Circuit diagram of the pre-stage of the tube

Photocoupler

The sound made with the tube is round, vocal, and musical. I believe that the majority of music enthusiasts know that the middle and low-end audio used by the majority of the family will feel the sound quality is not easy to listen to for a long time, and even feel irritating. In fact, this is all digital sound and transistor amplifiers whose metal sound is at fault. In response to this shortcoming, enthusiasts with certain acoustic theory and hands-on ability will make the pre-stage of the tube to push the post-stage transistor amplifier to obtain a round and beautiful tone.

At present, the more famous pre-existing lines of electronic tubes are: Marantz 7, Ma Sangshi, Mai Jingtu C-22, and Tian Maoshi, JADIS, SRPP, etc., who introduced the author to the enthusiasts after several months of design and repeated debugging. Before the final stage of the tube was finalized, the line structure was referenced to the front stage of the Ma Sangshi electronic tube.

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

Manufacturing circuit of the pre-stage of the electron tube

In order to play the most beautiful sound of 6N3 and reduce the interference of AC noise, the filament voltage is powered by DC, and it is not 6.3V, but the sound quality of 5670 (6N3) is best with 5.9V-6V (this is a gallbladder experiment) At the same time, in order to protect the life of 5670 (6N3), the filament power supply circuit uses a soft-start circuit (see figure): because the filament of the tube is low in the cooling (room temperature) state, the red heat is more High impedance, this characteristic makes the current flowing through the filament at the moment when the filament power is turned on is very large, and it returns to normal after a few seconds. Therefore, it is common for some tubes to suddenly brighten at the moment of starting the power, and then slowly turn dark. For a long time, of course, there is no benefit to the durability of the filament. Generally, the filament is burnt more than this situation. For this problem, the author uses a delay soft-start power supply circuit. The principle is 0V, 1V, 2V..., several seconds when starting up. The normal voltage is restored after the clock, so that the high current pulse at the time of starting up can be avoided and the tube is protected.

The operating voltage of the tube preamplifier is powered by 60V. And most enthusiasts will be puzzled: those famous tube preamps are not using two or three hundred volts? The author's design is to enter the load resistance according to the static curve of 5670 (6N3) tube, the gate bias is changed and then choose 60V Because the typical high-voltage and low-capacity filter capacitors of two or three hundred volts of the tube circuit are the main culprit. In particular, the high-voltage noise has the most impact! The low-capacity filter capacitor makes the power AC noise signal mixed into the audio... I chose 60V low-voltage power supply to improve the signal-to-noise ratio.

Selection of electronic tubes and components: 1. The tubes suitable for low voltage operation are 5670 (domestic model is 6N3), 6DJ8/ECC88 (domestic model is 6N11), and the two tubes have their own advantages: the sound is 6670 (6N3) with 6DJ8. Clear and pure, and 5670 (6N3) is more mellow, finally I chose 5670 (6N3), note: 6DJ8 and 5670 (6N3) feet are different, can not be directly replaced, about 6DJ8 and 5670 (6N3) The information can be downloaded to the audio data. 2, the circuit board is designed by circuit design software, the components are arranged neatly, symmetrically, elegant and generous, and adopt 1.5-1.6mm thick epoxy glass fiber circuit board, and adopt silver plating process, printing blue oil Upscale. 3, T1 should add a larger radiator. 4, the power supply part of the electrolytic capacitor withstand voltage must have more than 80V, the tube filament power supply capacitor is not so high. 5, 0.1uF capacitors use Philips blue high-grade MKP capacitors (also can use WIMA or Thomson MKP capacitors) 6, resistors: power supply part with 1/4W five-color ring metal resistors, the audio part of the use of the United States DALE resistors The sound quality is very good, and the selection of the resistance values ​​of R1 and R2 is the most important (the best choice is the bile taste, the noise is low, otherwise the noise is large, the layout is limited to this detailed description, and the finished board produced by the factory has numerical values ​​printed on it. It is not given here to prevent peer piracy. 7. Signal coupling capacitor is very important: you can use Swedish RIFA capacitor or German WIMA capacitor, Philips blue MKP capacitor. Different brands have different timbres, and I like the enthusiasts' own tricks. I don't describe them much here.

Audition process: the sound source uses the Marantz CD machine, the rear stage power amplifier uses 400W dual 32V ring cattle with ME-308 current negative feedback class A power amplifier, the front tube of the tube uses a 50W double 22V ring cow to connect the two groups of 22V into a single The 44V is connected to the power supply part of the front stage of the electron tube, and the power supply part of the tube filament is connected with a set of 11V voltage of the transformer. After passing through an ALPS blue advanced volume potentiometer, the sound source is connected to the front stage of the electronic tube, and then the signal amplified into the front stage of the electronic tube is connected to the audio input end of the ME-308 current negative feedback type A power amplifier. Note: the signal line must be With high-quality shielding wire, all the welding is checked and the power is turned on. After 1 minute or so, the tube is preheated. First, the noise problem is tested. It is found that the noise is very small when the potentiometer is completely closed or fully open (measured by multimeter with millimeter) With a noise voltage of only 20mV, it is impossible to hear noise when sitting in the normal listening position when the night is quiet. When the potentiometer is halfway open, it is slightly larger (note: the noise problem is different for different power amplifiers. The ME-308 current negative feedback class A power amplifier has very little noise, and the integrated power amplifiers such as TDA7294/TDA7293 and LM4766 have large noise. Some, the reason is that the set is successful, the gain is large, and the noise is smaller.

After trying the noise, I began to try the music. I entered the Hugo CD "Love Bamboo" and put it into the CD player. After listening and comparing with the front stage of the transistor, I found that the front of the tube is distinct from the front of the transistor, and the space is rich and high. Frequent and delicate, it doesn't feel harsh when listening for a long time, and the human voice is sweet and the low frequency is natural. It is confirmed that the sound amplified by the pre-stage of the tube will have a clear and distinct layering, rich sense of space, delicate high frequency, natural low frequency, sweet intermediate frequency! Solve the shortcomings of the transistor amplifier, and filter the sound source. The residual digital signal makes the music signal more pure, and the large dynamic and soft distortion characteristics of the tube can make the sound more soft! The investment method with less investment and quick surprise will surprise you!

The cost is not much (of course, if you use a SOREN big S capacitor of tens of dollars and a resistor of five or six yuan, you need more than three hundred pieces), you can get a charming bile, it is really a small investment upgrade. method. To this end, the author has made an article for the imitation of DIY enthusiasts, but declined commercial imitation. In this paper, the electronic tube bile taste is better than the acoustic circuit. The author named SP-27 tube bile-like sound circuit (produced by Meishun Electronics Factory in Yunan County, Guangdong Province). If you want to see the photo of the pre-stage of this tube, please in January 2003. Check it out after 1 day.

Tube: A common basic electronic component. There are typically three poles, one cathode (K) for emitting electrons, one anode (A) for absorbing electrons emitted by the cathode, and one grid (G) for controlling the flow of electrons to the anode. The tube housing is typically a glass tube, and the current conduction in the middle utilizes the effect of an electric field on the flow of electrons in the vacuum to obtain signal amplification or oscillation.