High quality bookshelf speaker DIY Friends who like to play audio always have strong hands, and I am no exception. Although there are a lot of finished equipment at home that makes me enjoyable, my hands are always a little "itchy" and I always want to make a good equipment by myself. And after a long time in the circle, there is always a fever. The first brother respectfully asks you to help you buy or make equipment. I like the latter the most, because it not only greatly satisfies your "vanity", but also fills your own. Hands on. This is not the case, Xiao Wang of the partition unit in the early stage came here to ask me to help him design a pair of thousands of speakers. Oh, under the premise that the funds are in place, of course I am full of promises. 1. Selection of units I have made several speakers with different units in the past, both domestic units and foreign units. I have accumulated some production experience. I feel that although the cost performance of the domestic unit is good, its performance is indeed comparable to that of foreign countries. There are some gaps in brand-name units, especially tweeters, so I still turn my attention to foreign brand-name units in the selection of units. Second, set the crossover point The high and low units of the two crossover speakers have a certain amount of knowledge. It is necessary to balance the direction of the unit, the distribution of power, and the consistency of the sound. Senior enthusiasts know that the piston vibration area of ​​the unit is usually limited to the middle frequency band of the rated frequency band. This frequency band is generally from a multiple of the resonance frequency of the unit to half of the upper limit frequency. The various distortions in this frequency band are minimum, and the crossover point The setting should be determined according to the rated frequency range of the unit used. Generally speaking, the crossover point of the two crossover speakers is mostly 2 ~ 4kHz, which requires the upper limit of the frequency response of the mid-bass unit as high as possible, and the lower limit of the tweeter unit is lower. The upper limit of the high frequency of the 5-inch mid-bass unit can generally reach about 5kHz. Because the frequency extension of the Vifa P13WH-00-08 unit is good and there is no obvious resonance peak, it is easy to select the tweeter. However, I think the lower limit of the frequency response of the tweeter is still as low as possible, because according to the principle that the mid-bass crossover point should be selected at half of the upper limit, the crossover point of the Vifa P13WH-00-08 unit should not exceed 3kHz. If the directivity should be considered to meet certain high fidelity requirements, the crossover frequency should not exceed 2.5kHz. Figure 5 is the final crossover network of this speaker, which consists of a low-pass filter (LPF) and a high-pass filter (HPF). The first-order (6dB / oct) frequency divider is divided into two types of 3dB and 6dB landing point cross type. This speaker adopts 3dB landing point cross type. Since the asymmetrical frequency divider should meet at -4.5dB, when using the formula of -3dB drop point to calculate the filter component value, the low frequency fc should be moved to 0.9fc accordingly. The fc of the high-pass filter should also be moved to 1.1fc accordingly. 3. Design and manufacture of the cabinet After selecting the unit and setting the frequency divider, the cabinet can be designed. Considering that there must be a certain low frequency energy, this box is designed as an inverter box. Here, the size of the cabinet and the size of the opening of the inverted hole are designed based on the Thiele / Small parameters of the unit. Finally, the volume of the unit and the sound absorbing material should be added, and the margin should be increased by 10%. The effect of the frequency converter element resistance on the Q value of the speaker. Therefore, it is better to design the inner volume of the cabinet to be larger and leave room for adjustment. Finally, add a layer of dense cotton wool sound-absorbing material with a thickness of about 20mm to the left, right, top and bottom of the cabinet, and the inner wall of the rear baffle to make the sense of low frequency moderate, so that the bass is clearer and the height changes instantly. 4. Conclusion The box can be used in an environment of about 15m, with a sensitivity of 88dB. It can be well promoted by a 20W amplifier or a transistor amplifier of about 50W. As far as my own sense of hearing is concerned, the analytical power, transients and dynamics are all good, the dyeing is very low, the sound is relatively smooth, clear and transparent, and the density of details is high.
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The room where Xiao Wang used to enjoy music is a little more than 13m2. He usually listens to some classical symphonies and jazz in addition to some cover "salute songs". Taking into account these factors and budget, I emphasized that one was that the room was a little smaller, and the other was that the budget was a little lower. If you want to make a sound under this premise, you can only make a bookshelf box of about 5 inches. This caliber and price of bookshelf boxes should be based on smooth and natural frequencies, and you can't force how thick the low frequency is. A face of reverence, I do n’t know if he really understood or did n’t understand, anyway, it ’s all up to me to deal with it. After I took over the task, I could only do it well and not lose his trust.
Select the bass unit first. In fact, there are many options for imported brand-name units of about 5 inches, such as H702 and H775 and H907 of Seas, Eton 5-880 / 25HEX of Eton of Germany, 8530K of ScanSpeak of Denmark, and Vifa of Denmark. Weifa) 's P13WH-00-08 and Germany's Visaton (Wisatong) W130S8Ω are among the choices, but considering the price, only Vifa's P13WH-00-08 and Visaton's W130S 8Ω are available. Considering that the preparation and dynamics of classical symphony are relatively large, if the unit's ability to withstand is not strong, the distortion will increase, so I decided to choose Vifa P13WH-00-08. This is an excellent unit. Although its reputation is not as great as the products I mentioned earlier, in fact its performance is not inferior, and its high cost performance also makes it appear in many high-end speakers.
In fact, the selection of Vifa P13WH-00-08 also has such considerations: choosing an excellent unit can guarantee a good foundation, and can also refer to the existing design scheme of the unit as much as possible to reduce the overall design difficulty. The third is its sensitivity. On, it is easier to match the amplifier.
The appearance of Vifa's P13WH-00-08 is shown in Figure 1. It is one of the factory's top 5-inch mid-bass units. Its caliber is 5.5 inches. It uses a powerful magnesium alloy basin holder. The magnet is thick and has a diameter of 92mm. It is almost equal to the size of its diaphragm. This is its One of the reasons why the sensitivity (88dB) is higher in the same caliber. Its diaphragm is polypropylene, equipped with a soft dust cap, and its magnetic pole has an air outlet to reduce the impact of air compression at the rear, so that the diaphragm maintains lower distortion.
The frequency response of P13WH-00-08 is straight and natural, and the mid-frequency response only drops smoothly at 12dB / Oct to 5kHz. Its off-axis response is also optimized, so simple frequency division can be used instead of frequency compensation circuits, not to mention 5 inches. The mid-bass unit's original split vibration is very small. The diaphragm of P13WH-00-08 is made very hard, and its damping is also very good, so its split vibration is smaller, the intermediate frequency is not as muddy as the typical 7, 8-inch polypropylene speakers, and the transient is excellent. In addition, the stroke of this unit is higher than that of other brands of the same caliber unit, that is to say, its dynamic is larger, and there is relatively little distortion when playing complex large dynamic classical music.
Figure 2 is a graph of the unit's amplitude-frequency response and impedance on an infinite baffle. It can be seen from the figure that the response is very straight, and the technical parameters of its related units are shown in the attached table. 
Figure 2 Vifa P13WH-00-08 amplitude-frequency response and impedance curve 
Some people abroad use the Vifa P13WH-00-08 unit to match the combination of Scan Speak's D2905 / 9300 tweeter, but this combination has exceeded the cost. The domestic market price of a Vifa P13WH-00-08 is about 380 yuan, while Scan ’s D2905 / 9300 tweeters are priced at about 600 yuan each. If you use this combination, Xiao Wang ’s cost of more than a thousand yuan is not enough, so You have to choose a tweeter with good performance and reasonable price.
Taking into account the price factor, this combination of tweeters used the British Morel (Magic Thunder) MDT-29 tweeter. Although this unit is only the cheapest Morel tweeter available on the market, it has a great power endurance, and other performance is also very good. Proud. Figure 3 Morel MDT-29 is a 28mm silk dome diaphragm unit made by hand coating with special materials. Its sound is soft and there is no lack of strength of the soft dome unit, the energy is also relatively rich, the density is superior, the listening Good feeling, has been used by high-end speaker manufacturers at home and abroad. Figure 4 is a diagram of its relevant parameters.
When the crossover point is selected at 2.5kHz or higher, it is conducive to playing part of the harmonics of the human voice and the instrument from the middle and low units, the consistency of the sound is good, and at the same time, the frequency response near the crossover point caused by the crossover And the distortion of the phase characteristics has relatively little effect on the sound quality. However, at this time, the lower limit of the treble should not be higher than 1250Hz. For example, the lower limit of the frequency response of the tweeter can be about 800 ~ 1000Hz.
From the frequency curve of the woofer in Vifa P13WH-00-08 in Figure 2, the rated bandwidth of the woofer is 60-5000Hz. From the relevant parameters of the Morel MDT-29 tweeter in Figure 4, the rated bandwidth is The 1/2 frequency of the frequency is exactly 2.5kHz, which has entered the vibration area of ​​the piston. The resonance frequency of Morel MDT-29 is 900Hz, and the factory recommends a minimum crossover point of 1800Hz. In this example, the 2.5kHz crossover point is nearly 3 times the lower limit frequency of the lower end, and it also enters the vibration area of ​​the piston with a large margin. In fact, the crossover point is set at 2.5kHz is ideal, so that both units can work in the piston vibration medicine, the combined frequency band distortion is less, to meet the requirements of high-fidelity speakers.
After the frequency dividing point is basically set at 2.5kHz, it is necessary to consider what type of attenuation slope frequency dividing method is used. Since the crossover point of 2.5kHz is far away from the upper end of the mid-bass piston vibration zone at 5kHz, in order to make the mid-frequency thicker and the mid-range more interesting, the low-frequency filter may consider the first-order -6dB / oct attenuation slope. The power endurance of Morel MDT-29 is up to 80W, and the 2.5kHz crossover point is also far away from its 900Hz resonance frequency. I originally wanted to use the first-order -6dB / oct attenuation slope as a high-pass filter, but considering the phase And linear adjustment, here we still set the attenuation slope of the high-pass filter to the second order -12dB / oct, which can also prevent too much low-frequency energy from entering the tweeter, and improve the clarity of the high-frequency.
Whether the unit sensitivity is consistent has an effect on the design of the crossover. Fortunately, the two units in this speaker have the same sensitivity. It is very easy to attenuate the sound pressure of a unit in the crossover. To ensure the balance of the sound pressure level of each frequency band, not only makes the design of the frequency division simpler, but also because the attenuation information of the high-frequency channel has no resistance, it can be completely retained.
Because the frequency divider is composed of asymmetric first-order low-pass filter and second-order high-pass filter. Their phase curves are not in different phases. The function phase curve of the high-pass part rotates to + 90 °, and the low-pass part rotates to -45 °. When the two units are connected in positive phase, the phase difference is 135 °. The sound at the crossover point cannot be Combined as a whole, there is a phase difference of -45 ° after the reverse connection of the treble, but this phase difference can be obtained by the + 45 ° phase difference generated when the high and low units are arranged on the same vertical plane of the front baffle Correction makes the system achieve a relatively linear phase sound effect. However, this type of synthesis requires that the crossover point be selected appropriately and accurately, and that the crossover point of the high and low frequency filters cross at -4.5dB, otherwise the synthesized frequency response will appear uplifted and degraded at the crossover point. 
The low pass L1 calculation formula is
L1 = 159R / fc≈159R / 0.9fc (mH)
Qualcomm L2 calculation formula is
L2 = 225R / fc≈225R / 1.1fc (mH)
C1 = 113000 / fcR≈113000 / 1.1fR (uF)
R in the formula is the nominal impedance of each unit. The nominal impedance of the two units in this box is 8Ω, so R = 8Ω, and finally calculated
L1 = 159 × 8/2500 × 0.9 = 0.565 (mH)
L2 = 255 × 8 / 1.1 × 2500 = 0.742 (mH)
C1 = 113000 / 1.1fcR = 113000 / 1.1 × 2500 × 8 = 5.14 (uF)
The load of the crossover network is a loudspeaker with a composite impedance characteristic including capacitance, resistance, and inductance. Its impedance changes with frequency. Therefore, the actual input impedance of the crossover network also changes with frequency. In this case, there will be offset and frequency characteristics of the crossover point, distortion of the phase characteristics and unstable power amplifier operation. To reduce these effects, an impedance compensation circuit should be added to the woofer to make it approximate a pure resistance equal to the rated impedance, reducing the amplitude frequency and phase frequency distortion at the crossover point. R1 and C2 in Figure 5 are the impedance compensation circuit. The values ​​of R1 and C2 can be calculated by the following formula
R1 = Zo (Ω)
C2 = Lbm / Re2 (F)
In the formula, Zo is the nominal impedance of the mid-bass unit, Lbm is the inductance of the voice coil (unit is uH), R is the DC resistance of the voice coil, R13 of Vifa P13WH-00-08 in this box = 5.7Ω, L = 0.7mH = 700uH. After calculation, R1 = Zo = 8Ω, C2 = 21.5uF.
Some components in Figure 5 have two values, and the component values ​​in parentheses are the values ​​actually adjusted. When making the frequency divider, pay attention to reduce the impedance of the high-end frequency, it is appropriate to use small-capacity capacitors in parallel to meet the requirements. The audio metallized polypropylene film high-speed capacitor for the C1 capacitor is sufficient, and the Benike electrodeless electrolytic capacitor for C2 is better. The L1 coil should not be wound with a small wire diameter, and it should be wound with an enameled wire with a diameter of 1.5 mm or more, to avoid the characteristic of the crossover point being deteriorated due to excessive DC impedance. Since L2 is not on the signal channel, the requirements can be lowered appropriately.
Vifa manufacturers recommend that the volume of P13WH-00-08 is 5L, the diameter of the inverted hole is 25.4mm, and the length is 38mm. At this time, the tuning frequency of the inverted hole is about 74Hz, and F3 is 80Hz. The recommended response is QB3, which is the worst of the flat expected responses, and the 25.4mm inner diameter of the inverted hole is too small. I reckon that the manufacturer's recommended value is designed according to the case of QL = 15.
I redesigned it. The reason why I did not list the related process of the design's content product and inverted hole here is to consider that the parameters obtained by the formula are very different from the actual production of the final adjustment, so there is no one Enumerate. To put it simply, the response adopted is SC4, which has a transient state better than QB3, and the content product value is 5.5L. The phase reversal hole is generally 10% to 40% of the cell diameter, but if the value is too small, the nonlinear distortion of the phase reversal tube will increase. At high dynamics, the ability to follow the output sound pressure will deteriorate, which means that the transient is low. Finally, the inner diameter of the inverted hole of this speaker is selected as 36mm, the length of the adjustment is 73mm, the tuning frequency at this time is 75Hz, the response of F3 is 75Hz under the condition of -3dB, for a 5 inch bookshelf box , Should not be considered low.
Some people say that due to the high air pressure in the closed box, it is easy to produce standing wave interference, which leads to sound staining and affects the clarity of low and medium frequencies. Therefore, the size ratio of the box is strictly required. I very much agree with this statement, but I think that the air pressure inside the inverter box is basically not much different from the condition of the closed box. It also requires a strict ratio of the size of the box.
The 5.5L internal volume mentioned above is the net volume, and the final volume of the cabinet should also include the volume occupied by the speaker unit, frequency divider, etc. Therefore, the actual volume of the cabinet can be 6L. The height, width and depth of the internal clearance of the cabinet are closely related to the sound quality. If the size ratio is not selected properly, it may cause two or even three axial vibration frequencies to overlap, resulting in standing wave interference that is difficult to eliminate, which makes the sound staining worse . In order to avoid the influence of standing waves, the size ratio of width (W), height (H) and depth (D) of this cabinet is 1: 2.1: 1.46, which is similar to 1: 2.1 that can eliminate standing wave interference: 1.5 size ratio.
After the volume and size ratio are determined, the height, width and depth of the box can be calculated. The calculated size should be kept to one decimal place to ensure that the error is not large. Figure 6 is the dimension drawing of the box. Except that the panel is made of 25mm thick medium density board, the other baffles are made of 18mm thick medium density board (MDF). The outer layer of the box is a light-colored veneer. The production of the cabinet is made by professionals in the Electrical City to ensure the processing technology and strength. 