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Discrete Class-A Topology
As a true elysia specialty, the audio path of the mpressor 500 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 often 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 500 follows this philosophy consequently. Its complete audio path is a new design which is based on high grade 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.

THD Boost
This switchable harmonics generator allows on-the-fly signal coloration and saturation effects capable of delivering a grittier flavor.

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Activating the THD Boost function raises the level in front of the gain control element. This generates additional harmonics inside the input stage of this amplifier, which has a significant influence on the sonic structure of a signal.

The mpressor 500 is a pure feed forward compressor, and the gain potentiometer is placed before the control element. This means that the intensity of the saturation can be raised or lowered by different settings of the gain controller.

What’s special: The detector in the sidechain of the compressor runs in parallel and is therefore not influenced by the THD Boost at all. The actual compression does not change, it is only the sound which is being influenced.

You generate a saturated signal which as a result is also reduced in dynamics, but the actual control of the compressor itself is always based on the original dynamics including all transients, impulses and so on.

Auto Fast
Inspired by its big brothers: 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 striking 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 500 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 compression 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 500 ;-)

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

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A specialty of the mpressor 500 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 a truly 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 500 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 custom discrete gain reduction cell of the mpressor 500 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 500 uses elysia’s special 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.

Another 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.

Stepped Controllers
All potentiometers of the mpressor 500 have 41 steps, which is a great help for recalling your previous sessions fast and precisely.

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The mpressor 500 features stepped potentiometers for all its parameters throughout. The 41 steps make a precise recall very easy, and they provide a useful range of possible settings at the same time. And you will just love the feel of them, too ;-)

Stepped controllers are something you will usually only find on much more expensive gear, special mastering editions, or in most of the cases – not at all. The mpressor 500 establishes this very handy feature in the regular serial version without any luxury up-charge.

Ground Layer Shield
One of the six layers of both PCBs is a full copper plane which helps to reduce interferences caused by electric fields.

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The PCBs of the mpressor 500 have a total of none less than six layers: While the first five are used for separately routing audio and power supply, the sixth one is a dedicated ground shield layer. But what's the idea behind this?

As the additional ground layer is placed extremely close beneath the traces which carry audio, it is very efficient as a shield against unwanted noise interferences caused by electric fields.

Technical Data
If you are interested in learning more about typical measurements and technical specifications of the mpressor 500, this is the right place to check.

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Frequency response:                 <10 Hz - 390 kHz (-3 dB)
@ 0 dBu, 20 Hz - 22 kHz
@ 10 dBu, 20 Hz - 22 kHz
@ 0 dBu, 20 Hz - 22 kHz, THD Boost
@ 10 dBu, 20 Hz - 22 kHz, THD Boost

0.04 %
0.19 %
0.24 %
2.20 %
Noise floor:
20 Hz - 20 kHz (A-weighted)
20 Hz - 20 kHz (A-weighted), THD Boost

-79 dBu
-87 dBu
Dynamic range:
20 Hz - 22 kHz

99 dB
Maximum level:

21 dBu
22 dBu

10 kOhm
68 Ohm
Power Consumption:

75 mA