Feiao K5 Measurement and Re-friction Machine

in the previous article, we demonstrated for the first time the dismantling of Feiao K5 and briefly introduced the circuit design of K5. The K5 provided by our friend Vulcan Chite has been returned after the motorcycle. We immediately got the K5 sent by our friend Servant Archer. So the measurement mentioned in the previous article will be carried out in this article, and this K5 change will be shared.

measurement

At first, we guessed from the sense of hearing that the peripheral circuit of the TPA6120A2 might be designed unreasonably, resulting in cross-crossing distortion. However, this test proved K5's innocence, and we were shocked by K5 's excellent performance from the test performance.

In the test, in order to ensure the performance of K5, we did not use the original switching power adapter, but used our own linear power supply box for power supply. All test wires are Japanese Canare, and the connectors are Yongsheng.

test signal source is the FG-100 DDS function synthesizer of Beijing EZM Studio. The waveform output by DDS synthesis technology is very accurate and perfect, which is better than the signal source that uses the oscillator to generate the waveform at the same price or even more expensive. EZM's products are exported to foreign countries and have become a powerful tool for EE at home and abroad. The designer is Mr. Zhang Mao, an instrument design engineer retired from the state-owned enterprise, and he is also a senior audio player.

In order to test the output of the ear amplifier, we specially made this switchable load with 2 sets of resistors inside, which can be switched between 32 Ω and 300 Ω, representing the performance of the ear amplifier under low resistance and high resistance loads respectively.

oscilloscope is a dual-channel 100M digital oscilloscope DS1102E by RIGOL Puyuan Jingdian. The left and right channels are respectively input into one channel for measurement.

First of all, we carry out a sine wave test of 1kHz, with a load impedance of 32 Ω, a high gain gear and a maximum undistorted input.

in the waveform, we see that the performance of the sine wave is very perfect, there is no cross-over distortion that we previously guessed, and there is no obvious distortion.

high gain gear maximum undistorted output Vpp = 15.0V. After the waveform is enlarged, it is still quite perfect and no defects are found.

Next, let's look at the performance of other gears:

gain gear, the maximum undistorted output Vpp = 5.6V, which is nearly 3 times different from the high gain. It can be said that the volume will increase sharply when switching from the gain to the high gain. Please adjust the volume carefully when switching gears.

low gain gear, Vpp = 2.6V, less than half smaller than medium gain gear.

we arrange the waveforms of Senior high school low third gain horizontally, we can feel them intuitively.

Next, we will perform a square wave test. Through the square wave test, we can intuitively understand the frequency response range of the amplifier, the rising rate, the presence or absence of overshoot, ringing and other undesirable phenomena caused by unreasonable design. First we enter a 1kHz square wave:

very satisfactory square wave waveform, almost completely straight, rising speed is quite fast (according to Texas Instruments official Data-sheet, the rising rate of TPA6120A2 reaches a surprising 2500V/μS), and there is no overshoot and ringing phenomenon at all.

amplification is also very satisfactory, which means that the design of the amplifier is very reasonable and accurate, and represents extremely high quality. Here is a comparison of the waveform performance of some four-digit earphones that we have tested before. Through comparison, we can find that the advantage of K5 is really too great.

but careful friends may point out how there are some small protrusions on the waveform. In fact, those protrusions are WIFI signals of the wireless router nearby induced from the 1-meter-long test wire. The signal waveform is canceled out through the A- B mode of the mathematical channel, leaving only the interference waveform, and the RFI signal waveform can be observed:

this also reminds us of the importance of shielding transmission cables in HIFI system. If a balanced transmission method is used, these interfering signals can be canceled to a large extent. This is also an important reason why balanced transmission is adopted in high-end and professional audio equipment.

20kHz square wave signal performance. You can see a little round. The rising edge also began to have a significant slope decrease. However, this is still much stronger than the performance of several earphones previously measured by the author. As the frequency rises, affected by the voltage change rate of the amplifier itself, the waveform amplified by the square wave will become more and more close to the sine wave waveform.

here, we continue to give the waveform performance of some four-digit earphones for comparison:

By comparison, I guess everyone should understand why we admire K5 so much, write ten thousand words for it so persistently, and spend hundreds of yuan to modify and add linear power supply. Because at the price of 698 yuan, this earpiece has already gone beyond the existence of the base to become a super-divine product. Its almost irrefutable waveform performance, appearance, function, and operation have surpassed some of the thousand-yuan-level earphones we recommended in the past. However, the following comparison will surprise everyone even more, and we were equally surprised when measuring.

100kHz square wave waveform is still so beautiful, which has far exceeded the frequency response range of human hearing and even the frequency range that the body can feel. What is the performance of some other earphones at 100kHz?

I can't help but want to put a facial expression bag.. Compared with K5, the ultrasonic performance of these earphones is simply the scene of a car accident, which is horrible.. Although we cannot demand that an audio equipment can well replay signals above 20kHz, the bandwidth limit of general audio equipment is also cut off at higher frequencies, and there should be a reasonable signal amplification effect at 100kHz. These amplification capabilities are likely to be directly related to the performance of high-frequency microdynamics. Next, let's look at how K5 performs at higher frequencies.

200kHz square wave is still so beautiful.. The speed against the weather is attributed to the ultra-high performance new generation operational amplifier OPA1642 and integrated ear amplifier TPA6120A2. In fact, we later tested the frequency of 500kHz or even higher, and K5 has not yet lost the array. If the frequency exceeds 0.2MHz, it can already be regarded as radio frequency, while K5 can still handle it. Is this the rhythm that can be used as a low-frequency radio frequency power amplifier.. But such a high frequency response range will also bring some side effects. In order to improve the signal-to-noise ratio in the audio range and prevent the intrusion of high-frequency useless signals and interference in the environment or front-end equipment, general audio equipment will set a low-pass filter, so that audio signals below 20kHz are almost completely Passed, and higher frequencies are bypassed to the ground. We can't see whether K5 has a design LPF on the PCB, but obviously there is no such effect from the frequency response. Therefore, we will add a pair of low-pass filter capacitors to the Line In port in the following reform. The square wave amplification effect of other earphones at 200kHz is probably as follows:

However, under such circumstances, we cannot criticize other earphones. After all, these earphones have been set up with reasonable LPF. Affected by LPF network, the signals of these ultrasonic frequencies will be strongly attenuated and compressed, and the performance in this frequency band is not representative.

we casually tried the performance of ultra-low frequency. When inputting a 1Hz sine wave, because the conversion frequency is too low, the protection circuit will think that there is a DC input, so each wave will trigger a protection, you can see that the blue halo of the panel turns red, and the relay crackles on and off. When you enter 2Hz, the protection will not be triggered. When we input 3Hz, we got very good waveform performance:

So it seems that the frequency response range of K5 has reached a super wide 3Hz-0.5MHz..

After measuring the waveform, we also carried out a high power test with heavy load. When the load impedance is 32 Ω or less, if the full voltage is output, great power will be generated, and the temperature of the TPA6120A2 and power supply circuit will quickly rise to a destructive high temperature. If the time exceeds 1 minute, the device may be burned. After conducting such a test for about 30 seconds, we already felt that the temperature was too terrible. We immediately switched the load impedance to 300 Ω, and the circuit stopped heating quickly and slowly returned to the normal temperature. It seems that if you push a large-size headset with low impedance and low sensitivity, such as HE6, or even AKG K1000, the circuit will generate strong heat, and the machine should be disassembled to add an effective radiator to the above heating components. In general, the temperature is normal, so no other heat dissipation measures are needed.

BTW, a detail not described above, there are many marked points on the back of the PCB:

these contacts are used for automatic or manual measurement using instruments on the production line, and the working voltage, waveform and other parameters at these points can be directly used to judge whether the circuit is working normally and whether it is qualified for quality inspection. The advanced automatic production line can also use the Pass/Fail detection system (Americans like to call Go/NO Go) combined with test instruments (including oscilloscope, desktop multimeter, desktop bridge, programmable power supply, programmable load, etc.) to automatically screen out unqualified products and let qualified products pass.

carefully observing the traces on the back, it can also be judged that the in-line devices on the circuit board are soldered by local wave soldering instead of manual soldering by other manufacturers. This is obviously a mass automatic production method. Among the domestic audio electronics brands, only companies like Feiao can support such large shipments and output. This also directly reduces the production cost of K5, which is an important reason why we can buy it at a low price of 698 yuan.

change

Next, I would like to share the plan of K5 in this example. First, as in the previous article, replace the main filter capacitor, regulator output decoupling capacitor and gain op amp decoupling capacitor with ELNA RE3 series.

and in order to better decoupling high-frequency response capability and micro-dynamic performance, filter the high-frequency noise of the regulator, and add a pair of EPCOS 0.1 μF thin film capacitors to decoupling:

Then, in order to improve the low-frequency coupling and low-frequency response, the capacity of the input coupling capacitor is increased. Different from the SILMIC stepless electrolytic capacitor in the previous example, this time we used the super-large redundant space of K5 chassis to insert two pieces of fermented bean curd: the same EPCOS 2.2 μF MPK capacitor as Big Lehmann:

you have also seen the Line In LPF low-pass filter capacitor mentioned above, a pair of 52pF mica capacitors:

In this way, the frequency of LPF attenuation will allow normal input to DSD-level broadband audio sources, while higher frequency interference will be attenuated and compressed to a certain extent. At this point, the motorcycle modification is basically completed. We re-fixed the plastic board, cleaned the flux and oxide remaining on the back of the PCB with detergent, and then re-installed it.

such a large main filter capacitor and two large fermented bean curd can be perfectly put in, you can feel the redundant space of K5 chassis. You can even put down a circuit board for a linear regulated power supply.

such a large redundancy, it has a lot of room for improvement and is more playable. At the same time, the overall appearance is still small and exquisite, which does not take up much desktop space. The design really makes us full of praise. These large capacitors have been added, and they feel much heavier and 233 than before after installation.

was changed, I went on the computer to listen to it. When it was first installed, the high frequency broke through the sky. However, it was much softer and normal to listen to it after going out for dinner. Later, I listened to it all night. The high frequency was quite normal. I don't quite understand what the reason is, but it seems that the new capacitor has not been charged and discharged, and the internal physical and chemical state may be slightly different from the used capacitor.

low frequency have been greatly improved. The low frequency is more normal. With a solid foundation, the listening is softer and more comfortable. There is no bad listening feeling mentioned in the previous article, and it is basically no longer as cold and hard as before. It should also have a good listening feeling with headphones as thin as AKG. The power is abundant, large dynamic and micro dynamic, and transient all have good performance. I believe you don't need any other gorgeous explanation after reading the above waveform performance.

, we once again appreciate and sincerely recommend Feiao K5. After basic modification and K5 with external linear power supply, it has greatly surpassed many four-digit desktop earphones on the market in terms of sound, appearance, function and operation.

Moreover, K5 does not pick headphones. Whether it is a 32 Ω load or a 300 Ω load, the waveform and output voltage we tested are not affected. After actual audition, whether it is portable water, rain VX, Aihua M38, rambler H840, innovative live2, iron triangle MSR7, Sony MDR-1A, Goethe Alice, large-size and high-power low-resistance moving coil pioneer SE-A1000, flat-panel earphone HE-400i, and high-resistance moving coils K701, HD600, HD650, HD800,DT990, DT880-600 Ω, all performed.

even with the cheapest entry player like Feiao X1, you can get a very satisfying sound. It is a desktop system solution that the poor can also consider. You can Plug and Play all kinds of earphones. It is very worry-free and is an impeccable balm within 1,000 yuan. If it is matched with X7, you can get better operation experience and sound. Friends who are still watching can start without hesitation.

Author

Rady, co-founder, senior consultant, designer and director of Ultrady studio of Shuiyueyu Technology. 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 serve as bar services.

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