What constitutes those smooth sounding equalizers some are raving all about? In fact the answer is pretty much simple but maybe disenchanting to someone else: It’s in the equalizers transfer curve and (almost) nothing else. That equalizer transfer curve determines the actual frequency and phase response and generally speaking, in an analog filter model the frequency response implies the according phase response (and vice versa). In the digital domain this holds not true in general as shown by linear phase filter implementations. Additional effects like the actual transient response or additionally generated harmonics are then the icing on top (if desired) but may appear quite subtle or even negligible if we just look into rather transparent devices.
For now, if we just look at the transfer curve, a very simple rule applies: Smoother (broader) transfer curves equals in a smoother sound experience. To be clear, the term “smooth” does not imply any notion of “best” or “superior” at all. In fact there are audio domains where you won’t succeed with such an EQ and some obvious examples are room acoustic or hearing loss correction as well as audio restauration. “Smooth” in this case simply means a rather artifact free sound experience so that it might not be that obvious in the very first place that audio signal filtering has already been applied.
If someone is after such curves then looking into the mastering department is not a bad idea but there are lots alternatives out there as well of course. Just as an example in the figure above some plots are showing some basic characteristics of the recently appeared Dangerous Music BAX EQ. The device features low and high frequency shelving filters based on a Baxandall design. Note that they do not gain any higher or lower than a 6dB change in magnitude. Instead of the high shelf cutting the lowpass filter of the device is shown in the diagram (it also has a highpass filter which is omitted here) which offers a 12dB per octave curve with Butterworth characteristics. Some gentle and truly awesome sounding curves.
When speaking about smooth peaking/notching filters one has actually quite a number of options to choose from and some famous ones are coming from the mixing desk department. The Neve proportional EQ designs (e.g. as in the 1081) are a good example and an even softer curve design can be found in the Harrison console EQ. Another good example is the presence/absence filter of the Siemens w295b cassette (as shown in the diagram above) which had a lot of use in the german broadcast industry around 1970. Some people today are arguing that such curves are not focused or steep enough to properly balance a mix but countless great records are proving them wrong of course. It seems that the problem today is more the fact that not that much time, budget or attention is paid to the recording process itself anymore or to carefully select some proper sound sources (concerning quality and its fit into an arrangement). This is where people usually start complaining about that “phasy” sound of the equalizer they need afterwards to fix all that mess but I would rather name the game audio restauration and not mixing anymore. Believe me or not, but phasing isn’t the problem here.