Production of the pre-stage of the tube

The sound produced by the electronic tube has a round sound, sweet vocals, and strong music. I believe that all music fans will know! The low-end and mid-range sounds used by the majority of households will feel unpleasant sound for a long time of listening, and may even feel irritating and irritating. In fact, these are digital sounds and transistor amplifiers. In response to this shortcoming, enthusiasts who have certain sound theory and hands-on capabilities will make the front stage of the tube to promote the transistor power amplifier of the rear stage, in order to obtain a round and beautiful sound.

At present, the more well-known tube pre-level circuits are: Marantz 7, Ma Dianshi, Mai Jingtu C-22, Hotan Maoshi, JADIS, SRPP, etc. Here I introduce the audiophile to the author after several months of design and repeated debugging Only the final stage of the tube was finalized, and the circuit structure is based on the predecessor of Ma Dianshi.

The schematic diagram of the preamplifier of this tube is shown in Figure 1: two-stage amplification is adopted, the magnification is 10 times, the stereo design, the power supply and the amplifier stage are designed on the same circuit board, and the main power supply and the filament voltage are both designed as soft start circuits : The principle is that 0V, 1V, 2V ..., and the normal voltage is restored after a certain period of time at startup, so that the large current pulse at startup can be avoided and the electron tube is protected.

In order to play the most beautiful sound of 6N3 and reduce AC noise interference, the filament voltage is DC power supply, and instead of 6.3V, 5.9V-6V is used to supply 5670 (6N3) with the best sound quality. Conclusion), and in order to protect the life of 5670 (6N3), the filament power supply circuit uses a soft start circuit (see figure): because the impedance of the filament of the tube is very low when it is cooled (room temperature), it is more High impedance, this characteristic makes the current flowing through the filament very large at the moment the filament power is turned on, and returns to normal after a few seconds, so it is common for some tubes to suddenly turn on when the filament is turned on, and then slowly dim. For a long time, of course, it is not good for the durability of the filament. Generally, the filament is burned out and this situation is related. For this problem, the author uses a delay soft start power supply circuit. The principle is that the power is switched from 0V, 1V, 2V ..., a few seconds The normal voltage is restored after the clock, so that the large current pulse at startup can be avoided, and the electron tube is protected.

The working voltage of this tube preamp is powered by 60V. And most audiophiles will be puzzled: don't those famous tube preamps use two or three hundred volts? The author's design is based on the static curve of the 5670 (6N3) tube, and the load resistance and grid bias voltage are changed to 60V, because the typical high voltage of two to three hundred volts and the low-capacity filter capacitor of the tube circuit produce noise. Culprit. In particular, high-pressure noise has the strongest impact! The low-capacity filter capacitor allows the AC noise signal of the power supply to be mixed into the audio ... The author chooses 60V low voltage power supply to improve the signal-to-noise ratio.

Selection of electronic tubes and components: 1. Electron tubes suitable for low-voltage operation are 5670 (domestic model is 6N3), 6DJ8 / ECC88 (domestic model is 6N11), two kinds of tubes have advantages: using 6DJ8 makes the sound more 5670 (6N3) It is clearer and purer, and 5670 (6N3) is more mellow. In the end, I chose 5670 (6N3). Note: 6DJ8 and 5670 (6N3) are different in position and cannot be replaced directly. About 6DJ8 and 5670 (6N3) The data can be downloaded from the audio data. 2. The circuit board is designed with circuit design software, the components are neatly arranged, symmetrical on the left and right, and are beautiful and elegant. The epoxy glass fiber circuit board with a thickness of 1.5-1.6mm is used, and the silver plating process is used to print blue oil. Upscale. 3. T1 should add a larger radiator. 4. The electrolytic capacitor of the power supply must have a withstand voltage of more than 80V, and the capacitance of the tube filament power supply is not so high. 5. The 0.1uF capacitor uses Philips blue advanced MKP capacitors (WIMA or Thomson MKP capacitors can also be used) 6. Resistance: the power part uses 1 / 4W five-color ring metal resistors, and all audio parts use American DALE resistors , The sound quality is very good, the selection of R1, R2 resistance is the most critical (the choice is the best, the guts are the best, the noise is low, otherwise the noise is large, the layout is not described in detail here, the finished products produced by our factory are printed with values , Not given here to prevent peer piracy) 7. Signal coupling capacitors are very important: you can use Swedish RIFA capacitors or German WIMA capacitors, Philips blue MKP capacitors. Different brands have different timbre, specific preferences for enthusiasts have their own magic, which will not be described here.

Listening process: Marantz CD player is used as the audio source, 400W dual 32V ring cattle with ME-308 current negative feedback Class A amplifier is used for the rear stage amplifier, and the two groups of 22V are serially connected into a single with a 50W dual 22V ring cattle for the front stage of the tube. 44V is connected to the power part of the front stage of the electron tube, and the filament part of the electron tube is connected with a set of 11V voltage of the transformer. The audio source is connected to the front stage of the tube through an ALPS blue advanced volume potentiometer, and then the signal amplified by the tube prestage is connected to the audio input terminal of the ME-308 current negative feedback Class A amplifier. Note: the signal line must be Use high-quality shielded wire. After all welding is completed, check and turn on the power. After about one minute of preheating of the tube, first test the noise problem. It is found that the noise is very small when the potentiometer is completely closed or fully open (measured with a multimeter AC millivolt file) There is only a noise voltage of 20mV, and people can not hear the noise when they sit at the normal listening position at night. When the potentiometer is opened to half, it is slightly larger (note: the noise problem is different for different power amplifiers. The ME-308 current negative feedback Class A amplifier has little noise, while the integrated amplifiers such as TDA7294 / TDA7293 and LM4766 will have more noise. Some, the reason is that the gain of the amplifier is large, and the noise will be lower if you turn it down.

After listening to the noise, start to listen to the music: Put the Hugo CD "Love Bamboo" into the CD player. After listening and comparing it with the transistor pre-stage, the tube pre-stage is clearer than the transistor pre-stage, and the sense of space is rich and high. The frequency is delicate and durable. Even if you listen for a long time, you will not feel harsh, and the vocals are sweet and natural in low frequency. Confirmed this conclusion: the sound amplified by the tube preamp will get a clear sense of hierarchy, rich sense of space, delicate high frequency, natural low frequency, sweet mid-frequency! Solve the shortcomings of the transistor power amplifier, can filter out the residual digital signal of the audio source, and make the music signal more pure. The large dynamics of the tube and the characteristics of soft distortion can make the tone softer! The rapid upgrade method with less investment will surprise you!

Technical area detailed analysis of Doherty power amplifier design load traction principle (top) detailed analysis of Doherty power amplifier design how to improve efficiency (bottom) analysis of voltage follower input and output error analysis Solve the effect of power amplifier on emi

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