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100% Discrete Class-A
As a true elysia specialty, the mpressor is completely based on discrete circuitry and does not use any integrated circuits at all.

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To explain the advantages of discrete circuitry we’d like to present a comparison that might seem a little odd at first: audio technology vs. cooking! If somebody uses instant meals exclusively, he will have to accept whatever comes out of the box. A creative cook, however, focuses on his own special recipes and ingredients.

In this respect, integrated circuits (ICs) are pretty similar to packet soups: they are cheap, mainstream and they simply do not match haute cuisine. So if you want to design an analog audio device 100 % according to your own demands and ideas without any compromise, there will be no way around a discrete design.

The mpressor follows this philosophy consequently. Not only all audio paths, but also the power supply, sidechain and even the driver for the LED meter have specially been designed for this unit and they are built by the exclusive use of discrete components. A truly unique recipe!

In addition, our creative compressor operates in permanent class-A mode. This means that the transistors are always conductive, resulting in the absence of crossover distortion and providing a pristine sonic base: the general sound character is always wide, open and punchy.

Auto Fast
Inspired by its big brother: the switchable semi automation for a perfect attack on the basis of the value set by the user.

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The attack parameter is a very crucial factor for the operations of a compressor. Choosing the right time settings is very important, but depending on the dynamic progress of the source material this can be a difficult task – no matter if single tracks or complete mixes are processed.

If a very short attack time is chosen, the compressor is able to catch the short peaks, but on the other hand the sustaining signal will also be processed, which might result in audible distortion. Longer settings reduce distortion significantly, but then the compressor is too slow for catching fast impulses.

This is where the Auto Fast function comes into play. For example, if you set the attack to 80 ms and then engage the Auto Fast mode, the attack time will be shortened automatically on fast and loud signal impulses. The compressor reduces the signal quickly and prevents it from slipping through.

Then the attack time directly and automatically returns to its original setting. In Auto Fast mode the compressor can be very fast, but only when it is really needed. This function influences the attack parameter on short and loud impulses only; in all other cases the original setting of the controller has priority.

Anti Log
This alternative characteristic of the release curve follows an antilogarithmic course instead of the standard linear progress and produces a much more audible result.

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It is the time constants and especially the release parameter that decide if the processing of a compressor is obvious or unobtrusive to the ear. Mastering applications, for instance, require a discreet performance as a general rule. Here you will find mostly logarithmic or linear release curves.

It is characteristic of a logarithmic release that the time constant shortens when the amount of gain reduction increases. The advantage of this behavior is that short and loud peaks (e.g. drums) have a fast release time, while the remaining material is processed with a slower release time.

But if intentionally striking and creative compression is the goal, it definitely makes sense to turn things upside down. In the Anti Log mode of the mpressor the curve behaves just the other way round: If the threshold value is passed and compression starts, the release time will be longer at the beginning. If the input signal starts to decline, however, the release time will become faster as a result.

A special circuitry makes this behavior independent from the absolute amount of gain reduction. No matter if the compressor reduces 10, 15 or 20 dB, the curve will always stay the same at the beginning and it will only become faster at the end. With this feature you can create many exceptional compression effects just by the push of a button!

Negative Ratios
The characteristic curve bends and goes back down! Heavy pumping, backward sounds, etc. – perfect for very cool compression effects.

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Negative ratios – what exactly does this mean? To get a better understanding of this function, it makes sense to realize what the ratio control of a ‘normal’ compressor does:

• 1:1 The signal remains linear, there is no compression going on.
• 1:2 After crossing the threshold, an increase of 2 dB at the input will be compressed to an increase of 1 dB at the output.
• 1:∞ After crossing the threshold, the output signal is constantly held at the threshold level without reacting to further increases at the input (limiter).

At a negative ratio, the characteristic curve bends and returns back down after crossing the threshold. The louder the input signal, the lower the output signal – perfect for groovy compression effects. To get a grip on the extreme ‘destruction’ this can cause, engaging the Gain Reduction Limiter is just the right idea.

Beyond infinity – made possible by the mpressor ;-)

Audio Filter
Just as the alpha the mpressor features a Niveau Filter. According to its creative approach, it here allows even stronger sound shaping results.

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This filter is a specialist in changing the overall sonic character of a signal with ease. It features two controllers per channel and is capable of producing convincing results flexibly in no time at all. Whenever a classic shelving filter would be too limited and a fully parametric filter would be too much, the Niveau Filter is the perfect tool.

Its main function is to change the proportions between high and low frequencies. The principle is quite similar to a pair of scales: Dependent on the gain setting around a variable center frequency, the high frequencies are boosted whereas the low frequencies are attenuated (or vice versa).

By simultaneously boosting and cutting the selected frequency areas, it is much easier to influence the character of a track (‚bright‘ vs. ‚dark‘) compared to using other types of equalizers. The center frequency can be shifted continuously between 26 Hz and 2.2 kHz or between 260 Hz and 22 kHz respectively (when the x10 switch is activated).

The characteristics of the filter change in the extreme positions of the EQ Gain controller: the fully counter-clockwise setting will give you a low pass filter; fully clockwise position will result in a high pass filter. The overall level can drop quite noticeably then, but this can be corrected with the gain controller easily.

External Sidechain
You want to create frequency dependent compression or have it accented by the groove? The switchable sidechain inputs make nearly everything possible!

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The external sidechain enables the compressor to control its processing totally independent from the audio material running through it. If the SC Extern switch is active, compression will not be triggered by the signals from the regular audio inputs anymore, but by different signals which are fed into the additional sidechain input connectors.

If, for example, a duplicate of the input signal is processed with an equalizer and then fed into the sidechain input, the result will be frequency-dependent compression. Another example is to send the bass drum of a drum machine into the sidechain input in order to achieve nice groovy compression that is pumping in sync with the music.

The creative options are almost infinite. Compression can be exactly on time or totally against it, which can of course be varied on the fly. Single instruments can be given more space in a mix according to its rhythm. All of a sudden, static sounds become vivid and sound really interesting!

The external sidechain inputs are equipped with a high pass filter that has its cut-off frequency at 80 Hz with 6 dB per octave. This has been done in order to reduce the influence of the low frequencies on overall compression by a certain amount in order to keep the effect as balanced as possible.

Gain Reduction Limiter
This novel limiter is not placed in the audio path as usual, but restricts the control voltage of the compressor instead.

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A specialty of the mpressor is the Gain Reduction Limiter for the control voltage. This limiter is not placed in the audio path where you would usually find it, but in the control path of the compressor. When it is activated, it limits the control voltage according to the setting of the GR Limit controller. This means: No matter how high the input level might become – the amount of gain reduction will never exceed the value which you have set.

For comparison, imagine a fader on a mixing console with your hand moving the fader to ‘play compressor’. If now the fader was limited by a piece of duct tape at -10 dB, for example, it could only reduce the signal up to this value. If the input level dropped below this limit, the fader would be moved up correspondingly.

However, if the input signal got even louder, the fader could not be moved down any further because of the duct tape limit, and then the output signal would become louder again in correspondence with the input signal.

Loud parts in an arrangement can keep their dynamics, as they will not be compressed beyond the limit of the Gain Reduction Limiter. Some very nice special effects like ducking or upward compression can be achieved with this easily by only reducing the quieter parts without changing the original dynamics at the same time.

Analog Dynamic LED Meter
The gain reduction meter modulates its LEDs in their brightness in order to show the action of the compressor in an analog way: fast and with smooth transitions.

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The gain reduction meter is a very important visual tool for evaluating the operation of the compressor in addition to what your ears tell you. A lot of devices make use of sometimes more, sometimes less precise VU meters. But because of the inertia of the needle these meters are only useful with moderate time parameters.

Another popular form of meter is the LED chain. Unfortunately it has a disadvantage, too: When the standard driver units are used, the change between two values always happens abruptly. A single LED in the chain can therefore only show an imprecise value in a defined interval. Hectic flicker indicates that the actual value must be somewhere in between.

The mpressor solves these problems by using an analog dynamic variant that combines the benefits of both VU meters and LED chains. This meter is based on LEDs, too, but a special circuit design makes it possible to show intermediate values by modulating the brightness of the LEDs.

This means a true analog way of showing the operation of the compressor: very fast, but with smooth transitions. The user gets an important tool for precise gain reduction monitoring – finally the relationship between acoustic and visual perception feels just right.

Transconductance Amplifier
The mpressor’s custom discrete gain reduction cell provides low noise, extremely fast time constants and all the punch in the world.

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A crucial part in the development process of a compressor is to design its control element that reduces the audio signal controlled by voltage. This is also where the main technological concepts show the most obvious differences.

The mpressor uses elysia’s fully discrete and temperature compensated Transconductance Amplifier (TCA). A differential pair of transistors that uses a modulated current source to affect the amount of amplification builds the core of this module. A few extra transistors were added in order to further decrease noise and unwanted influences of the control voltage.

The benefits of discrete electronics design can once again show their strengths by tailoring the sound via the concept of the circuitry and the choice of components. A further advantage of the TCA topology is the very fast response to the control voltage. This is the basis for a compressor that can even handle rapid changes in dynamics and which provides extremely fast time constants.

Also remarkable is the fact that the make up gain was designed as an amplification of the input signal. This is running parallel to the sidechain path and therefore does not have any unwanted influence on the processing itself. The essential advantage of this arrangement is that the background noise always stays at the same level, no matter how much amplification is applied.

T12 Heater
The single transistors are kept at a constant temperature by the use of this heating system which helps to avoid parameter drifts.

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Some delicate parts of the discrete circuitry can be influenced by the surrounding temperature easily. The main reason for this circumstance is the single transistors that can react very sensitively to variations in temperature (which can – depending on the place of installation and operating time – happen by all means).

With the T12 Heater elysia presents a system that arranges constant conditions and reduces the thermal fluctuation to a minimum. This system was inspired by high-precision measuring instruments. It features up to 12 discrete transistors in a massive copper ring that is warmed up to a definite temperature.

Once the system has reached its working temperature, it only needs little current to keep it at the same level. An electronic control circuit is responsible for a low variance of only a few degrees. The procedure is known from high end tube gear: the mpressor should be granted a short warm-up time to experience it absolute top form.

Technical Data
If you are interested in learning more about typical measurements and technical specifications of the mpressor, this is the right place to check.

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Frequency response:                 <10 Hz - 33 kHz (-3.0 dB)
@ 0 dBu, 20 Hz - 22 kHz
@ +10 dBu, 20 Hz - 22 kHz

0.04 %
Noise floor:
20 Hz - 20 kHz (A-weighted)

-84 dBu
Dynamic range:
20 Hz - 22 kHz

109 dB
Maximum level:

+25 dBu
+27 dBu

10 kOhm
68 Ohm