In the system terminal of the audio family, it is the conversion of electrical energy to acoustic energy, and both speakers and headphones play such a role. Compared with speakers, headphones have many limitations. For example, although the bass is rich, it only blasts around the eardrums, without the shock of the speaker. In addition, there is a sound field, which seems to be always turning in the forehead and brain, and it is easy to fatigue after listening for a long time. But headphones also have their inherent advantages. The headphones are basically a lightweight diaphragm that covers the full frequency, and there is no crossover problem caused by most speakers consisting of multiple units. The frequency response curve is relatively straight and has excellent transients, which can easily capture the details in the music. There is also a relatively low price, the speakers should achieve the same sound and transient, the price does not know how many times to turn up. In addition, although it can only be enjoyed by one person, it can listen to music at a high volume without affecting others. Therefore, you can't afford high-priced speakers, and you are demanding sound quality, or the home environment is not limited to a good listening environment. Headphone fever is really a shortcut. In addition, it is undoubtedly helpful to exercise your own ability to identify sounds and form your own listening standards. two. The amp 6P9P made by 6P9P is a side-heated oxide cathode output pentode. The purpose written in the manual is wide-band power amplification, indicating that this tube is used more in the high-frequency field, but it is actually accessible. 6P9P is rarely used in all kinds of electronic tube machines. Some people have used this tube in audio amplifiers in recent years, but it is still rare. I have no previous experience in using this tube before, but this time I tried to use it in the HD600 amp, but the effect was satisfactory. three. Selection of components and debugging of the circuit The circuit of the whole machine is simple, and there are not many parts, but the quality and quality of spare parts are closely related to the sound quality, and they cannot be careless. The resistance used in the main amplifier circuit is mainly the carbon resistance that the author likes, including R1, R2, R3, R4, R6, R7. In the power supply: R8, R9, R10 and the cathode resistance of the power tube use the big red cannon. Features of high reliability. four. Listening evaluation After two consecutive weeks of uninterrupted warm-up, the amp can be put into use. The appearance is shown in Figure 4. The audition equipment is as follows: The sound source is the early CDO-701ES from SONY, the signal line is self-welded with Teflon oil dispersing line, the amp is the unit, and the headset is the author's HD600. Since this pair of headphones has been in use for some time, it has entered the state early and can objectively evaluate the performance of this amp. CDP-701ES is an early product. The price was up to 260,000 yen. Although it has been used for more than 20 years, it has now entered the twilight year, but it is still worthy of the top product of the year. It has excellent craftsmanship, gorgeous sound and complete performance. Today's product is one of the author's favorite collections, and the sound performance is also very good with HD600. Fives. Postscript Soon after the machine was ready, HD600's upgraded replacement product HD650 appeared, which is said to surpass HD600 in all aspects. The author endured it with patience, after all, the lack of concentration, bought the HD650. From the appearance point of view, these two brothers from the same door are almost from a model, very similar, but the connection HD650 is thick and the plug is also more delicate. In order to have an objective comparison result, the Kenwood band radio amplifier that was bought in the early years and now basically retired was carried out, tuned to a radio station, and the HD650 was plugged into its headphone jack. Turn the volume to a suitable position and let the HD650 work for more than a month without power interruption, which is comparable to the HD600 of the same brother. The HD600 has excellent analytical power and a bright tone, but the female voice is a bit overdone, and the HD650 is just right; the thickness of the HD650 is better than the HD600 when listening to male voices, but the air feeling of the HD650 is slightly insufficient when listening to jazz. Mid-range alcohol and dense are equivalent. The low frequency is the most obvious improvement of HD650. The texture and control are more improved than HI) 600, and the sense of urgency is better. The depth is still dominant in HD600. The excessively low frequency of HD600 makes it impact the music, highlights the underframe, and discounts the control. That is to say, with thickness and depth, it sacrifices a little effort.
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One. New to Sennheiser I like listening to headphones, especially high-quality monitor headphones. Among many brands, Sennheiser's monitor headphones are particularly fond of. As early as 1945, the German company Sennheiser successfully designed microphones and headphones with excellent performance, which were well received by the industry. No matter in terms of sound quality, quality, comfort and craftsmanship, Sennheiser has its own uniqueness. The OrpheHS headphone system it produces is almost the reference standard for the world's best headphones. I came to know this brand through audio newspapers and magazines in the early 1990s. I learned about the HD580, the "leader" of the dynamic headphones. After hearing the HD580 in the home of a local enthusiast, I was deeply shocked by the small size. After the huge scene. The silky treble is ethereal and ethereal like a fairyland; the dominance of the low frequency makes it impossible to imagine how the bursts of "qi waves" came out of these tiny headphones. What is most unforgettable is the truth of the sound, as if listening to the singer's singing and the musician's playing at zero distance. For the writer who had no real contact with high-end equipment at the time, and was happy with a bunch of earth cannons at home, this kind of sound is like Xianle, not from the world. Since then, the author has a preliminary understanding of what is really "good voice".
At that time, the price of HD580 was close to 3,000. For the author who had just graduated from work for a few years, it was a "huge sum", but it was precisely because of that listening experience that he had a goal of "struggle" in his heart. Later, I also knew that there was a more amazing HD600 behind this HD580, and since then I have a knot: when to cross the HD580 and directly own the HD600.
The changes in more than 10 years are huge. At that time, few people dared to imagine that they could own a private car in this life, and now this unimaginable thing has become or will become a reality in more and more families. Thanks to the country's growing prosperity and strength, our dreams one after another became a reality. Once upon a time, I suddenly realized that having a pair of HD600 is no longer a dream. I bought a pair of Sennheiser HD600 more than 3 years ago. Things have changed, and I have been exposed to good equipment. When I actually used the HD600, I was no longer excited when I listened to the HD580.
The circuit program is very simple, as shown in Figure 1. There are two levels of amplification before and after. The front is the SRPP circuit voltage amplification, and the latter is the single-ended output circuit. 
The first stage uses a 6N3 connected to SRPP line for voltage amplification. The SRPP circuit has small distortion and strong control, and the high and low ends have a good extension. It was popular in China more than 10 years ago, but due to its overtone sound, the sound tended to be bright, and the low and medium frequencies were thin and thin. In recent years, it has been greatly left out and has become a symbol of no taste. In fact, everything in the world has two sides. At that time, the audio predecessors transplanted the SRPP circuit from radio frequency to audio, which was due to its excellent mid-to-high frequency characteristics. Although the low frequency is a little thin, but it has good control, so it is listening. There is an autumn-like refreshment in it, and the thick and heavy HD600 can just be resolved in this refreshment. It can be said that using the SRPP circuit here is for the HD600 to get a balanced tone here.
Both 6N3 and 6N11 are used for high-frequency medium-μ tubes, which are both suitable for SRPP circuits. In comparison, 6N3 has a larger internal resistance and a smaller transconductance value than 6N11, but 6N11 has less gall odor in the sound. The SRPP circuit shows the taste of the transistor more, and the 6N3 charm is better in comparison, and the price is more advantageous, so 6N3 is selected for the voltage amplification stage of this machine.
The current value of this level is not very large, about 3.4mA, which is adjusted according to the characteristics of HD600 and the author's listening habits. The cathode 430Ω resistor of 6N3 does not have a parallel capacitor and introduces a little negative current feedback. Although a little gain is sacrificed, it is exchanged for a more stable working state. The voltage magnification of this stage is about 17.5, which is about 25 dB. The second stage uses a wide-band five-pole power tube 6P9P, connected to the triode state, there is no special purpose to use 6P9P, that is, there is a ready-made tube on hand, and I want to try it, and the characteristics of this tube are also good, should be able to meet The need for this amp. Since this 6P9P has few users, it is relatively unpopular and the price is very low.
The basic parameters of 6P9P are as follows: filament voltage 6.3V, filament current 0.65A, limit screen voltage 330V, limit second grid voltage 330V, limit screen power consumption 9W, transconductance 11.7mA / V. Since 6P9P is connected to a triode in this circuit, there are no ready-made parameters for the 6P9P in the triode state. We can calculate according to the I / u curve under the triode state. Refer to Figure 2. 
First calculate the transconductance value. The concept of transconductance is: under the condition that the screen voltage is fixed, the grid voltage changes by 1V, and the screen current changes. Taking the screen voltage of 160V as an example in the figure, assuming that the grid voltage changes from -6 to 0V, the screen current changes from 20mA to 110mA, so from the formula S = â–³ I / â–³ U We can calculate that S = (110-20) / [0-(-6)] = 15mA / V.
Then find the internal resistance of the transistor state 6P9P. The concept of internal resistance is the amount of change in screen voltage caused by the change in unit screen current under the condition of fixed grid voltage. Still referring to FIG. 2, it is assumed that the grid voltage is fixed at -3V, the screen current changes from 90mA to 40mA, and the screen voltage changes from 240V to 160V. According to the formula Ri = △ u / △ I, the internal resistance Ri = (240-120) / (0.09-0.02) = 1714.3 Ω can be calculated.
With the values ​​of transconductance and internal resistance, according to the formula μ = s × R, it is easy to find the amplification factor μ = 15 × 1.714 = 25.7.
The 6P9P in the power amplifier stage of this machine adopts the self-bias voltage form, which is the bias voltage form that the author likes to use when making the single-ended amplifier. Compared with a fixed bias, the self-biased sound is more mellow and more charming. Self-bias voltage is safer than power tube. Unlike fixed bias voltage, once the negative pressure goes wrong, the whole power tube burns red, and a new tube can be burned in a few minutes. However, the output power of the power amplifier tube operating under self-bias voltage will be less than the fixed bias voltage under the same conditions. In other words, its advantages are achieved by sacrificing efficiency. Fortunately, the power required by the headset is not large. For the headset, the earphone has a large power reserve here, and it does not care about the loss.
The cathode resistance of 6P9P is 400Ω, the quiescent operating current is 19.5mA, and the maximum screen current Ia = 39mA. It is estimated from the I / U curve in FIG. 2, when Ua = 0V, the saturation voltage drop is about 95V. It can be roughly calculated that the best load impedance of the power amplifier stage under the working voltage of the machine = (214-95) /0.0195=6102.6Ω, the output transformer value is 6.5kΩ, the output power under this state and this load is about 1.1W, so The power of the HD600 is huge, and the thrust is not a problem at all.
The selection of capacitors is also very particular. The only coupling capacitor C1 selects WEST-CAP axial oil-immersed capacitors. The sound performance is more comprehensive, with both analytical power and charm. The cathode capacitor of the power tube 6P9P uses the electrolytic capacitor of the MBI brand. This is a rare brand that is found in the flea market. It is reserved after comparing with several famous brand capacitors. The power filter capacitor C3 uses Nichicon from Japan, and C4 and C5 use Philips Dong hexagon.
The tube 6N3 uses the T grade produced in Beijing in 1980, and the 6P9P uses the 6Î 9C produced in Nanjing in 1960, which is the early domestic 6P9P. 5Z2P uses Liuzhou products from 1975. All the tubes are ready-made, and the market is also very popular. The power transformer and output transformer are designed by the author himself and customized by a well-known audio transformer manufacturer in China. The power transformer power is 150W, and the output transformer is wound with a core of 25.4mm and a stack thickness of 30mm. Its structure is shown in Figure 3, and the static parameters are as follows: primary side inductance 16.7H and leakage inductance 15.2mH. It is not difficult to calculate that the upper limit of high frequency is close to 70 kHz and the lower limit of low frequency is 62 Hz. Although the lower limit of the low frequency is relatively high, it is under full power. The amp uses 1/10 of the power to push the HD600 to have a very high sound pressure, which is usually not used at all. In such a low power state, the low frequency There is no such thing as a dive. 
The inductance in the power circuit uses the old products of Shanghai Radio No.27 Factory. I originally wanted to omit this inductance, but the result of the omission was a huge increase in hum, even if it was replaced with a resistance of hundreds of ohms, the single-ended circuit could not cancel out the hum, plus the high sensitivity of the headset, it was proposed to the power supply High demand.
Commissioning is also very simple. After the circuit welding is completed, first plug in 6N3 and 6P9P, turn on the power, observe whether the filament is normal, under the premise of normal filament, shut down and plug in 522P, and then measure the voltage at several key points in the circuit: main filter The capacitance is about 241V at the back end of the inductor; the voltage at the anode of the 6N3 upper tube is 208V; the voltage across the cathode resistance of 6N3 is 1.6V; the voltage at the cathode resistance of 6P9P is about 7.8V. Due to the discrete nature of the power tube, try to use a few more cathode voltages. 
The effect of HD600 is self-evident, but it is not suitable for any amp. If the amp is not matched well or the quality is not good enough, the effect is plain and uncommon, giving people the feeling of "justification". But the potential of HD600 is huge. It needs to be tapped by players. It also includes the matching of different CDs and signal cables. The amp is even more important. This is the first time to use 6P9P as an amp. But when the amp was connected to the HD600 and the music sounded, the author felt relieved, and finally did not waste his effort.
First of all, I feel that the signal-to-noise ratio can pass. We all know that single-ended lines are more difficult to deal with hum, and single-ended amps are more demanding because of the high sensitivity of headphones. The remaining communication sounds of this machine are very weak and cannot be felt accidentally.
Secondly, I am worried about the low frequency problem, this amp solves it better. Low frequency is not only the strength of HD600, but also a shortcoming that is difficult to deal with. Its low frequency is very rich, extremely thick and unpredictable. If it is not handled well, it is easy to make people think about it. The design and production process of this amp is targeted, and the actual listening effect also proves that it has achieved its purpose. I felt profound when playing Promenade of the 9th track of DolJian Sample Vol II. This is a piece of music played on the organ in the church. It is magnificent and profoundly unusual. But it is really hard to express the low-frequency texture of the organ. In the entire audio system, no matter which part of the low frequency band is wrong, it will be a mess when performing this piece of music. Here, the performance of the 6P 9P single-ended amp push HD600 makes me feel very satisfied. The huge momentum can be easily expressed. The rare thing is that the low frequency texture of the pipe organ is very real.
In the high frequency band, the analytical power of HD600 is beyond doubt. When the equipment is high-end enough, the HD600 has transparency close to electrostatic headphones. The high frequency of this amp is better than the charm, and the richness unique to single-ended power amplifiers is well reflected here. In the interpretation of the record commonly known as "Famous Music" has been verified. This album was released in concert to commemorate the 250th anniversary of the death of the great violinist Stadi Vari. All the tracks are played with the famous Strad piano. This amp is matched with the HD600. The noble tone of the piano is vividly expressed. The IF can hardly find obvious defects for the HD600. In this combination, no matter the density of the human voice or the texture of the instrument, it can perform well. From the female voice of Cui Yanguang to the male voice of the eight eyes, it can be reflected in more detail here.
In general, HD600 to HD650 is an improvement. Under the premise that all kinds of software are better and better at high frequencies, HD650 undoubtedly has better adaptability, but unfortunately, HD650 has not completely surpassed. 