High-quality active subwoofer design and implementation

As we all know, bass is the basis of music signals, which greatly affects the listening atmosphere. Without bass signals, the sound will appear light and unreal. In regular home theater playback, subwoofers are a very important component. Without the support of subwoofers, the sense of presence will be completely lost, which means it is unreal. Many audiophiles generally use speakers with 6.5-8 inch woofers. The low frequency limit of these speakers is relatively low. Although the bass sounds powerful, the energy and extension ability are insufficient.

Therefore, I suggest that if conditions permit, it is better to choose medium-to-large floor-standing speakers to get richer low-frequency response, and when building a home theater, subwoofers should be taken into consideration. Of course, if the original system does not have rich low-frequency effects, you can also add a high-quality subwoofer to improve the playback effect. However, a better subwoofer is expensive. Since we have the ability to design and make bookshelf speakers or floor-standing speakers ourselves, can we also make a better subwoofer ourselves? The answer is yes. Interested readers may wish to follow my example.

The concept of an ideal subwoofer
Before making it, we should have a basic concept of what a "better subwoofer" is. The author believes that there are several aspects to measure the quality of a subwoofer.
1. A good subwoofer must be actively amplified The
so-called "active amplification" means a built-in amplifier, while a passive subwoofer does not have a built-in amplifier. There is only a passive crossover in the box, which must be shared with the main speaker or equipped with an additional amplifier. The passive subwoofer uses the volume control of the pre-stage to determine the volume. If the sensitivity or volume of the subwoofer is not equal to that of the main speaker, it will cause confusion in the sound field, uneven frequency response, and poor sound and image positioning. At this time, the placement of the subwoofer cannot solve this problem, and these problems are difficult to improve. In addition, the vibration mass of the large-caliber subwoofer unit is definitely greater than that of the main speaker unit, so the sound speed is slower. After adding this subwoofer, the effect is often very muddy.
Active subwoofers are specially designed for bass playback. Its working characteristic is that the signal directly enters the preamplifier with active crossover. The frequency below 100 Hz is amplified by a dedicated bass amplifier and then drives the subwoofer. The frequency above 100 Hz is sent to the amplifier after crossover and broadcast by the main speaker after amplification. At this time, there should be an independent volume control to control the ratio of the subwoofer volume to the main speaker volume. The
formal addition of a subwoofer is that the subwoofer works below the crossover frequency (for example, 100 Hz or 120 Hz), and the main speaker works above the crossover frequency. However, such a crossover should be set after the signal source output and before the main channel preamplifier. Therefore, some advanced subwoofers are equipped with a pair of left and right channel output terminals, but in daily use, many people directly connect the preamplifier output to the subwoofer.
From this point of view, the unit and internal magnetic circuit structure, dedicated low-frequency enhancement technology, crossover amplifier, cabinet, etc. used by active subwoofers are all for low-frequency reproduction. Therefore, the performance of active subwoofers is not comparable to that of passive speakers.

2. The sub-bass volume should be sufficient and the extension should be low enough
The function of the sub-bass speaker is to make up for the lack of low frequency of the main channel speaker.

3. The sub-bass unit should be able to withstand high power without distortion As
we all know, the larger the diaphragm area, the deeper the low frequency dives. At the same time, in order to achieve sufficient energy, most sub-bass units are over 10 inches. The larger the diaphragm of the unit, the greater the relative mass will be. The movement of the unit is not easy to control. At this time, a larger control energy, that is, a stronger input power, is required. Otherwise, the movement of the diaphragm cannot be controlled, which will make the sound blurred. If the speed of the unit reaction cannot keep up with the speed of the music, it will cause distortion in the low frequency band, making the sound unclear, and at the same time make the low frequency weak and not powerful enough.

4. Have a well-designed and solid cabinet structure
The sub-bass is the frequency band with the most serious distortion in the speaker playback, but the low frequency band actually also contains many details and textures. For example, the low frequency of the double bass and the organ is different. It is definitely not a blind "powerful" feeling of booming and blurry. The excellent performance of the sub-bass can also easily hear the various clever changes in the hall sound, details and timbre. The fast-response sub-bass can reproduce the condensed impact and effectively improve the analytical power and connect well with the mid- and high-frequency bands. The length, width and height ratio and solidity of the cabinet will directly affect the detail clarity, analytical power, sense of speed, transient response, etc. of the sub-bass, so it should not be taken lightly.

Material selection, design and production
Based on the above points, this DIY active subwoofer will achieve the effect comparable to high-quality commercial machines through careful material selection, careful design and production.
First of all, the bass unit we selected is the Hi-FiRESEARCH 12-inch W12 bass unit (as shown in Figure 1). It is characterized by the use of high-loss, high-compliance, ultra-thin, fatigue-resistant rubber folding ring and German KEVLAR reinforced paper-based composite material diaphragm, with high heat dissipation coating, no eddy current loss, high-carrying power 76 mm voice coil and high-temperature resistant SV line and integrated high-density aluminum basin frame. Its symmetrical magnetic field (SMD) drive system can reduce the mutual modulation of voice coil inductance and back electromotive force, coupled with ultra-long stroke linear displacement design, so that it has the advantages of high power bearing, low fo (resonant frequency) and Qts (quality factor, total Q value), good transient, excellent low-frequency response, full and powerful bass, high clarity and resolution, large dynamics, and low distortion, which is very suitable for active subwoofers.
The main parameters of the W12 woofer are: rated impedance 8Ω; resonant frequency (fo) 28 Hz; rated power 150 W (maximum 300 W); sensitivity (2.83 V, 1 m) 90 dB; total Q value 0.42; vibration mass 89.7 g; equivalent volume (Vas) 156 L. Figure 2 is its structural dimensions.

After selecting the unit, the power to drive the woofer should be considered. In order to control the movement of the woofer well, the author believes that the power output of the power amplifier should not be less than the 150 W rated power of the woofer, but should not be greater than its maximum 300 W input power. Therefore, the output power of the power amplifier used should be around 200 W (8Ω). For a general listening room of dozens of square meters, this power is sufficient if the design is reasonable.
The power amplifier used for the subwoofer should not only have a large driving force, but also have small transient distortion and fast response speed. The constant current amplifier drive used here uses linear elements to sample the current flowing through the speaker voice coil and feed it back to the amplifier input, so that the amplifier drives the load in a fixed current mode, which is a good way to solve the problem of nonlinear distortion or transient distortion in the amplifier.
There are many advantages to using constant current amplifier drive. First, the current input to the speaker voice coil is not affected by the speaker impedance, which simplifies the protection and improves reliability; second, the feedback sampling voltage is linearly related to the current flowing through the speaker voice coil, and there is no phase difference, which reduces the transient distortion inside the amplifier. The transient distortion index of the system depends on the transient characteristics of the speaker; it uses an amplifier with output load characteristics to drive a load whose impedance changes with frequency, which will enhance the strength and resolution of the sound and has a fast response speed. Figure 3 is an active servo power amplifier for subwoofers. It is specially designed for sampling based on the relevant parameters of the W12 woofer. This circuit can output about 200 W of RMS power. It adopts negative feedback constant current design for upper and lower symmetry. The characteristic of this circuit is that negative feedback is added to the emitter of the input stage transistor respectively, and combined with the respective bias circuits to form an independent upper and lower symmetrical circuit. The advantage is that the emitter of the front stage is not easy to mix with noise, and the power amplifier has a high signal-to-noise ratio.
The design characteristics of the servo balanced phase and volume control circuit are that the signal output from the sound source or the front stage is sent to the input end of the operational amplifier IC1A through the L and R inputs. The switches K1 and IC1B are used to control the positive and negative phases of the signal so that it has a more correct phase match with the main speaker. The circuit composed of the triple band switch K2 and IC1C is composed of a Butterworth third-order low-pass filter with a total of 4 low-frequency input cutoff points of about 40 Hz, 55 Hz, 75 Hz, and 100 Hz.

Figure 3 Active servo power amplifier for lifting bass speakers
Figure 4 is a power supply diagram used for active servo power amplifiers. Audiophiles know that a good power amplifier spends as much effort on the power supply as on the circuit. In order to achieve better sound effects, the machine uses a high-quality R-type transformer of about 650w. The leakage magnetic flux and efficiency of the R-type transformer are better than those of the common toroidal or EI transformers, and the temperature rise is low. It is obviously superior to the toroidal or EI transformer when placed inside the subwoofer with a relatively complex use environment. The front and

rear processing circuits of the machine each have their own independent power supply, and the windings are separate. Among them, the power supply of the control circuit uses a three-terminal voltage regulator to output a positive and negative 15V power supply to IC1 and IC2. In order to reduce the internal resistance of the power supply, the rear power amplifier uses 4 pairs of medium-capacity filter capacitors in parallel to obtain a faster response speed. In order to further reduce the mutual interference of the magnetic field in the box, the entire active servo power amplifier is installed in a 30 cm×35 cm×9.5 cm thick aluminum metal chassis, in which the radiator is installed on the surface of the box to facilitate heat dissipation.
After obtaining the volume occupied by the active servo power amplifier, we can easily calculate the volume of the inner cavity of the subwoofer. From the main relevant parameters of the W12 woofer, this unit is more suitable for the bass reflex box. In order to obtain a relatively flat rather than rendered low-frequency response, a better transient response and good low-frequency extension, the SC4 response design is used when designing the box. The SC4 response design is a better choice in the bass reflex box design scheme. Its characteristic is that the box is relatively large, but it has a very low tuning frequency and relatively good transient response and low distortion output characteristics. Some readers may ask why the better SBB4 response design is not used? SBB4 is of course better, but after comprehensively considering the relationship between the sound quality of the two designs and the size of the box, we believe that the SC4 response is sufficient to meet the high-level listening requirements.
The bass reflex box designed with the SC4 response is good, but it requires the woofer to have a lower Q value. The total Q value of the w12 woofer is 0.42, which basically meets this requirement. The subwoofer is designed by looking up a table. Table 1 contains the design data of SC4 with low Q values ​​from 0.37 to 0.44. Readers can also use this data to design SC4 response bass boxes with different Q values.

According to Table 1, we can quickly find the internal volume of the subwoofer.
Calculate the empty volume Vb of the cabinet:
Vb= Vas÷α=156÷0.9113=171.2(L)
Calculate the tuning frequency of the cabinet:
fb= (fh÷fo)×fo=1×28=28(Hz)
Calculate the low-frequency cutoff frequency f3 of the speaker:
f3= (f3÷fo)×fo=0.937×28=26(Hz)
Calculate the length of the bass reflex pipe L:
Lv =2350Dv2÷(fb2×Vb)-0.73Dvwhere
Dv is the opening diameter of the bass reflex pipe. In order to obtain good linearity, the value here is 100mm. After calculation, Lv≈102.1 mm.
The values ​​calculated according to the box volume formula are all net volumes. When actually calculating, 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. In this way, the final volume of the box is V=171.2+10+9+4=194.2 L. In actual production, the box size can be calculated as 195 L.
The height, width, and depth ratio of the clearance inside the box is 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, and aggravating the sound coloration. In order to reduce unnecessary influences, the height, width, and depth ratio of this box is 1.3:1:1.2. After calculation, the height, width, and depth of the clearance inside the box are 65 cm, 50 cm, and 60 cm, respectively. The box is made of 25 mm thick medium-density fiberboard (MDF). In order to improve the rigidity of the box, a support plate is provided inside the box, and the front baffle is made of two
25 mm thick medium density fiberboard (MDF) is glued together to reduce the sound coloration caused by the resonance of the box board and obtain a clearer sound effect. Figure 5 is the production size diagram of the speaker. When making it, please note that the contact surface of each baffle should be processed straight, the joint surface should be coated with wood glue to make the contact surface close, and tightened with wood screws to ensure that the joint is tight and reliable without air leakage. In order to balance the weight of the front and rear baffles, the active servo power amplifier is set on the rear baffle, and the radiator is an exposed design. From the rear baffle layout diagram of the box, it can be seen that the inverted hole is opened at the back, that is, the box should not be placed too close to the back wall when it is positioned.

There are two general methods for placing sound-absorbing materials in speakers: pasting and filling. The inverted speaker often uses the pasting method. The amount of sound-absorbing material used in this box is to lay mineral wool wrapped in cotton cloth about 5 cm thick on the box wall. If you feel that the low-frequency energy is too much, the sound-absorbing material can be appropriately thickened. In this way, the total Q value (Qts) of the speaker decreases, which can change the low-frequency thickness, make the bass clearer, and improve the transient response speed. In order to obtain clear and high-resolution bass from the subwoofer, it is advisable to use metal nail feet, preferably copper cone feet.

Suggestions for use
During use, if the low-frequency input cutoff point is adjusted too high, it will overlap too much with the low-frequency band of the main speaker, and the low frequency will appear muddy and sluggish in some frequency bands, which will affect the speed and clarity of the low frequency. On the contrary, when the low-frequency input cutoff point is adjusted too low, the low frequency will also be incoherent, making people feel that the main speaker and the subwoofer are "working separately". Therefore, choosing a reasonable low-frequency input cutoff point will enable the main speaker and the subwoofer to achieve a smooth low-frequency transition, and the low-frequency volume will increase but will not feel too abrupt or rendered, making the overall momentum seamless. The general adjustment principle is
If the -3dB low-frequency lower limit of the main speaker is 6O Hz, it is advisable to select a 55 Hz low-frequency input cutoff point, and it can also be roughly adjusted in this way. If the woofer of the main speaker is 4 to 5 inches, a 75 Hz low-frequency input cutoff point can be selected; if the woofer is 6 to 7 inches, select a 55 Hz low-frequency input cutoff point or lower.
Don't adjust the volume of the subwoofer too loud (preferably not more than 10 o'clock) to prevent the subwoofer from being too dominant and reduce distortion. When adjusting the phase, first play a piece of music with a fuller low frequency, then set the phase at 0° and 180° respectively, and compare the volume of the low frequency at this time. The one with full volume, clear sound and cohesion is better. After the adjustment, the subwoofer has been integrated into the whole system, and you will feel its power.
It costs about 2,500 yuan to make an active subwoofer like this yourself, but its sound effect is comparable to that of a commodity box that costs twice as much, and it is real. Friends in need may wish to make it yourself to have enough food and clothing.