Popularization of basic audio knowledge

With the development of multimedia audio-visual, speakers are an important part of audio equipment and their importance is gradually being recognized by everyone. However, think about whether you have budgeted 700 yuan for the sound card and only 100 or 200 yuan for the speakers when you build or upgrade your computer. This is probably a common thing for most people. However, in the scientific configuration of multimedia equipment, what price range of speakers meets your needs, how to understand and choose speakers, and identify counterfeit products, this is exactly the problem that this article will solve for you.

　　The structure of the speaker. Ordinary multimedia speakers are mainly composed of the following parts:

　　1. Shell. Common speakers are mainly made of wood or plastic (some professional speakers are cast and filled with cement, steel or sand). Wooden speakers are made of composite medium-high density boards, and the thickness should be more than 10mm. Compared with plastic speakers, they have better anti-resonance performance, and the speakers can withstand greater power and are not limited in size. Plastic speakers are relatively low in cost and are one-time molded products. They can be rich in design but are limited in size, relatively small, and can withstand relatively small speaker power, which is suitable for multimedia speakers. Poor-quality speakers are mainly caused by insufficient density of the density board, thin board, or brittle plastic with sand holes, which is easy to crack.

　　2. Power supply. The circuit in the speaker is a low-voltage circuit, so first you need a transformer that converts high voltage into low voltage, then use two or four diodes to convert AC into DC, and finally use capacitors to filter the voltage to make the output voltage smooth (in ordinary speakers, voltage regulator tubes are generally unnecessary). To be honest, these parts are not noticed by people, but their importance to the speaker is no less than that of the host power supply to your computer. The transformer is generally fixed at the bottom of the main speaker (this is also the reason why the main speaker is heavy), and it is required to have sufficient power output. Inferior products often cut corners here. Taking into account some loss and efficiency factors, it can be calculated that if the rated power of the transformer is 100W, the power of the amplifier chip it can actually drive smoothly must be below 45-40W, so by calculating the power relationship between the speaker transformer and the amplifier, it can also verify whether the actual rated power of the speaker can reach the nominal value. The rectifier and filter capacitors are both on the circuit board. The large filter capacitor (several thousand microfarads) should be an electrolytic capacitor, and the larger the better. A large capacitor or two medium-capacity capacitors in parallel can be used to achieve filtering, while small capacitors (less than a few tenths of a microfarad) are used to make up for the lack of high-frequency filtering by large filter capacitors. The rectifier part is generally not a big problem, but in order to reduce costs, the filter capacitors of inferior speakers are obviously insufficient, and some are even less than 2000 microfarads; and small capacitors are likely to be ignored by them.

　　3. Power amplifier part. This part consists of the pre-stage operational amplifier and the post-stage power amplifier. The operational amplifier only plays the role of voltage amplification. In order to prepare for power amplification, it amplifies the voltage amplitude of the input signal to above the minimum value required for power amplification. In addition to the frequency range and distortion, the most important requirement for it is that the amplification factor must be sufficient. For the power amplifier chip, it can be called the core of the speaker. The key lies in its rated power. According to the standard, the rated power of the marked speaker should not exceed the typical value of the power amplifier chip. If there is a higher part, it is the "thriftiness" of the inferior speaker.

　　4. Circuits for special sound effects and functions. This part is not available in all speakers. A speaker may have one or several of them, or none of them. This includes the digital-to-analog conversion circuit of the USB speaker, three-dimensional sound field processing chips such as SRS, APX, Spatializer 3D, etc., active electromechanical servo technology circuits and BBE high-definition plateau sound playback system technology circuits, etc. From a technical level, small workshops cannot make USB speakers, so manufacturers who can make USB speakers have a certain level of technology and should be trustworthy. From the perspective of three-dimensional sound field processing technology, all the chips used are ready-made chips from abroad, it is just a matter of circuit interface and position. What I want to say here is that some low-end speakers also have 3D sound field functions. Their 3D effects are not achieved by chips, but by an extremely simple feedback circuit. The effect can be imagined.

　　4. Speaker unit. Generally, wooden speakers and better plastic speakers use two-way frequency division technology, that is, two speakers, high and mid-range, are used to achieve sound playback in the entire frequency range; while some plastic speakers used as surround speakers on X.1 (X=2, 4 or 5) use full-band speakers, that is, one speaker is used to achieve sound playback in the entire sound range. Common mid-range speakers are mostly coated paper cones, ceramic paper cones, paper-based wool cones, loose-pressed cones, bulletproof cloth cones and PP cones. They have their own characteristics: paper cones have the advantages of simple and natural sound, low cost, easy to mix with other materials, and a combination of rigidity and flexibility. The disadvantages are poor moisture resistance and difficult to control consistency; bulletproof cloth cones have the advantages of wide frequency response and can withstand large power, which is the first choice for those who love strong bass, but because of its heavy material, it has poor playback effect on small and weak signals, and the effect of light music is not very good and the cost is high; wool woven cones are softer in texture and lighter in material. The vibration of the voice coil with the same energy will make its amplitude larger, so it has better performance for small and weak signals and perfect performance for soft music. The disadvantage is that the bass effect is not good, and the performance of rock music and marches is not satisfactory; PP (polypropylene) membrane has good toughness and consistency, low distortion, and good performance in all aspects, but the price is slightly higher; loose-pressed cones are mainly used in subwoofers, and the sound playback performance in the low-frequency band can be said to be a choice. The tweeters have silk membrane and PV membrane soft dome, which are used in mid-range and low-end speakers respectively. Since the speakers used in the multimedia field must be magnetically resistant, the design of the speakers adopts a double magnetic circuit, and a magnetic shield is added behind the speakers to prevent the leakage of magnetic lines. In order to save costs, the speakers of counterfeit speakers are likely not magnetically resistant!

　　5. About the design. Ordinary speakers can be divided into two types: bass reflex and sealed. A sealed speaker is a closed box with no openings inside the box connecting to the outside world; while a bass reflex has a cylindrical bass reflex tube installed on the front or back panel of the speaker to connect the inside and outside of the box. It works according to the principle of the Helmholtz resonator, and its advantages are high sensitivity, high power tolerance, and wide dynamic range. Since the sound waves on the back of the speaker can also be released from the bass reflex port, its sound efficiency is higher than that of a sealed box. The same speaker installed in a suitable bass reflex box will have a low-frequency sound pressure 3dB higher than that in a sealed box of the same volume, which means that it helps the performance of the low-frequency part of the speaker, so this makes the bass reflex box widely popular.

　　6. About subwoofers. For subwoofers, in order to ensure the performance of the speaker, the box must be strictly sealed, so a three-cavity design should be adopted, that is, there are two cavities behind the speaker. As shown in the picture, a foreign X.1 subwoofer adopts a two-cavity design of a single speaker, that is, there is only one cavity behind the speaker, plus a cavity in front of the speaker, so there are two cavities.

　　7. About flat-panel speakers. Several manufacturers, represented by Yasuda, have launched ultra-thin flat-panel speakers. It should be said that the advantage of this product lies in its versatility rather than the performance of the speaker. Because it is a flat panel, it is not as large as a traditional speaker, so its placement and installation are extremely flexible, and you can even stick it on the wall. However, in terms of performance, this product has no advantages at all, and its sound quality is even inferior to that of low-end wooden speakers, which is hard to praise. There is nothing magical about flat-panel speakers. Since it is a speaker, it must rely on the vibration of some substance to drive the vibration of air molecules and spread the sound. Here, this substance is a flat panel. What replaces the vibration of the flat panel? It is also a voice coil. In the final analysis, its principle is the same as that of ordinary speakers. It is achieved by the vibration of the voice coil to drive the vibration of the sound medium.

　　As a famous computer peripherals expert, Logitech's products such as mouse, keyboard, camera and even game controller are sought after by many computer users. In fact, multimedia speakers are also Logitech's "forte". However, although the previously launched Xuanyin and Shengmei series products are very distinctive in technology and design, they do not seem to have a great impact in the market, which makes computer users seem a little unfamiliar with Logitech speakers.

　　In 2001, Logitech spent 120 million US dollars to acquire Labtec, a famous audio brand in North America. At the same time, it also acquired Labtec's 20 years of unique audio patent technology. After nearly two years of research and absorption, Logitech finally released a revolutionary audio product in 2003 that combined the technologies of the two companies - the Z series.

　　The Z series uses many exclusive patented technologies such as the continuous vented band-pass bass design (Series Vented Band-Pass), and is also equipped with the most popular advanced technologies in the audio industry.

　　Logitech Z-680 speakers are positioned as high-end 5.1-channel speakers and are THX certified. Z-680 speakers come with data cables for DVD players, CDs, PS2, XBOX and PC 2, 4, 6-channel sound cards. Z-680 speakers have built-in Dolby Digital and DTS decoders and support 5.1 channels. As a rare 6-piece sound system that has passed THX certification, Z-680 is not only the flagship of the series, but also the leader of this type of system currently on the market.

　　In fact, the Z-680 is very special from the packaging, because it seems that no other multimedia speakers have such a large size and weight. It takes a lot of effort to take it out of the box. In fact, the Z-680 has exceeded the application scope of multimedia and is more positioned as an entry-level home theater home audio system. Its various functions are also aimed at multi-functional applications in the home.

　　The Z-680 consists of eight components, including a subwoofer, four satellite speakers, a center speaker, a music control center, and a remote control. The overall appearance is silver-gray, which looks very modern and is very suitable for modern home environments.

　　When it comes to Swans speakers, I believe everyone will first think of its classic M200. But that was more than two years ago. After two years of hard waiting, audiophiles finally got Swans' new near-field monitor-level speaker - T200a. The T in the product number represents the speaker series with a simple black and white non-resonant triangle (Tri) geometric structure, and a represents the active system.

　　The two main boxes of T200a are exactly the same. The shape breaks through tradition and is extremely avant-garde. The main colors are black and white, with sharp contrast and great visual impact (see the title picture). The appearance breaks through the traditional cubic shape. The front panel is a polyhedron composed of three triangles, which has the flavor of fashionable electrical appliances, so it is easier to open a breakthrough in the market of families and individual users. The front panels of the three sides of the entire speaker are black piano lacquer. A white panel is embedded in the middle of the front panel, and there is a round hole for the LED indicator below. When the speaker is working, a blue light is faintly glowing under the black mask, which not only decorates the speaker but also serves as a power indicator.

　　Speaker unit

　　The midrange unit of T200a is arranged at the top, while the tweeter is at the bottom. The midrange unit of T200a adopts the 5-inch M5N magnesium alloy long-stroke magnetic shielding speaker of Swans (see Figure 1), and the tweeter is the 25mm natural fiber braided membrane neodymium iron boron magnet tweeter of Swans.

　　The rated power of the M5N mid-bass unit is 35W, and the maximum power is 70W. Its diaphragm adopts an advanced aluminum-magnesium alloy integrated metal structure diaphragm, and the surface of the diaphragm is sprayed with a special damping layer, and the high-compliance support system has very good rigidity and dynamic stability. The M5N adopts a high-power, high-temperature resistant, eddy-current loss-free Kapton voice coil skeleton and a high-temperature resistant SV wire voice coil, a shielded magnetic circuit system and a long-stroke linear displacement design. The unique symmetrical magnetic field (SMD) driving technology of HiVi puts the voice coil in a symmetrical driving magnetic field, thereby obtaining a symmetrical and balanced driving force, which can significantly reduce the inductance of the voice coil to reduce the confrontation of the back electromotive force on the output stage, so that the control characteristics of the speaker are improved, the sound is fuller and more powerful, and the distortion is reduced.

　　The TM1A-T tweeter uses a top-grade German natural fiber spherical diaphragm, with a flat frequency response, high frequencies up to 20kHz, and a sweet and delicate tone. The unit uses a shielded high-performance NdFeB magnet, and the sandwich anti-magnetic structure can effectively suppress the leakage magnetic field and create a miniaturized linear uniform magnetic field. The TM1A-T uses an American liquid magnetic cooling high-temperature resistant aluminum alloy skeleton and a copper-clad aluminum wire voice coil, which has a large power bearing capacity and a strong sound-absorbing back cavity design, which reduces the nonlinear distortion of the unit near the resonant frequency.

　　Functional design and interface

　　In terms of functional design, besides the switch, T200a only provides the only adjustment of the total volume, but this is also a good thing for bookshelf monitor speakers. Because for a monitor-level speaker, any modification of the sound is redundant. T200a is quite professional in audio input. It has both balanced input and unbalanced input (see Figure 3). Our ordinary lotus head, that is, two-core RCA connector, belongs to the unbalanced input method, which is also the most commonly used connection method on our PC.

　　Design structure

　　T200a abandoned the design concept of main and sub-boxes in traditional multimedia speakers and adopted the design of a single-channel independent power amplifier. Therefore, the left and right boxes are exactly the same in appearance, box structure and internal circuit design. This can ensure that the sound characteristics of the two boxes are completely consistent, which is an absolute improvement. The T200a box is a phase-inverted design, with the phase-inverted port opening backwards and located below the heat sink on the rear plate. The box uses an inverted triangle that is slightly tilted backwards as the front sound baffle. The baffle has a small radiation area and a large internal space in the box, which ensures the interpretation effect of the low-frequency part in the design.

　　The cabinet of T200a is extremely heavy, and it feels twice as heavy as multimedia speakers of the same size. The cabinet is made of high-density board with a thickness of about 15mm, instead of the composite density board used in ordinary multimedia speakers. The cabinet manufacturing avoids the possibility of resonance caused by sound waves. In addition, there are a lot of attracting cotton inside the cabinet, which artificially increases the damping environment of the sound inside the cabinet, reduces the reverberation time of the cabinet, makes the mid-low frequency part cleaner, and makes the output of the mid-low frequency not feel dragging.

　　T200a has done a good job in sealing the speakers and the back panel. After removing the tweeter and midrange speakers, we can see that they are both equipped with rubber gaskets to strengthen the tightness between the speakers and the cabinet, eliminating the possibility of leakage. The designers of T200a have independently enclosed the tweeter in a small space, which cannot be directly connected to the cabinet. This avoids a lot of reflections of high-frequency sounds above 2kHz in the cabinet, ensuring the relatively clean sound quality of the mid- and low-frequency parts.

　　It is not difficult to see from the overall diagram of the back panel (see Figure 4) that the circuit part is composed of four parts from top to bottom: the front-stage operational amplifier, the post-stage power amplifier/rectifier filter, the transformer and the AC input.

　　The transformer used in T200a is a 75W, ordinary E-type transformer with dual +15V output terminals (see Figure 5). The single voltage +15V power supply is used in the whole circuit to avoid the asymmetry of the positive and negative power supply parameters and destroy the symmetry of the circuit, which can better achieve full balanced amplification. Generally speaking, positive and negative power supplies will have better performance in dynamics. The filter capacitors used in T200a are two electrolytic capacitors with a withstand voltage of 25V and a capacity of 10,000 microfarads (see Figure 6). This capacity of capacitor has never been seen on any multimedia speaker before. Generally, it is good for a multimedia speaker to use a filter capacitor of 4700 microfarads. After filtering, a rectifier tube is used for rectification, and then a part of the power is sent to the upper op amp circuit to provide power.

　　The circuit design of the power amplifier of T200a still follows the traditional clichés. The op amp stage still uses the ordinary 5532 (see Figure 7).

　　The volume control of the op amp part of T200a uses a common rotary carbon film potentiometer, and the casing is grounded, which can suppress some noise interference, which is commendable.

　　The power amplifier of T200a is driven by two LM3886TF bridged and mutually pulled. LM3886TF is a 68W mono high-performance power amplifier. Each LM3886 can provide 38W of power when Vcc is positive or negative 28V, but the power output of +15V voltage provided by the power supply is only about 20W. After mutual pulling, the output power can reach 40W in theory, which is naturally enough for the nominal 75W power of the transformer power supply. It can be seen that the designer has reserved enough power reserve in the design to prevent clipping distortion and overload distortion in large dynamics. However, insufficient Vcc voltage cannot make LM3886 work in the best state, which will more or less have some negative impact on the performance of the power amplifier.

　　As for the peripheral components of LM3886, it is recommended to use imported tantalum capacitors, CBB metallized non-inductive capacitors, five-color ring precision metal film resistors and other professional luxury-grade components, and use heat sinks to assist in heat dissipation. Here, T200a uses a considerable number of metallized non-inductive capacitors and metal film resistors. However, there are also a large number of ceramic capacitors, and there is only one polypropylene capacitor. Therefore, in terms of the selection of peripheral components, T200a also has a lot of room for enthusiasts to polish.

　　The frequency division part of T200a adopts the traditional LC circuit. The frequency division circuit of each unit is composed of one capacitor, one inductor and one resistor. The two frequency division circuits of the tweeter and the midrange unit are independent of each other.

　　Sound quality evaluation

　　After more than ten days of burn-in and audition, we have come to some conclusions. Overall, if we evaluate the T200a in the field of multimedia speakers, its sound quality performance is unprecedented. In the five-year history of multimedia speakers, no product has surpassed it, and even the M200 from two years ago is far behind it.

　　The sound quality and tone characteristics of T200a are just like its design positioning as a monitoring speaker. It is neither hot nor cold, and no special effects are incorporated to modify and carve the original color of the sound source. After running-in, the treble of T200a has been very pleasant to the ear, and there is no occasional metallic sound and burrs. The midrange is delicate and round, and the bass is solid and steady. In short, the uniformity of each range is so smooth and natural that it is almost impossible to find peaks and valleys. Its diving depth in the low-frequency part can make people feel insufficient. Some low-frequency details that can be heard with high-end 2.1 speakers are simplified a lot under the interpretation of T200a. However, the low frequency of T200a is very clean, without the feeling of chaos and turbidity. The resolution of T200a is still quite good, but the dynamics are not as good as expected. This is also restricted by the power output that does not put the power amplifier circuit in the best working state.

　　In short, this T200a is a product for high-end individual users and professional audio producers. It can also be regarded as an entry-level "professional-grade" monitoring speaker. As for its sound quality and timbre and the quoted price of 2,360 yuan, is it worth it? Only after users listen to it can they draw this conclusion.

　　The most famous speaker unit manufacturers are Dynaudio, Scan-speak, Focal, Audax, Seas, Morel, ETON and JBL. In addition, KEF, Diatone, B & W, Altec, LPG, Visaton, Thiel, Elac, MB and other manufacturers also produce excellent units.