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Elysia - XFilter 500 April, 2014 pan60: Let's chat about EQs. What a broad subject! It seems to me it is a broader topic then, let's say, preamps or even compressors. elysia: Oh, this is really a wide field, indeed. It's hard to find a start, as there are so many aspects to cover: Topologies (analog, digital, active, passive); Purposes (corrections, enhancements, placements of signals); or Situations (recording, mixing, mastering, playback, acoustics). But, what we can say, for certain, is that an EQ is one of the most universal tools we have in music production. It helps solve problems or adds creativity. Ideally, it does both at the same time. It also helps if we can understand that almost all music we hear is artificial. But, only in the best meaning of the word, as the vast majority of signals are being EQd before being published – as opposed to of being all 'natural'. This starts even earlier with the frequency response of mics, pickups, and so on. But, that is something for a different article, at another  time. The bottom line is that we want to, and often need to, change the way a signal is perceived or how much space it takes in the frequency spectrum. And bending its response is a very powerful way of achieving this. pan60: First off, let's talk about the different types of EQs. I mean, there is this basic separation between active and passive EQs. So, what's the deal with this? elysia: This separation refers to the actual filter section of a specific EQ. With very few exceptions, so-called passive EQs include active amplifier stages (tube or transistor-based) as well as compensating the loss in gain -- which their filter network causes. However, the actual filters, in such an EQ, use certain combinations of inductors, capacitors and resistors only for changing the frequency response. These are, of course, all passive components. Networks often only need a single amplifier stage for making up the lost gain. They are considered to sound very pure. Some of them are also popular for their somewhat special frequency curves, such as the historic Pultec design. Active EQs joined the game a little bit later due to the availability of components needed for developing more complex circuitry. Even though it is not always the case, they tend to be  more flexible than their passive counterparts. The next question would be - which one is better, right? The answer is the same as with the question “Tube or Solid State?”  it depends. And it depends on a lot of things, like your individual taste, the material you want to use the EQ on, what you want to achieve with it, and especially the specific product you are planning to use.I have listened to some very good passive EQs working sheer magic, while others were rather mediocre. And exactly the same is true for the many active EQs I experienced. Let's not make a religion out of this... pan60: What about the Q factor? Wide or narrow, fixed or adjustable, constant or proportional – what can you tell us about this magic letter? elysia: Well, first of all, the Q factor tells us how wide or narrow the peak of a bell filter is – a high Q value stands for a narrow peak, while lower values determine broader curves. Once this parameter becomes selectable, we have an influence on the range of frequencies around the chosen center frequency. In practical use, it often turns out that we like to boost with wider curves, as it is pleasant to the ear. On the other hand, cutting often needs more precision and therefore higher Qs. But, the actual implementation of this into an EQ really depends on what will be the main application. For example, some bass preamps follow the above and always boost with a low Q and cut with a narrow Q -- without giving the user any access. Yet, it works just fine. The mastering guys, though, often depend on having the choice of Q as flexible as possible and, therefore, selectable through wide ranges.These are just two examples, but they already demonstrate there is no perfect method to implement the choice of Q which would ideally meet the demands of all possible users at the same time. Sometimes you want to work fast and don't want to interrupt the creative process by fiddling around with your knobs. Other times you want to address certain problematic frequencies and really depend on precision and flexibility/complexity. The question is, if you have a constant Q (the 'broadness' of the curve remains the same, no matter how much you boost or cut) or a proportional Q (the curve becomes more narrow, the more you boost or cut) basically results from the topology of the filters. But, apart from all technological details, the most important requirement is that all individual filter curves also work in perfect tune with all other filters in a specific EQ at the same time.  However, this requires quite a bit of fine-tuning. We like the two selectable Q factors (0.5 and 1.0) on our EQs, as they are very musical and useful in a variety of different situations. This allows, the mid bands to work fast and efficiently, yet you still have the option to tweak their response according to your material.  pan60: Please explain phase issues when it comes to EQs, and how these can influence a signal. Is this a problem we have to live with, or can it be avoided or corrected? elysia: Oh boy, an answer to this question would fill a book! To start, almost all analog filters change the phase of a signal. That's also true of speaker chassis, pop filters for microphones and so on... so, I'd call it a natural phenomenon. Coming back to our subject of EQs, the question regarding how strong the phase is influenced depends on the topology and  parameters of the specific filter. On a side note, there is also a type of filter that only changes phase without influencing the frequency response: The allpass filter.The human ear does not detect much of a difference when a signal is being processed through an allpass. This is because the spectrum of harmonics is analyzed instead of phase relations between the individual harmonics. Phase starts to play an important role, especially when two individual signals are mixed. Just take the snare drum as an example. It is often recorded with one mic on the top and one on the bottom. If phase is not set correctly, in this case, very audible cancelations will be the result. And a coherent phase is also important when processing stereo signals. With some exceptions in the digital domain, most EQs are not really linear when it comes to phase... but, we should not worry about this too much. A lot of great sounding recordings have been made with them. ;-) pan60:  A very nice piece of gear, as expected. Elysia delivers, yet again, in a very cool 500 form product. The basis for more gear lust! The xfilter 500 would be a welcome addition to anyone’s gear collection. I have had it on a two buss mix for some time, now, running a number of mixes through -- and I am loving the results! A big thumbs up guys! If you are looking for a two buss EQ, don’t hesitate to check the xfilter 500 out :) .
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