High-quality active subwoofer design based on Hivi's Hi-FiRESEARCH 12-inch W12 woofer

As we all know, the bass is the basis of the music signal, which greatly affects the atmosphere of listening. The lack of bass signal sounds light and unreal, but in the regular home theater playback, the overweight low speaker is an important part. If there is less emphasis on the subwoofer, then completely lose the sense of presence, that is to say not true. Many enthusiasts generally use 6.5-8 inch woofer speakers. The lower frequency limit of these speakers is relatively low. Although the bass sounds powerful, the energy and extension ability are insufficient.

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Therefore, the author suggests that if there are conditions, it is better to use medium and large floor-standing boxes to get richer low-frequency response, and when building a home theater, the subwoofer should be taken into consideration. Of course, if the original system does not have rich low-frequency effects, you can also add a quality subwoofer to improve the replay effect. However, a better subwoofer is very expensive. Since we have the ability to design a bookshelf or a floor box, can we also make a better subwoofer? The answer is yes, interested. The reader may wish to follow my gourd painting.

The ideal concept of overweight subwoofer
Before production, we have a basic concept of what is a "gooder subwoofer". The author believes that there are several aspects to measuring the quality of a subwoofer.
1. A good overweight subwoofer must be active amplification. The so-called "active amplification" is built-in amplifier, while the passive subwoofer has no built-in amplifier. There is only a passive crossover in the box, which must be shared with the main speaker. Also equipped with power amplifier. The passive subwoofer uses the volume control of the front stage to determine the volume. If the sensitivity or volume of the subwoofer and the main speaker are not even, it will cause the sound field to be chaotic, the frequency response is unbalanced, and the sound image can not be positioned. At this point, the placement of the subwoofer can not solve this problem, and these problems are difficult to improve. In addition, the vibration quality of the subwoofer large-diameter unit is definitely greater than that of the main speaker unit, so the sounding speed is slower. After adding this subwoofer, the effect is often very turbid.
The active subwoofer is designed for bass replay. Its working characteristic is that the signal goes straight into the front stage with the source frequency division. The frequency below 100 Hz is amplified by a dedicated bass amplifier to drive the subwoofer. The frequency above 100 Hz is divided and sent to the amplifier, which is amplified and broadcast by the main speaker. There is a separate volume control to control the ratio of the subwoofer volume to the volume of the main speaker.
A regular add subwoofer is a subwoofer that operates below the crossover frequency (for example, 100 Hz or 120 Hz), while the main speaker works above the crossover frequency, but such a crossover should be placed after the source output. Before the main channel preamp, some advanced subwoofers have a pair of left and right channel output terminals, but in daily use, many people directly input the subwoofer directly from the preamp output.
From this point of view, the unit and internal magnetic circuit structure used by the active subwoofer, dedicated low-frequency boosting technology, as well as frequency-dividing amplifiers, cabinets, etc., serve low-frequency reproduction. Therefore, the performance of the active subwoofer is not comparable to that of passive speakers.

2. The ultra-low frequency sense should be sufficient, and the extension should be low enough. The function of the subwoofer is to make up for the low frequency of the main channel speaker.

3. The subwoofer unit can withstand high power without distortion. It is well known that the larger the area, the deeper the low frequency of the diaphragm, and in order to achieve sufficient energy, most of the subwoofer units are more than 10 inches. The larger the film, the higher the relative mass, and the movement of the unit is not easily controlled. In this case, a larger control energy is required and the input power is stronger. Otherwise, the movement of the diaphragm can not be banned, and the sound will be blurred. If the speed of the unit reaction cannot keep up with the speed of the music, the distortion of the low frequency band will be caused, the sound will be blurred, and the low frequency will be weakened. force.

4. There must be a reasonably sturdy box structure. The subwoofer is the most distorted frequency band in the speaker playback, but the low frequency band actually contains a lot of details and texture. The low frequency of the bass cello and the organ is not the same, it is not a bang. Blurring a blind "mighty" sense, the super bass with excellent performance can easily hear various ingenious changes such as the hall sound, details and sounds. The fast response subwoofer can reproduce the cohesive impact and effectively improve the analytical power. And the medium and high frequency bands are well connected. The length, width and height ratio of the cabinet and the sturdiness will directly affect the subtle bass's detail definition, analysis power, speed sense, transient response, etc., so you can't wait for it.

Material design and production In view of the above points, this DIY active overweight low-range speaker will be carefully selected and carefully designed to achieve the effect of high-quality commercial machines.
First of all, the woofer we chose is the Hi-FiRESEARCH 12-inch W12 woofer (see Figure 1). It features high loss, high compliance, ultra-thin, fatigue-resistant rubber folding ring and German KEVLAR reinforced paper-based composite diaphragm with high heat dissipation coating, no eddy current loss, high load-bearing power of 76 mm voice coil and High-temperature SV line and integrated high-density aluminum frame, its symmetrical magnetic field (SMD) drive system can reduce the mutual modulation of voice coil inductance and back EMF, and the ultra-long stroke linear displacement design makes it bear large power. Fo (resonant frequency) and Qts (quality factor, total Q value) are low, transient is good, low frequency response is excellent, bass is full and powerful, high definition and resolution, dynamic and low distortion are suitable for Source subwoofer.
The main relevant parameters of the W12 woofer are: rated impedance 8Ω; resonant frequency (fo) 28 Hz; rated power 150 W (maximum 300w); sensitivity (2.83V, 1m) 90 dB; total Q value 0.42; vibration quality 89.7 g; equivalent volume (Vas) 156 L. Figure 2 is a structural view of its structure.

After selecting the unit, you should consider the power of the woofer. In order to control the movement of the woofer well, I believe that the power output of the power amplifier should not be less than the 150 W rated power of the woofer, but it should not be greater than its maximum 300W. Input power, so the power amplifier output power used should be around 200 W (8 Ω). For a typical tens of square meters of listening rooms, this power is sufficient if the design is reasonable.
The power amplifier for the subwoofer has not only a large driving force, but also a small transient distortion and a fast response speed. The constant current power amplifier driver used here uses a linear component to feed the current sample flowing through the speaker voice coil to the input end of the power amplifier, so that the amplifier drives the load in a fixed current manner, so that the nonlinear distortion or transient in the power amplifier is well solved. The problem that distortion cannot be balanced.
There are also many advantages to using a constant current power amplifier. First, the current that is transmitted to the speaker's voice coil is not affected by the speaker's impedance, which simplifies protection and improves reliability. Second, the feedback sampling voltage and the current flowing through the speaker's voice coil. In a linear relationship, there is no phase difference, which reduces the transient distortion inside the power amplifier. The transient distortion index of the system depends on the transient characteristics of the speaker; it is the load that is driven by the power amplifier with the output load characteristic. The strength and resolution of the sound, the reaction speed is also fast. Figure 3 shows the active servo power amplifier for overweight subwoofers. It is designed based on the relevant parameters of the W12 woofer. The circuit can output RMS power of about 200 W, and adopt negative feedback constant current design for upper and lower symmetry. This circuit is characterized in that negative feedback is respectively applied to the emitter of the input stage transistor, and is combined with the respective bias circuits. The upper and lower symmetrical lines have the advantage that the emission stage of the front stage is not easily mixed with noise, and the power amplifier has a high signal to noise ratio.
The design of the servo equalization phase and volume control circuit is that the signal output from the sound source or the front stage is sent to the input terminal of the operational amplifier IC1A via the L and R inputs, and the switches K1 and IC1B are used to control the positive and negative phases of the signal to make it The main speaker has a more correct phase match. The line consisting of the triple band switch K2 and IC1C is a set of four low frequency input cutoff points of about 40 Hz, 55 Hz, 75 Hz, and 100 Hz composed of a Butterworth third-order low-pass filter.

Figure 3 Active servo power amplifier for lifting subwoofer Figure 4 is the power supply diagram for the active servo power amplifier. Enthusiasts know that good power is not less than the line. In order to achieve better sound, the machine uses a R-type high-quality transformer of about 650 w. The leakage flux and efficiency of the R-type transformer are more common than the ring. The type or EI type transformer is better, the temperature rise is low, and the inside of the overweight low-range speaker with relatively complicated use environment is obviously superior to the ring type or EI type transformer.

The front and rear processing lines of the machine each have their own independent power supply, and the windings are separated. The power supply of the control circuit is powered by three-terminal voltage regulation, and the power supply of positive and negative 15V is supplied to IC1 and IC2. The power amplifier is used to reduce the power supply. Resistance, parallel output with 4 pairs of medium-capacity filter capacitors for faster response. In order to further reduce the mutual interference of the magnetic fields in the box, the entire active servo power amplifier is mounted in a 30 cm × 35 cm × 9.5 cm thick aluminum metal chassis, in which the heat sink is mounted on the outer surface of the cabinet to facilitate heat dissipation.
After obtaining the volume occupied by the active servo power amplifier, we can easily find the inner cavity volume of the overweight subwoofer. From the main relevant parameters of the W12 woofer, the unit is more suitable for the inverter box. In order to obtain a relatively flat rather than a rendered low frequency response, there is a better transient response and a good low frequency extension in the design cabinet. The time is based on the SC4 response design. The SC4 response design is a better choice in the inverter box design. It is characterized by a large cabinet, but it has a very low tuning frequency and relatively good transient response and low distortion output characteristics. Some readers asked why not adopt a better SBB4 response design. SBB4 is of course better, but after considering the relationship between the sound quality of the two designs and the size of the cabinet, we believe that the SC4 response is sufficient to meet the high level of listening requirements.
The inverter box designed with SC4 response is good, but it requires a lower Q value for the woofer. The total Q of the w12 woofer is 0.42, which basically meets this requirement. The overweight subwoofer is designed according to the look-up table. Table 1 contains the low Q value SC4 design data from 0.37 to 0.44. When designing SC4 response inverter boxes with different Q values, the reader can also use this data design.

According to Table 1, we can quickly find the internal volume of the overweight subwoofer.
Find the empty volume Vb in the box:
Vb= Vas÷α=156÷0.9113=171.2(L)
Find the tuning frequency of the box:
Fb= (fh÷fo )×fo=1×28=28(Hz)
Find the low frequency cutoff frequency of the speaker f3:
F3= (f3÷fo )×fo=0.937×28=26(Hz)
Find the length of the inverter tube L:
Lv = 2350Dv2 ÷ (fb2 × Vb) - 0.73Dv
Where Dv is the opening diameter of the inverted tube, in order to get a good linearity, here the value is 100mm, calculated, Lv ≈ 102.1 mm.
According to the formula of the volume formula of the box, the calculated values ​​are all net. In actual calculation, remember to add the volume of the built-in amplifier (about 10 liters) and the volume of the speaker (about 4 liters), and the volume of the reinforcement support can be set. At about 9 liters, the final volume of the cabinet is V=171.2+10+9+4=194.2 L. In actual production, the cabinet size can be calculated according to 195 L.
The height, width and depth ratio of the clearance inside the cabinet are closely related to the sound quality. If the size ratio is improperly selected, it may cause two or even three axial vibration frequencies to overlap, resulting in hard-to-eliminate standing wave interference, which makes the sound dyeing worse. . In order to reduce the unnecessary influence, the height, width and depth ratio of the box are 1.3:1:1.2. After calculation, the height, width and depth of the clearance inside the cabinet are 65 cm, 50 cm and 60 cm respectively. The box is made of 25 mm thick medium density fiberboard (MDF). To increase the rigidity of the box, a support plate is placed inside the box, and the front baffle is bonded with two 25 mm thick medium density fiberboard (MDF). Get up, to reduce the sound of the box resonance, get a clearer sound. Figure 5 is the production dimension drawing of the speaker. Please pay attention to the flat contact surface of each baffle when manufacturing. The bonding surface should be coated with wood glue to make the contact surface tight, and tighten with wood screws to ensure that the joint is tight and tight. . In order to balance the weight of the front and rear baffles, the active servo power amplifier is located on the rear baffle, and the heat sink is exposed. As can be seen from the rear baffle layout of the box, the inverted hole is open behind, that is, the box should not be placed too close to the back wall when placed.

Generally, the sound absorbing materials in the speaker are placed in the pasting method and the filling method. The inverted speaker is often used in the pasting method. The sound absorbing material of the box is placed on the wall of the box and covered with cotton cloth about 5 cm thick. Mineral wool. If you feel that the low frequency energy is too much, you can properly thicken the sound absorbing material. In this way, the total Q value (Qts) of the speaker is reduced, and the low frequency thickness can be changed to make the bass clearer and improve the transient response speed. In order to get a clear, high-resolution bass for the subwoofer, the box should be made of metal studs, preferably with copper feet.

Recommendations
In use, if the low-frequency input cut-off point is adjusted too high, the low frequency band overlaps with the main speaker too much, and the low frequency appears cloudy and dragging in some frequency bands, which will affect the speed and sharpness of the low frequency. On the contrary, when the low-frequency input cut-off point is adjusted too low, the low-frequency will also appear inconsistent, making people think that the main speaker and ultra-low speaker are "each work." Therefore, choosing a reasonable low-frequency input cut-off point will make the main speaker and ultra-low speaker achieve a smooth low-frequency transition, the low-frequency sense will increase but it will not feel too abrupt and rendering, so that the overall momentum is integrated. The general adjustment principle is that if the main speaker's -3dB low frequency lower limit is 6O Hz, the 55 Hz low frequency input cutoff point should be selected, and this can also be roughly adjusted. For example, the woofer of the main speaker is 4 to 5 inches, and the 75 Hz low frequency input cutoff point is optional; if the woofer is 6 to 7 inches, the 55 Hz low frequency input cutoff point or lower is selected.
Do not adjust the volume of the subwoofer too large (preferably no more than 10 o'clock) to prevent the subwoofer from being overwhelming and reducing distortion. When adjusting the phase, first put a piece of low-frequency music, then set the phase to 0° and 180° respectively. Compare the sense of low frequency at this time, and it is better to have a full volume, clear sound and cohesion. When adjusted, the subwoofer is integrated into the entire system and you will feel its power.
It takes about 2,500 yuan to make an active ultra-low speaker like this, but its sound is comparable to the price of a commodity box, and it is real. If you have a friend in need, you may wish to do your own food.