The pre-stage needs to be warmed up for more than 3 minutes before it can be connected to the post-stage, otherwise there may be DC input during the warm-up process, resulting in post-stage protection!
【Circuit description】
This time I recommend a Jadis JP-200 tube preamplifier with balanced timbre and musical appeal.
1. Introduction of circuit principle
The left and right channels of the circuit diagram are completely in phase. The l2AX7 used by the original machine is a small-signal audio voltage amplifying tube, which has the characteristics of high μ value and high internal resistance. Originally mainly used for MM phono amplification and input-level amplification, its sound is beautiful, bold taste, the domestic model is 6N4.
The values of all the components in the circuit are faithful to the original. It is amplified by a two-stage common cathode in full parallel connection. The additional cathode follower is the output, and negative feedback with a large loop is applied. By applying negative feedback with a large depth loop, the gain of the entire preamplifier is determined to be around 11.
The pre-amplifier has high requirements on the power supply, the capacity of the power transformer is insufficient or the power supply filtering is not good, and it is easy to induce hum and other noise. Even if it is only a little bit, after a few tens and hundreds of times of amplification of the post-stage power amplifier, it will seriously affect the sound quality.
The power supply part is slightly modified on the basis of the original machine circuit. The high voltage part adopts field effect fast diode rectification, the sound quality is very close to the bile rectification effect, the resistance and capacitor are not shaped, and the tube is regulated to supply power. The reference voltage is provided by the crystal regulator tube of l50V, the 6N2 electron tube constitutes the differential error voltage amplification, and the beam-emitting quadrupole 6Pl4 is connected as a triode connection method as a voltage adjustment tube, forming a series voltage regulator circuit of the electron tube. The output voltage is stable, the ripple coefficient is small, the dynamics of the rectification of the charging transistor are combined with the characteristics of the gallbladder of the tube voltage adjustment, and the purpose of balancing the tone is achieved. The low-voltage filament power supply part also adopts DC stabilized voltage power supply, using the original ST7812 three-terminal adjustable voltage regulator integrated block with an output current of 1.5A. This integrated block is a high current and stable voltage regulator integrated block, which easily supplies a total current of 0.45A Three 12AX7 filaments. The electron tubes of the amplifying circuit part and the voltage stabilizing circuit part are separately regulated and supplied with power to reduce mutual interference. There are two power transformers, each of which is responsible for the power supply of one channel with sufficient margin.
Second, production and debugging
There is a saying in the industry. I think that the amplifier made by "shelter welding" is better than the one made by printed circuit board. The author thinks this is a matter of opinion. Regardless of the manufacturing method used, the placement of each device and the layout of the circuit have a great influence on the sound quality. The key is to deal with the above two points reasonably, and it is fine to use "shelter welding" or circuit board production. Consider that the local circuit is more complicated and more components are used. This unit uses a circuit board design.
An amplifier is first and foremost a stable amplifier, so the selection of components is very important. It is not necessary to use imported components, but it must use brand new products from regular large manufacturers. And strictly follow the "derating use" principle. When selecting components, a large margin should be left to ensure that each resistor does not generate heat when working, and each capacitor does not overvoltage when the commercial power exceeds 10% fluctuation. The two power transformers use 90W square transformers, the actual total power consumption is less than 50W, leaving twice the margin. The advantage of this is that the load of the electric transformer is very light, the internal resistance of the transformer has little effect on the circuit, and the heat of the transformer is small, and the work is stable and reliable. The signal channel resistance is made of domestic high-precision copper pin metal film resistors, and its resistance value error is very small, and there is a power margin of 4-6 times. To minimize the thermal noise when working. The input coupling capacitor and the inter-stage coupling capacitor are all selected from Hong Kong antique TE MKP film capacitors according to their withstand voltage, and the potentiometer uses APLS blue plastic shell electrical appliances. (The circuit board does not contain a potentiometer, only the whole machine is included) The electronic tube is selected from the J grade of Shanghai brand 6N2 and the J grade of Sugon 6Pl4. The grounding method strictly follows the star point grounding method, and the grounding position is determined through repeated experiments. During the actual work of this preamplifier, the exchange sound is very small, and the amplitude is comparable to the thermal noise. Only when the volume of the post-stage power amplifier is turned to the end, the hum and noise will appear weak at the same time, and the filament DC power supply is indispensable.
Three, package selection instructions
The finished board does not contain the electronic tube as the circuit board in the picture
The finished board contains all the tubes: including Dawning 6P14*2, Shanghai 6N2*2, Dawning 12AX7B*6
The finished board contains the voltage regulating part of the electronic tube: including Sugon 6P14*2, Shanghai 6N2*2, excluding Sugon 12AX7
[Material Description]
●All parts of this product are original, unpretentious, and pure quality; the use of high-quality original parts is not only a guarantee of safety and quality, long service life, but also a strong guarantee that the actual performance can meet the design goals. The pursuit of quality is our consistent aim.
●Resistance uses five-ring copper pin precision metal film resistors, and high-power resistors use metal oxide film resistors. The carbon film resistor is not used because the thermal noise of the carbon film resistor is large.
●PCB adopts 1.6mm thick double-sided board, 1OZ (35um) copper thickness, gold plating process to ensure good passing performance of large and small current. PCB quality. PCB size: 250MM*210MM, highly containing 9CM tube.
●Adopt field effect fast recovery diode rectification, 6P14 and 6N2 bile voltage regulation, after multi-stage filtering, provide stable and pure DC power to the front stage, more suitable for areas with large power fluctuations. The effect of the fast recovery diode is similar to that of bile rectification, but the rectification current is larger. The left and right channels are independently powered, and the power is surging and the symphony effect. Bile rectification is more lyrical and vocal.
●After all the tube filaments are 7812 DC stabilized power supply, combined with one point grounding and reasonable wiring, it is frugal and will not produce hum. The peak noise of the whole board is only 3MV. Filament wiring has been laid on the board, no need to fly any wires, it will not appear messy, often because the filament wiring board often causes interference, and the noise floor will be high, so many stores are drawing wires when flying the board. That is a last resort. The layout of the board's wiring usually takes many experiments and improvements before it is finalized, and the cost is relatively high.
●Standard electronic tube (choose package including electronic tube): voltage adjustment tube is Shanghai 6P14*2, Shanghai 6N2*2, amplification tube is Sugon 12AX7*6, all are brand new tubes.
●Voltage requirements: AC 330V-0 100MA; 6.3V 2A; 13V 2A;
●The signal coupling capacitor adopts German WIMA and Hong Kong TE MKP copper pin film capacitors.
●High-voltage filter capacitors from the Netherlands 150UF 450V×12
●Pure copper base plated with pure copper.
[Refer to the circuit diagram, the power part has been modified]
matching transformer
matching transformer
