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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Completion of the CD player. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Audio equipment The low-cost NAD 502 CD player has improved the analog node and separated the power supplies. Subjectively (according to the authors' information) in comparative tests, the modified player was in no way inferior to devices of a higher price category in terms of sound reproduction quality. One of the signs of our times is the revolutionary change in sound recording formats. Both in everyday life and in the professional field, digital technologies for recording and reproducing sound information are replacing analog ones. Along with the objective improvement of the electrical parameters (large dynamic and frequency ranges, etc.), digital technology made it possible to process and copy phonograms in a high-tech manner and ensured their long-term preservation. Almost every lover of high-quality sound recording now, among other sources, has a CD digital audio (hereinafter referred to as CD) player. However, with all the advantages of CDs, the most demanding part of listeners, who also have equipment of the middle price category, notices such unpleasant features of digital sound reproduction as unnaturalness, "coldness", "hardness" of sound, as well as fatigue during prolonged listening. Hard annoying sound is even called "digital". In this regard, some audiophiles point to the inherent defect of the CD format - low information density of the data stream at a sampling rate of 44,1 kHz and a bit depth of 16 bits. They are partly right... Meanwhile, it should be noted that the digital audio equipment of the highest price group (High End) has a rather high quality and natural sound even from the point of view of an experienced listener. Obviously, the hardware of such players most fully implements the capabilities of the popular format. Of course, the CD format is also not ideal, and it is being replaced by more advanced (and more expensive) ones - DVD audio, SACD. But it is possible to partially solve the problem of high-quality digital playback for a music lover now by upgrading (from English: upgrade) the equipment at his disposal for playing CDs. First, about the shortcomings of the design of the players. Digital sources of music programs have given rise to new types of distortions, most of which a person does not hear, and the rest either annoy or tire him, and conditionally inaudible distortions impoverish the timbre of the musical signal. Consider the most common factors in low-cost equipment that affect the occurrence of these distortions: 1. Unreliable (with dropouts) reading of information from the disk, which leads to interpolation of lost data and simplification of sound. An insufficiently rigid case, not protected from the penetration of external vibrations, further complicates reading. 2. Simplified digital-to-analog converter (DAC). 3. The output stage of the player with low overload capacity for input and output. 4. Mutual pickups in power circuits, pickups on the analog part from the digital part and pickups from the drive systems of the transport mechanism. 5. Simplified power supply. 6. Poor quality electronic components. Here are possible ways to eliminate the above disadvantages in the order they are listed. Vibration protection of the player case can be improved as follows. Attach a sheet of laminated chipboard, cut to the dimensions of the case, to the stamped base of the case from below with screws or self-tapping screws. Regular plastic legs must be replaced with self-made ones, consisting of glued discs of dense rubber 10 ... 15 mm thick with a diameter of 20 ... 25 mm and a cork 5 ... 7 mm thick with a diameter of 15 ... 20 mm (Fig. 1).
The body should be reinforced with a spacer on which the top cover will rest. Glue the top cover in two or three places with guerlain. More advanced readers are invited to adjust the transport mechanism in those players in which adjustment elements are provided for this. As a criterion for assessing the quality of the settings, it is proposed to take the error-free reading of a heavily scratched disk that used to fail. Already carrying out only these operations will give a quite noticeable improvement in sound quality. You can really improve the performance of even a modest DAC by supplying it with a separate +5 V power supply, preferably with a separate low-power mains transformer and a parametric stabilizer on discrete elements (an integrated regulator is not recommended for these purposes). The connection of the primary winding of this transformer to the mains wires through an anti-interference low-frequency filter is selected according to the minimum of audible interference (Fig. 2).
The most serious attention should be paid to the finalization of the output buffer stage, since the quality of sound reproduction to a large extent depends on its properties. It should be borne in mind that this very important node of the sound reproduction path is usually not subjected to total integration in one chip by the manufacturer, but is performed as a separate cascade on integrated op amps and discrete elements, so it is as accessible as possible for an upgrade or replacement as a whole. The output stage of the analog path of the players is usually built according to the traditional scheme, consisting of a low-pass filter and a buffer amplifier, which is necessary to reduce the output resistance of the low-pass filter and minimize losses and interference to the power amplifier. Often the output stage is made combined with a filter. A low-pass filter (usually of the second - fourth orders) is necessary in order to get rid of overtone interference with a sampling frequency of 44,1 kHz and its harmonics in the output audio signal. Higher-order filters do a good job of providing a steep (up to 24 dB per octave) drop in the frequency response beyond the audio range and have a linear frequency response in the frequency range of 20...20000 Hz. However, such filters introduce significant phase distortions into the signal, which is perceived by the ear as a decrease in the transparency of the sound, when a dull, "clamped" sound is formed. In addition, they require the selection of elements or the use of precise (and therefore expensive) components. For our purpose, we chose a second-order low-pass filter with a controlled cutoff steepness, combined with a power amplifier (Fig. 3).
Above the frequencies of the audio range, the filter provides a frequency response drop of 12 dB per octave, and the phase distortions that inevitably occur during filtering are minimized by ear with a cutoff slope control. From the point of view of simplicity, as well as ensuring the purity and transparency of the sound, such a solution seems to be the most acceptable. The output stage of the DA1 op amp and the power amplifier, which provides low distortion in a wide frequency band, operate in class A mode. This required an increase in the energy intensity of the power supply and heat sink for the output transistors. As a heat sink, you can use the metal case of the player, while the transistors are installed on it through insulating gaskets. It is also useful to power the analog output stage from a ±15 V power supply with a separate mains transformer, subject to the above conditions (see Fig. 2). The separation of network transformers and stabilizers is caused by the need for maximum decoupling of digital and analog power supply units. In the digital part of the player, there are signals of sufficiently high frequencies (their spectrum is up to several megahertz). High-frequency components penetrate the power circuits into the output stage, and as a result of intermodulation with the amplified signal, a kind of "digital" coloring of the sound is created. The same requirements also determine the separation of the power source for the servo drive of the transport mechanism. Peak values of currents in the servo drive can reach 2...3 A, and the induced interference in the common circuits has a very noticeable level. Moreover, the propagation of electromagnetic fields inside the player's case makes it necessary to shield the analog stages. The screen should be made of soft permalloy and connected to the body. It is desirable to supplement the power circuits of digital nodes with blocking ceramic capacitors with a capacity of 0,015 μF. Transformers and stabilizers must be selected with a margin of power, taking into account the peak values of the load current. For the same reason, capacitors with a capacity of 5000 ... 10000 uF are used in the power filters. In order to reduce costs, manufacturers often use low-quality electronic components in their equipment, which significantly affects the sound quality. Therefore, if there is a desire and opportunity, in all circuits of the signal path of your device, it is useful to replace the standard capacitors with capacitors of well-known audiophile brands (for example, Rubicon, Black Gate, Nichicon). Ceramic, lavsan, polypropylene capacitors, carbon resistors of well-known companies, high-frequency diodes should be used. The audible result of the upgrades is likely to exceed expectations even when comparing the upgraded unit to higher priced CD players. In the output stage of the filter, the resistor load (R8) can be eliminated and the quiescent current of transistors VT1, UT2 can be reduced to 20 ... 30 mA by increasing the resistance R7 to 22 ... 30 Ohms. An additional 75 Ohm resistor between the filter output and the output connector of the device will be useful to eliminate RF interference both from the PKD to the UMZCH, and from the outside to the OOS filter circuit. Authors: S.Demin, D.Senatorov, Orel
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