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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Say a word about the poor squeaker. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Speakers Traditionally, the division of the midrange and treble bands (or midbass-treble) is produced by passive crossovers (crossovers). This is especially convenient when using ready-made component sets. However, while the performance of the crossovers is optimized for this kit, they are not always up to the task. An increase in voice coil inductance with frequency results in an increase in head impedance. Moreover, this inductance in the "average" midbass is 0,3-0,5 mH, and already at frequencies of 2-3 kHz, the impedance almost doubles. Therefore, when calculating passive crossovers, two approaches are used: they use the real value of the impedance at the crossover frequency in the calculations or introduce impedance stabilization circuits (Zobel compensators). A lot has already been written about this, so we will not repeat ourselves. Tweeters usually lack stabilizing chains. In this case, it is assumed that the operating frequency band is small (two or three octaves), and the inductance is insignificant (usually less than 0,1 mH). As a result, the increase in impedance is small. In extreme cases, the increase in impedance is compensated by a 5-10 ohm resistor connected in parallel with the tweeter. However, everything is not as simple as it seems at first glance, and even such a modest inductance leads to curious consequences. The problem lies in the fact that the tweeters work in conjunction with the high-pass filter. Regardless of the order, it has a capacitance connected in series with the tweeter, and it forms an oscillatory circuit with the inductance of the voice coil. The resonance frequency of the circuit is in the operating frequency band of the tweeter, and a "hump" appears on the frequency response, the magnitude of which depends on the quality factor of this circuit. As a result, coloration of the sound is inevitable. Recently, many models of high sensitivity tweeters (92 dB and higher) have appeared, the inductance of which reaches 0,25 mH. Therefore, the issue of matching the tweeter with a passive crossover becomes especially acute. The simulation environment Micro-Cap 6.0 was used for the analysis, but the same results can be obtained using other programs (Electronic WorkBench, for example). Only the most characteristic cases are given as illustrations, the rest of the recommendations are given at the end of the article in the form of conclusions. A simplified model of the tweeter was used in the calculations, taking into account only its inductance and active resistance. This simplification is quite acceptable, since the resonant impedance peak of most modern tweeters is small, and the frequency of the mechanical resonance of the moving system is outside the operating frequency band. We also take into account that the frequency response for sound pressure and the frequency response for electrical voltage are two big differences, as they say in Odessa. The interaction of the tweeter with the crossover is especially noticeable for first-order filters, which are typical for inexpensive models (Figure 1):
It can be seen that even with an inductance of 0,1 mH there is a pronounced peak in the frequency range of 7-10 kHz, which gives the sound a characteristic "crystal" color. Increasing the inductance shifts the resonant peak to lower frequencies and increases its quality factor, which leads to a noticeable "click". A side effect of an increase in the quality factor, which can be turned to good use, is an increase in the steepness of the frequency response. In the region of the crossover frequency, it is close to filters of the 2nd order, although at a large distance it returns to the original value for the 1st order (6 dB/octave). The introduction of a shunt resistor allows you to "tame" the hump on the frequency response, so that some EQ functions can also be assigned to the crossover. If the shunt is made on the basis of a variable resistor (or a set of resistors with a switch), then it is even possible to carry out operational adjustment of the frequency response within 6-10 dB. (picture 2):
However, first-order filters provide too little attenuation outside the operating band, so they are only suitable for low input power or a sufficiently high crossover frequency (7-10 kHz). Therefore, in most serious designs, filters of higher orders are used, from the second to the fourth. Consider the possibility of influencing the frequency response for second-order filters, as the most common ones. For clarity, a model with a large inductance is used. The same results are obtained with traditional tweeters, only the filter parameters and the degree of impact on the frequency response will be different. For tweeters with low inductance, a shunt is not required. The first way is to change the quality factor of the filter at a constant crossover frequency due to the ratio of the capacitance and inductance of the filter (Figure 3):
It is difficult to simultaneously change the capacitance and inductance in the crossover, so this method is inconvenient for quick adjustment. However, it is indispensable in cases where the required degree of correction is known in advance, at the design stage. The second way is to adjust the quality factor using a shunt (similar to the previously discussed method for a first-order filter). In this case, the initial quality factor of the crossover filter is selected high (Figure 4):
The third way is to introduce a resistor in series with the tweeter. This method is especially convenient for tweeters with an inductance over 100 mH. In this case, the total impedance of the "resistor-tweeter" circuit changes insignificantly during regulation, so the signal level practically does not change (Figure 5):
Conclusions
The proposed methods for frequency response control are also applicable to filters of higher orders, but since the number of "degrees of freedom" there increases, it is difficult to give specific recommendations in this case. An example of changing the frequency response of a third-order filter due to a shunt resistor is shown in Figure 6:
It can be seen that the frequency response takes on a different form, which significantly affects the timbre of the sound. By the way, about 20 years ago, many "home" three-four-way speakers had switchable frequency response "normal/crystal/chirp" ("smooth-crystal-chirping"). This was achieved by changing the level of the MF and HF bands. Switched attenuators are used in many crossovers, and in relation to the tweeter, they can be considered as a combination of series and parallel stabilizing circuits. Their impact on the resulting frequency response is difficult to predict, in this case it is more convenient to resort to modeling.
Figure 7 shows the diagram and frequency response of the third-order filter developed by the author for the Prology RX-20s and EX-20s tweeters. The design used capacitors K73-17 (2,2 μF, 63 V) and homemade inductors. To reduce active resistance, they are wound on ferrite rings. Core type unknown: outer diameter 15 mm, magnetic permeability of the order of 1000-2000. Therefore, the adjustment of the inductance was carried out using the F-4320 device. Each coil contains 13 turns of insulated wire 1 mm in diameter. The sound quality turned out to be much higher than the original one, and the regulation of the frequency response fully corresponded to the task. However, it should be noted that the filter turned out to be problematic: the input impedance has a pronounced minimum, and the amplifier protection may trip. Author: A.Shikhatov
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