Reading my reviews, it is easy to see that I have been following the products of the Kiev laboratory with interest for many years. - custom intra-channel monitors (the so-called headphones, made of acrylic on the cast of the ears of specific users, you can read more here). Increasing the complexity and quality of their products, the laboratory staff does not forget about the research work. Recently, they presented a completely new technology for correcting the phase and impedance in their headphones, I propose to understand what it is and why it is necessary.
I asked the representatives of the laboratory how their technology worked, and based on their stories I tried to write an explanation.
First we need to clearly understand how the headphones work. I once wrote a fairly large article on this topic, so I will summarize it briefly. The basis of almost all emitters used in headphones is the principle of conversion of electrical energy into mechanical energy using permanent and variable magnets. That is, an electrical signal passes through a conductor placed in a constant magnetic field, which causes the appearance of an alternating magnetic field around it. These two fields interact, and the conductor is driven into mechanical motion. This movement in one way or another is transformed into vibrations of air, which we perceive as sound.
To increase the strength of the magnetic field, the conductor is most often wound into a coil. Where there is an inductance coil, an interesting physical quantity called reactance comes into play. Unlike conventional resistor resistance, characterized by a voltage-to-current ratio, the resistance of reactive elements (inductors and capacitors) cannot be measured so easily. The usual resistance of an ideal coil tends to zero, the capacitor tends to infinity.
For such elements, the famous mathematician Heaviside in 1886 introduced the concept of complex (or total) resistance, also called impedance. Impedance, unlike resistance, varies depending on the frequency of the signal passing through an electrical circuit. Most often in the characteristics of the headphones indicate the impedance for a signal at 1 kHz.
The first conclusion: the headphones have a non-linear impedance, depending on the frequency.
While impedance varies with frequency, the same happens with phase. In this case, the “phase” is the time shift between the peak values of the applied voltage and current. The greater the phase shift, the greater the reactive component of the load. As a rule, the greater the phase delay, the more difficult it is to manage the load. A conventional resistor has an almost zero phase delay over the entire audio frequency range. That is, the voltage and current passing through the resistor are in phase, without a time shift between each other.
Dynamic headphones, as a rule, have almost linear impedance and phase curves. Here, for example, graphics for headphones Monster Turbine Pro Gold.
Many high-end on-board monitors use balanced anchor technology (Balanced Armature emitters). Due to the small size of the emitters, it is possible to use two, three or more (in some cases up to eight) emitters in one earpiece. By analogy with stationary speakers, there is a subwoofer, “midrange”, “tweeter” in one earpiece. The greater the number of speakers, the greater the number of frequency bands into which the entire audible frequency range is divided. Separation is carried out using electric filters (electric crossover), acoustic filters (acoustic crossover) and acoustic resonators (horns, reverse horns).
The armature emitters themselves have a rather “reactive” impedance curve and an uneven phase due to the large inductance of the coil and the permanent magnets in the internal structure. As an electric crossover, passive electric filters of the 1st and 2nd order are used based on capacitors, inductances and resistors. Because of this, the impedance curves and phases of such headphones can be anything.
Conclusion two: multi-head reinforcement headphones most often have non-linear impedance and phase graphs.
As a rule, the more uniform the impedance and electrical phase curves of headphones, the less the influence of the output impedance of the sources on the final frequency response of the headphones. Thus, due to the linearity of the electrical characteristics of the plug-in dynamic headphones, they are the least demanding of sources. Conversely, the more irregularities in the headphone impedance curve, the more the source output impedance affects their sound. The above graphs show rather large bursts of the impedances of different headphones, sometimes reaching 80-100Ω. As can be easily seen in the graphs, on the same sections of the frequency response there are delays between the change in voltage and current (phase). It is in these areas when imposing phase shifts, uneven impedance of the headphones and the output impedance of the amplifier due to its circuitry, changes in the final amplitude-frequency characteristics (AFC) of the headphones appear. Simply put, there will be a difference between how the headphones should sound and how they sound at a given source. This raises the problem of "inconsistency" of some multi-driver models with some amplifying paths. There is such an empirical "rule of one-eighth": the source output impedance should be no more than 1/8 of the headphone impedance. That is, for modern sensitive headphones with an impedance of 16Ω, the output impedance of the player should be less than 2Ω, and better still less.
Of course, now there are a lot of good players with output impedance, tending to zero and allowing such problems to be maximized, but there are a number of difficulties. First, close to zero impedance reduces the effect of unevenness, but does not remove it completely. Secondly, in some cases it is not possible to use such sources. For example, Ambient Acoustics very often make their monitors for musicians who use them with professional equipment, in which they do not always set themselves the goal of achieving low output impedance.
Let us return from theory to practice, and more precisely to Ambient Monitors. For several years, Ambient Acoustics gradually moved forward, improving their products, and trying to achieve their goals. Improvements were both external (casting quality, the use of different materials and inserts, the manufacture of our own connectors) and internal (using printed circuit boards for crossovers, maximum use of acoustic rather than electrical damping).
The next step was the desire to maximize the impact of monitors, regardless of the source used. The only way to achieve this was to ensure that the impedance and phase plots were as linear as possible.
The main task was to smooth the resonant peaks of the electric circuit of the driver system and electric crossovers in each model, and thereby reduce the influence of the internal circuitry of the sources on the final sound of the monitors, increasing the “naturalness” of the sound. The result of the annual work and "running in" was the introduction of phase correction technology and the impedance curve in all custom monitors.
Of course, the details of the implementation of the technology are not disclosed. Its main feature is that it practically does not affect the final frequency response of monitors, and “works” exclusively in the field of electrical parameters. The maximum "evenness" within 10 units (see figure below) of the entire line of monitors was achieved in the studio AM3 Reference Edition. For other models, this figure is in the range of 20-40 units (which is also very good).
In order to check how it sounds in practice, I gave my Ambient Monitors AM6 HiRez to modify. In addition to the correction, in the laboratory I was made a new design (white carbon with a blue case and silver inscriptions - they look just great), and also made the ends of the sound guides from soft acrylic. The latter is another new development of the company, it allows you to achieve greater comfort during long wearing.
Tests were conducted with several players, including: HiFiMan 901, Rio Karma, Sandisk Sansa Clip +. Also, headphones were tapped from iPhone 4, iPod Video 5.5G and Yulong DA8 DAC.
Changes are best heard at “simpler” sources, although they can be heard on top players. The bass has become more resilient, the sensation of the impact is stronger and more collected, it is especially well heard on the quality recordings of the drums (the same “Ondekoza” sound very cool). On simple sources, the brightness has significantly decreased, the sound has become not so sharp, but this is not at the expense of detail. The construction of a virtual scene was also better, the positioning of sound sources was just incredible (on high-quality recordings).
The fact that Ambient Monitor 6 HiRez began to work better with simple sound sources is difficult to underestimate. I know some people who like sound recorders, and they use them as portable players. Now you can use AA products with them. I may be mistaken, but, in my opinion, no one from the manufacturers of “custom” in the world offered anything of the kind, so buying the Ambient Monitors becomes even more justified for lovers of high-quality portable sound.
The article is based on materials .
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