Feiao K5 Dismantling Machine and Motorcycle Reform Exploration

飞傲K5拆机与摩改探索

At the Guangzhou International HiFi Headset Show in 2015, Feiao released the new flagship player X7 and the K5 desktop earpiece base used with the new X series. At the exhibition site, the author experienced it first. At that time, I felt that the design of X7 and K5 bases was very excellent, with pleasing appearance, satisfactory operation and good sound performance. In the exhibition hall, Feiao provided full-size headphones such as HD600, HD650, K701 and HD800, while K5 could push the above headphones in a decent way. Later, my friend asked about the price. When I learned that the price was only 698 yuan, I was even more impressed.

later, I had many opportunities to try K5, and I felt that my voice was a little less while I was talking about it. Out of strong curiosity about the inside of this black box, searching for pictures and posts on the Internet has found nothing. So we decisively collected a K5 for the crab eating experiment, and got the friendship of our friend Vulcan Qiante, so we have this article...

listening to comments

K5 is obtained, we connect X7, X1 and AUNE X1S to listen to and comment carefully. Through careful research, it is found that the response of low frequency is poor, the amount of low frequency is small, and the sound seems to be a little "discrete" feeling, like the feeling of binaural beat frequency. The performance of other aspects has reached the level of desktop products above 1,000 yuan.

so we decided to dismantle the machine and study it and make some changes. At first we naively thought that its interior was as simple as other hundreds of dollars of 6210-based earphones on the market, but when we started dismantling the machine, we found that we were very wrong..

first look at its appearance. The design is simple but avant-garde and pleasing to the eye. It is ahead of the design of domestic HiFi digital manufacturers. A cool red/blue halo at the volume knob indicates the working status of the earpiece (there is also a Failure warning function, which will be discussed later). The front panel does not have any screws. The DOCK on the top has 2 screws and the rear panel has 7 screws.


飞傲K5拆机与摩改探索

飞傲K5拆机与摩改探索

飞傲K5拆机与摩改探索

飞傲K5拆机与摩改探索




Dismantling Machine

we didn't know its internal structure at first, we first removed the 7 screws from the rear panel, including the screws at the four corners of the panel, 2 screws fixed on the plastic plate (to be seen next), and a reinforced RCA socket screw. After unscrewing it, it was found that the rear panel was tightly embedded in the chassis and could not be removed from the connector. We are still unable to open the back panel with all our strength, and we are worried that prying open with sharp tools will hurt the appearance of the earpiece (other people's things..) So I dare not act arbitrarily. Turning to other outlets.

we unscrew the 2 screws of the top DOCK socket, the entire DOCK achievement can be easily removed. Be careful with the wiring! There is a wire to attach the cable to the PCB of the USB socket, open the fixing piece of the cable socket, and then loosen one end of the wire to easily separate the cable.

after removing the DOCK assembly, we can extend two fingers into the hole..) Touch the rear panel, then push hard (more dirty..) The rear panel flew out in response. Now you can spy inside.

飞傲K5拆机与摩改探索

飞傲K5拆机与摩改探索



realized that the internal design of K5 was not simple! The PCB and a piece of plastic board are fixed to each other with screws, and the front of the plastic board is fixed to the front panel with extremely strong industrial glue.

飞傲K5拆机与摩改探索

I pulled out the volume knob and saw that the original cool halo was only provided by two two-color LEDs and formed by diffuse reflection through the grooves of the shell. I immediately felt that the design had restored my confidence in domestic products.. After unscrewing the nut, a piece of iron is welded on the PCB and buckled with the plastic plate. The disassembly machine at the back needs to take it off with a soldering iron.

According to the previous experience of disassembling and assembling the table, the PCB can be pulled out from the back as long as the nut of the potentiometer is unscrewed. However, PCB and plastic board are tightly attached to the front panel and cannot be pulled out from the back.. Misled by this fixed thinking, my brain did not turn around for a while. How to take out the PCB made us have a problem. We have been thinking about how to take off the front panel, but did not think of taking out the whole inner core from the front..

later, a hammock hook was deployed to hook the hole above the potentiometer and pull it gently. The panel was not removed, but the whole inner core was pulled out from the front. At that moment, I was overwhelmed..


飞傲K5拆机与摩改探索

飞傲K5拆机与摩改探索
飞傲K5拆机与摩改探索



can see the whole picture of her after pulling out from the front. She still held the pipa half-hidden, leaving only the last piece of plastic. The plastic board is fixed to the PCB by 6 screws, and they can be easily unscrewed.

飞傲K5拆机与摩改探索

took off the plastic board, she lay naked on the stage and was watched to our heart's content. We were quite surprised at this moment. The complexity of the circuit far exceeded our original guess. There are 13 large and small ICs. And there are many 100 μF solid capacitors, which are basically filter capacitors and IC decoupling capacitors.

Analysis and Reform

飞傲K5拆机与摩改探索

main filter capacitor and LM317/337 is 1000 μF aluminum electrolytic capacitor of Taiwan Fengbin CapXon. At first, we thought it would be very spicy chicken. After we disassembled it, we carried out the measurement of the bridge tester. The results were satisfactory. ESR and Vloss values were very low and were not worse than many imported capacitors. For example, the measured parameters of electrolytic capacitors in the middle and low end of ELNA, Rubycon, Nichicon, Japanese chemical and other manufacturers were not comparable to it. However, the stability and life span of long-term use are not easy to say. Therefore, we replaced it later, using one 2200 μF and two 1000 μF low internal resistance ELNA RE3 series blue robe electrolytic capacitors instead.

飞傲K5拆机与摩改探索

Because the volume of 2200 μF is relatively large, it is impossible to install it on the front, so it is welded to the back and needs to lie down before it can be put into the chassis. After lying down, use 3M foam double-sided adhesive tape to fix the adhesive pad.

1000 μF was originally replaced by Panasonic FC electrolytic capacitor with better parameters, but it was found that it was too tall to fit into the chassis at all, so the same ELNA RE3 had to be used instead, so the overall visual feeling was better and cool and poisonous.



飞傲K5拆机与摩改探索

飞傲K5拆机与摩改探索

飞傲K5拆机与摩改探索


DC15V single power supply comes in, it is converted into a dual power supply through a complex virtual ground circuit, and then a pair of LM317/337 produced by Texas Instruments are used for voltage reduction and voltage stabilization. I don't know why the thickness of the heat sink of the 317 and the 337 are different, and the appearance of the package is also different, which may come from two different producing areas.

飞傲K5拆机与摩改探索

飞傲K5拆机与摩改探索



earpiece. This value may be relatively small, and judging from experience is likely to be one of the reasons why the low frequency performance is not good enough.


飞傲K5拆机与摩改探索

飞傲K5拆机与摩改探索


we have connected a pair of ELNA's SILMIC stepless coupling capacitors with a capacity of 2.2 μF behind it to reduce the capacitive reactance of low frequency and cut-off frequency. At the same time, due to the existence of 1μF thin film capacitor, according to the characteristics of flat frequency response of thin film capacitor, the response capability of coupling high frequency after parallel electrolytic capacitor is also excellent.

飞傲K5拆机与摩改探索

Ear IC uses a widely recognized Texas Instruments TPA6120A2. It can realize high-performance ear amplifiers with simple peripheral circuits. The output power and quality are high. Current feedback type A and B, ultra-high bandwidth, high dynamic range and ultra-low noise. However, the peripheral circuits here may cause Class A and Class B cross-over distortion (just speculation) due to improper parameter setting and small quiescent current, making the sound feel like a bit of binaural beat frequency as described above, a bit discrete and lacking line sense. In this example, we did not dare to change the circuit design at will, so we did not measure and modify it.

, the lighter the load of Class A and Class A and Class B amplifiers, the higher the load impedance, the hotter it is. When the load impedance is heavy, it is not so hot when the load impedance is low. This is related to quiescent current. Friends who are familiar with the principle should know clearly. The PCB under this IC is designed with windows and openings for heat dissipation, but because the IC is not close to the PCB, the heat may not be effectively transferred to the PCB. The full-size earphone with power in the hand (the impedance is not more than 100 Ω) does not feel excessive heat, and a little temperature is acceptable. Since there is no high-impedance full-size earphone in the hand, it cannot be tested at a higher temperature, so I choose to believe that the original design has controlled the temperature within a reasonable range and does not attach heat sinks to it.

Since this K5 has been returned, the above sine wave waveform output test and high resistance load temperature test will be performed on the next K5 received in a few days.

飞傲K5拆机与摩改探索

at the front end of the TPA6120A2, the operational amplifier with voltage gain is OPA1642 of Texas Instruments' new generation Sound-Plus high-performance, JFET input audio operational amplifier, which is better than the classic fever operational amplifier OPA2604. At this point, everyone should already feel that K5 circuit materials are extremely conscientious at this price.. But things are not over yet, and the voltage gain op amp at Line Out port is also a OPA1642. Balanced output is also fierce, which will be introduced later.

In order to further improve the low-frequency response, we removed the decoupling solid-state capacitor used by the 1642 here with the ear gain and replaced it with an ELNA RE3 electrolytic capacitor with the same capacity of 100 μF/16V. With the same appearance as the filter capacitor, the visual integrity is better. Looking at it is poisonous, which is beneficial to increase the brain placebo (laughter).


飞傲K5拆机与摩改探索

飞傲K5拆机与摩改探索

飞傲K5拆机与摩改探索



look at the Line Out port again. The DOCK port audio output level of X7 is very low. It is said that it is only 1.3V RMS, which is far from the standard audio level of 2V RMS. Therefore, in order to match with other audio equipment, the output level needs to be gained. As we have mentioned above, the op amp at LO port is also a OPA1642.


飞傲K5拆机与摩改探索

飞傲K5拆机与摩改探索


next to the LO port, there is a pair of balanced output circuits. For the same reason, gain is required before output, so the balanced output circuit also has a pair of high-performance audio operational amplifiers. I don't know why I don't choose the same OPA1642 to increase purchases and lower prices, but instead choose the LME49724 of National Semiconductor.


飞傲K5拆机与摩改探索

飞傲K5拆机与摩改探索

飞傲K5拆机与摩改探索



next to the audio output circuit, there is also a monostable multivibrator, CD4098BM of Texas Instruments. According to the official function description of Texas Instruments, this thing may be used to delay the trigger relay and should be used to realize the function of earphone protection. When turning on the machine, it should be due to turning red light to blue and turning on the earphone output.

, during the oscilloscope test, we found that when we encountered bad conditions such as input DC and signal peak cutting, the halo would also be triggered by blue to red. From this point of view, the design of K5 is very perfect and has the style of a large factory, which is very appreciated.

飞傲K5拆机与摩改探索

look back at the power supply. After LM317/337, there is also an adjustable voltage regulator, the LM22676ADJ of National Semiconductor. It seems that it should be powered for 2 relays or other ICs. It's not easy.


飞傲K5拆机与摩改探索

飞傲K5拆机与摩改探索


K5 is quite complicated and precise for other thousand yuan internal stations. All kinds of IC, some even the model cannot be determined. Without detailed explanation, just the picture above, you can feel it yourself ~(BTW, the photos of the above and the following small parts are all taken with the help of a magnifying glass, and the macro lens gear ring is broken and still has not been repaired.)

飞傲K5拆机与摩改探索

飞傲K5拆机与摩改探索

飞傲K5拆机与摩改探索

飞傲K5拆机与摩改探索

飞傲K5拆机与摩改探索

飞傲K5拆机与摩改探索




power adapter

Finally, we must talk about the power adapter. It can be said that this problem is quite serious. The original K5 switching power adapter looks good in workmanship, but when testing with rated power resistance load, it measured a terrible ripple voltage. Continue to the above picture and feel it.


飞傲K5拆机与摩改探索

飞傲K5拆机与摩改探索

飞傲K5拆机与摩改探索



this ripple.. It is the largest and most outrageous of all switching power adapters we have measured.. I don't know how this passed the factory inspection.. Vpp actually reached 2.1V, with a frequency of 1.35Hz and a period of 740 milliseconds.

in the Feiao player circle, the linear power supply will be greatly improved. From an electronic point of view, all aspects of a well-designed switching power supply will not be lost to a linear power supply, so the author always refuted them before, thinking that there is no need to buy a linear power supply. But we also switched sides after this test. Not because the linear power supply is too good, but the original switching power adapter is too poor. From the principle of switching power supply, its ripple is large, because the performance of this switching power supply is not enough, but also want to have instantaneous large dynamic output response capability. But such a low ripple frequency indicates that its response speed may still be insufficient.

In this low-power application, linear power supply is a low-cost solution that is easy to realize good power supply. A high-quality transformer can obtain very smooth DC output by using LT108X series high-performance linear regulator after rectifier bridge rectification. LT1083 provides 8A current, 1084 provides 5A current, and 1085 provides 3A current at different prices, but all new products cost tens of dollars a piece, so a linear power box worth over 200 yuan is worth owning.

summary

K5 desktop earpiece base, this product makes us feel bright at the moment, and rekindles hope for the domestic design. Whether it is appearance, internal structure, circuit design and workmanship, it is satisfactory, but the sound is still a little unsatisfactory, and the performance of the power adapter is even more unsatisfactory.

Next, we will conduct a load output test in the next K5 motorcycle modification exploration to determine whether there is obvious cross-over distortion in the ear. And further try to modify the parameters and devices of the circuit to obtain better performance, making this dark horse, which only sells for 698 yuan, a more competitive choice within 1,000 yuan. Welcome everyone to continue to pay attention. Thank you for your support.

Author

Rady, co-founder, senior consultant, designer and director of Ultrady studio. Studying in NSI.GD,Embedded Technology & Application. 6 years of burning age, 8 years of exposure to electronics and audio technology. Areas of study include electronics and embedded technology, music and audio headphones, drones and models, photography and choreography. Baidu Harmonica Bar and Air Model Bar used to work.

飞傲K5拆机与摩改探索


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