In PURPLE is something undoubtedly charming...


Just a few moments with this plugin is enough to sincerely desire to become a part of the Acustica Audio family.

Even the color which gives it its name contains a sort of suggestion that somehow tells us to prepare for a truly royal experience...

Ah, vintage tube passive equalizers...

Is there really anyone who has never dreamed of having one of them?

​With the development of technology we are witnessing a new revolution, which despite the efforts still can not cope with the phenomenon of the original tube constructions that despite many years of faithful service continues to enjoy popularity among new generations as home-grown and professional sound engineers...

What is so special about these kind equalizers?

Well, in short...

Everything! And now thanks to Acustica Audio we will be able to see this all, because PURPLE is an exceptionally faithful reproduction of two classic vintage tube passive type equalizers hiding in a very rich, greasy and full sound analog progenitor (at least that's what promise us the creators of this beauties).

Are all these claims true? Relax, in the course of this article I will try to answer this and all other emerging questions, but right now it's time for a little history...

The first passive tube equalizer was designed and manufactured by the Pulse Techniques Company and was the result of work of Ollie Summerland and Gene Shank.

Created in 1951 this handmade design, just like many of today's other iconic devices was born in a small garage workshop and unexpectedly for the creators themselves over the years has become  one of the most copied equipment in the history of processing audio.  Already on the day of its launch it was very well received.

Many from among the hand-made pieces almost immediately have been installed in radio stations, where it has become a key component for "that" final touch of the presented material. Interestingly turned out to be a component capable of improving the quality of the material, which was recorded earlier by using inferior equipment. It's another example of the unusual properties of these structures. Properties used to this day...

Over the time, this first passive tube equzalizer has formed the basis for both commercial and independent studios worldwide.

Today it has almost mythical status. For decades a synonymous with the best sound and the gold standard in the field of its coloring. Despite the passage of years, many engineers still can not imagine a control room without at least one characteristic immediately eye-gaze passive equalizer. Original first constructions achieve today a truly staggering values.

This unit is more than a equalizer. It's a religion. Everyone dreams about it, everyone wants it and everybody wants it to be...

None story about its creation can not be complete without providing one more fact. As it is said that everything is in the roots,

I think that it is worth noting also the place where both Summerland and Shank decided to start a business. 

The place as it turns out unique, perhaps not accidental, but in any case for sure symbolic...

Why? Because the home of Pulse Techniques was in Teaneck in New Jersey, which already prior to producing the first passive eqalizer managed to make their mark on the contemporary musical world map.

This is where the today legendary musician Les Paul (co-designer of the today's cult guitar which he gave the name) created the first multi-track recording studio in the world. You also can not fail to mention that Summerland and Shank for some time built their equalizers only manually. Their design (not counting the relatively minor modifications) are almost unchanged and was produced for more than thirty years.​ It most famous (occupying three rack spaces) blue incarnation was produced from 1961 to 1971 when was replaced by a slightly smaller silver 1A3. Although, the newer models have exactly the same features as its classic ancestor (many engineers mistakenly thought that every silver unit instead of tubes has transistors), in the environment of producers and sound engineers were convinced that only those original blue ones have "this" classic, timeless and the desired sound...


​As I'm not only a devoted user, but also a true fan of this type of equalizers, when I learned that this time its reproduction will be created by Acustica Audio I passed a very pleasant chills and the days have suddenly become a lot longer and full of impatient waiting for the first opportunity to test this bundle.


So much for an introduction, let's do a broader look at our today's heroes...

So after several very successful channel strips, Acustica Audio has decided to change course and focusing its full attention on an outboard gear, serving us right away a very special outboard. Outboard full of strong coloring properties which can safely be described as a sort of premium spice, without which even the best dish will not be tasted. 

You are probably wondering why this kind of kitchen terminology?

Just open the user manual attached to the PURPLE and you will notice that not only I did perceive that these equalizers.


All those who are waiting for a very thorough analysis of its sound in the context of real analogue hardware, in this place I have to upset. If you expect here that I'm trying to precise point both all the similarities and differences between PURPLE and true analog passive unit, you likely to be disappointed, because this time I will do not proved how close or how far it is to the original. When testing PURPLE, I simply realized that in the case of these equalizers this kind of criteria are not the most important. Of course I could do that too and I have no doubt that it would be beneficial for Acustica Audio, but I thought that if this is not the first (or not the second) time when I describing their products, today I will try to present "them" in a different way. So instead debunk myths and fight with both the literary and the proverbial windmills, I will focus on what this equalizers actually are and what they can do for us (and I assure you that they can do a lot). After all surely you already know that in the realm of the quality of Acustica Audio equalizers, there's not need to write much...

None of the plugins from AQUA series (which PURPLE is part of) gives us the mathematically developed artificial algorithms pretending to be the legendary equipment. Instead, the Italians give us the real (often built by himselfs) high-end hardware, which is then very carefully and multilevel sampled using specially developed technology.

The sound thus obtained equalizer simply could not be more true...

You need also to know that PURPLE are this kind of equalizers where strenuous searching after such similarities as well as differences in terms of real hardware recognizes their value.

​So the PURPLE bundle (both equalizers are also available separately) includes two units...

P1 (like the original analog unit) is a single-channel, dual-band tube passive eq, which is known for its very warm, powerful low end and wonderfully rounded and at the same time full of presence and incredible silk top, which is never harsh.

​M5 is a real expert on any problems encountered in the middle of the frequency spectrum.

This is an extremely important region, which is located not only the strength and warmth of our mix, but also the frequency (near the high mid) at which every human ear is particularly sensitive.

Tube design offers not only a unique character, but also a natural glue to each track.

The construction of this equalizer provides so getting the right balance between the warring with each other paths while avoiding the unpleasant and often ruinous for our mix the phase conflicts. M5 is therefore a vitamin for all audio tracks which are located in the middle band frequencies.  Greatly improves presence of all vocal tracks and successfully reanimates every bass - which many engineers begin their struggle with M5.

Both P1 and M5 are passive devices, which means that the level of the incoming signal at the beginning is reduced to later due to internal amplifier to be raised to its initial original value. From a technical point of view this design is called passive precisely because of the fact that the signal level remains the same both when you turn the unit on and off (which is especially useful when comparing with each the processed and unprocessed signal).

PURPLE bundle give us not only the classic sound of a passive equalizer. This equalizers gives us a sound that in the genuine and true way are dress our mix in the true royal purple. These equalizers are a class unto themselves.

"They have" the possibilities to effectively escape all the categories, that usually are taken into account when you go into another endless comparison of next plugin version of this classic equipment.  

PURPLE creates categories for himself, but first things first...


​The first thing that strikes you in the eye when you run this plugins is a very nice GUI (which in the case of Acustica Audio are sometimes perhaps a bit reluctantly, but always extremely original and deserve attention).































​Both PURPLE equalizers are not only beautiful. They look like a real hardware!

You just want to catch the potentiometers, which due to the high realism seem even somewhat protrude from the screen.

​Speaking of the interface...


The low-frequency section of P1 is a shelving EQ providing four selectable frequency bands (20, 30, 60 and 100Hz). 

What makes both design truly unique is their ability to simultaneously boost and undercut the same frequency. 

It quickly discovered that thanks to this extraordinary properties a very interesting and musical curves can be obtained, resulting by overlap of both undercut and conquered frequency. Complicated?

I assure you that it's not! You will see about it later...


High-frequency section of P1 features a unique boost-peaking type curve, which also offers us some (actually seven) choice of highly usefull frequencies (3, 4, 5, 8, 10, 12 and 16kHz). The upper band section is a bit more extensive so that the equalizer despite already an elderly age can still amaze with his versatility.

The upper band also has another very useful parameter (which does however not affect the lower frequencies) provided of bandwidth control, which change the shape of Q (from sharp to smooth).


The high frequencies also have the possibility of simultaneously undercutting, but here it happens symmetrically only in the case of three frequencies (5, 10 and 12kHz), which are done by rigidly set active shelf type of curve.

The available selection of three cutting frequencies is of course no limitation. All values are very useful and have been chosen very wisely.

The M5 Midrange EQ offers us two bands of boost (a low midrange peak bell and a high midrange peak bell) and one very musical Dip attenuation. Despite of a slightly different system and a different range of available frequencies we also have here the ability to simultaneously boost and cut a few chosen frequencies.

The range of possible frequency attenuation is very wide, but due to the design of the equalizer (only one knob responsible for undercutting) we are able to simultaneously raise and cut only one previously selected frequency.


​As for frequency, we can boost at: 200, 300, 500, 700, 1000Hz. A second set supports frequencies from: 1.5,  2, 3, 4 to 5kHz. 

At the same time we can cut the frequency of the following areas: 200, 300, 500, 700Hz and 1, 1.5, 2, 3, 4, 5, 7kHz.


​That's in the topic of possible settings. As the virtual tube's of our today's heroes are already warmed up, time to see how PURPLE behaves when working in the studio...


Confronted with a mix it turns out fast that PURPLE is all what is not and what will never be a surgical digital equalizer,

so it is greasy and full of character. You can hear clearly that the engineers of the Acustica Audio tried to make up as possible to get closer to the sound of the first timeless units. PURPLE pleasantly colors our sound, even if it undercuts.

PURPLE not only colors all, they actually create the sound again!


Really great fall harmonics generated in response to a signal fed to both equalizers.


I think a lot to say on this topic would probably have drummers, because it is on drums (and especially on the snare drum) very well appears the extremely vivid, true and dynamic harmonic response of both equalizers included in PURPLE.

Each (non-linear in terms of dynamics) snare hit generates slightly different harmonics and therefore also slightly different final sound of the snare, which varies depending on the strength and location where the drummer are hitting.

​While in the case of an ordinary algo plugins we get in this situation usually flat and linear (always the same kind of) harmonic response, the technology used by Acustica Audio enables us to enjoy the full dynamic harmonic response, closely related to the shape of the sound, introduced a living, changing harmonic response appropriate for the analog equipment!

This is something really worth praising. Especially since it was the element that's been missing so far in different passive tube EQ's plugins promising us to sound like a real classic analog legend.


​Developers of many of plugin versions of this passive tube EQs made it very difficult.

The proper capturing the right balance between the harmonic saturation and specific dirty sound (which is formed by heating the preamp tubes) causes a warmth and volume transmitted by trace, typical for real hardware units. It is also a very important aspect that lets you easily distinguish the true sound from the artificial generated digital one.

Of course much depends here on both the type and physical condition of the tubes in which "they" are situated at the moment of capturing the hardware, but the sound of digital plugins (especially those pretending to be an old tube equipment) is often unnaturally thin and at the same time exaggerated. As if too aggressive only in one particular spectrum (often in an area, that according to engineers forming the plugin is the most characteristic for typical sound of a "real" hardware) plugins the gains thus more or less pronounced artificially "programmed" mimic character.

Now, thanks to Acustica Audio indication of the true sound is no longer so simple. Very good reproduction of the preamp section makes PURPLE sound as if they have the original EEC Tubes - which are mounted on the first hardware units (and today almost inaccessible), whose presence despite the fact that we actually use a plugin seems to be almost palpable!

PURPLE gives us exactly what we should give a fat tube analog passive equalizer. It is not exaggerated or artificial. It does not sound harsh or fizz and most importantly... perfectly reproduces all difficult to obtain artifacts which are an integral part of the characteristic sound of the real tube construction. All this makes that PURPLE look like a full-fledged independent hardware, which can cope with all this what usually handles the real passive tube equalizer.

To see that PURPLE acts like a real hardware is sufficient to use some commonly-known techniques, which quickly shows whether and how close is the plugin to the analog equipment.


There are many tricks that are not only exquisitely reveal the full potential inherent in these types of equalizers, but also an effective way to expose everything that we do not usually see at the first contact with emulations of analog equipment.

​One of the best known and most widely used techniques for testing the fidelity of the "global passive" structure is the "Low end trick". For those who do not know what the low end trick is I explain...


it is a technique that allows to obtain a powerful, focused and at the same time warm bottom end.  


So, I take my kick track and significantly boost 30Hz (depending on the type of the kick we can also boost around 60Hz).

Then I cut the same frequency to the point where I got nice clean and powerful sound of the kick.

Thanks to this operation is generated very musical curve (depending on whether you choose 30 or 60Hz conquering the surrounding 65-80Hz change the range in which the formation of the curve).

Then I went to the M5, where I cut the frequency at 200Hz (in the case of rates depending on the type of kick you can cut 200 and 500Hz). In this way I received a powerful and very selective kick constituting a strong and well set on the foundation of my mix, which sounds just like achieved through the hardware!


So the first exam passed, but there's more...

PURPLE had coped well also with the technique used by the legendary Motown engineers (this is the secret why the artists of Motown sounded so good!), who kept used the passive tube equalizers not only for treating the main vocals, but also to cut the unnecessary frequency of all the instruments that make up the weaves known as the backing track.

​Also in this case, I received a true analog resulted in not only a beautifully polished lead vocals, but the full space of the second plan allows better placing the vocals in the mix.

Naturally this above techniques are only a small part of the possibilities offered by PURPLE. There are plenty of other techniques to acquire a beautiful snare, a great bass sounds or warm and silk overheads (where PURPLE simply has no equal). ​I encourage you to download the manual where you can find a lot of "real" recipes for a great range of tasty dishes, of course with P1 and M5 equalizers!


​Conducted my tests (which also by the lack of space is not described in detail here) led me to believe that PURPLE is not only very successful product, but even an indispensable one, especially for all those who have treated their adventure with the production of audio seriously. Both equalizers sound delicious. It is easy to forget that we are dealing only with plugins.



So, is PURPLE has some real weaknesses? Well, probably not really... weaknesses in the strict sense, in principle there is no.

With this package is just as in the case of the old tube analog equipment. Some love it, others will always find arguments to choose something else. The first thought immediately runs toward the hardware requirements, which despite engine optimization is still a lot. Well you have to simply prepare for this. The technology used by Acustica Audio simply requires that approach. As usual in life, something for something.​

Latency, especially on some weaker machines it can effectively limit our ability to use PURPLE during tracking.

However, if you have a decent computer, you will not encounter a greater problem...

PURPLE is really hard an analog thing, which despite appearances is not all may be his advantage. 


What does it mean?


Both equalizers included in the bundle add to the signal some artifacts that (what is worth emphasizing) are also integral and indivisible component of the sound of real physical construction of this type.

​If you know this type of equalizers and you know what you can expect from them, you'll probably be delighted however,

if you care primarily on a transparent and surgical type of EQ, then both the color and the sound character offered by PURPLE may be too intense. So weaknesses in the full sense of the word PURPLE rather not have.

Of course, as the digital versions of this type of equalizers in our market there is no shortage, and as they say, always appetite grows with eating. Certainly there will be those who have PURPLE expect will be more...


If we've talking about this...


A great bonus would be the opportunity to choose from the different sound characteristics allow us to obtain the color of the different models of this classic equalizer (Danish style, modern American style, etc...). Small switch contains the old or new values, certainly would affect the position of the plug on the market.

Admittedly, as is usual in the case of Acustica Audio, both equalizers have the ability to disable the preamp section and replace it with some some stuff from free Henry Olonga  MOJO programs however, this is not the kind of exit for anybody (those who do not have NEBULA are not able to run the libraries of Henry Olonga).

​In the realm of M5. He could be a bit stronger and sharper however, here the complaints come to an end...

​As I said a moment ago PURPLE is simply vintage and like a real analog can sometimes be (especially in the context of a more modern production) too vintage. Personally, I like such equalizers and I demand from them coloring.

​The sound of a real unit is very rich. A lot happens in it. The tubes working inside the hardware generates serval different nuances and artifacts extremely difficult to properly reproduce in the digital software domain. 

It's the kind of sound that despite almost instantly recognizable shape is never linear. Meanwhile, digital passive equalizers with varying degrees of success often go only a certain, often the most obvious to our ears part of its naturally full sound.​ PURPLE could successfully get out from under the wings of the Japanese, Americans, Englishmen or Germans (which are known from high quality products). So, what would happen domain aforementioned nations, it became the domain of the Italians, who have been consistently building its position and I can confidently say that it grow into one of the most important if not the most important place which are today the real power in the field of digital modeling. 

The only thing what's are really missing in PURPLE is the smell of real hardware gear, which always brings out the real physical analog devices. Of course you can always say that this is not a real analog, but only a structure in a more or less way based on the original diagram. And in a sense you were right, but I judge here primarily analog behavior and analog flavor of autonomous and totally original plugin which PURPLE actually is... 





​It is impossible to mention here all those who have used and still use real tube passive equalizers.

From jazz to rock and all other genres and subgenres... this style EQ works everywhere and for everyone.


There you have its motto, and you know what?


PURPLE by Acustica Audio works exactly in the same way!


No matter whether you are recording, mix or finalize the album. Whether you are in Los Angeles, New York, Sydney, London or Berlin, each of these cities today is the home of at least one passive tube EQ and PURPLE is a extremely successful software version of its original classic design. Version really faithful to the original! 

So before you check other commercially available incarnation of this timeless classic be sure to check out PURPLE from Acustica Audio. There's a good chance that this is  "the right" version that you are looking for...


By the way... 

if you do not imagine your mastering chain without a passive tube equalizer, I strongly recommend to check out "WHITE" (also from Acustica Audio), which is great, stereo-type, mastering PURPLE style EQ.




Go to: www.acustica-audio.com. There you will find everything you need.

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