a true original.

No matter if it is used for tracking or as a buss compressor or maybe even for mastering applications – the mpressor holds all the aces. Of course you cannot have a piece of gear with such a sophisticated build quality for the price of a plugin. It certainly belongs into the hands of pros who know what they are doing. The inner and outer workmanship is flawless, and the appearance of the mpressor is representative and elegant; a fact that customers will certainly appreciate with the given price tag. Boys: However good Mama Angelina’s cake might have been – you did a really amazing job on this one… Hats off!