I have heard some Own Mic tube amp, generally poor signal to noise ratio. Interrupt source and turn down the volume, speakers, there are always hum when you turn up the volume even more prominent, even in some commodities are also more or less tube amp this is a problem. So choke inductor to increase burning faithful, increase capacitance of the filter circuit CUHK Mote Mo, but it has little effect. Then left for the right place then, a little noise in a greatly reduced chance to treasure, it seems that thousands of low-noise ground point is to try out pilot million, mysterious, it was annoying to try Debu Chennai went by it . Another myth is that the stone machine has power supply, electronic filter circuit, the background noise is, of course, and tube amps do not, and it will add to the tube amp power supply, which makes the power supply is very complicated, the efficiency greatly reduced.
Eliminate background hum there are two ways. First off method, fire with fire, but when this drug does not work either-toxin, which must be the same wave, just a 180 degree phase noise source to offset the noise, although the method is also passive, but also effectively used properly. Such as the filament before the class balance of the circuit ground potential, push-pull output circuit board flow arms cancel each other out and so is the noise component of the application of this method. Another method is to find the source of noise to be removed from the root to prevent noise, this paper focuses on this method.
Popular over the past method of using the ground bus assembly tube amp, silver plated copper wire with a larger bus, because of its resistance is very small, to overcome the static noise has some effect, but in today’s Hi-Fi under the conditions of the high demand for this law has been outdated. Shown in Figure 1 for the more typical power amplifier circuit, in which rough outline for the ground bus. Now we analyze the noise sources, the rectifier output DC pulse current charge who C1, by bus back into the rectifier, then the left part of the ground point C1 are 100Hz pulse current through, the hardest hit of pollution, this period can not remember any bus amplifier circuit is connected. Smoothed by C1 100Hz pulse components is still a certain constant current charge by the L1 into the C2, C2 components in the 100Hz pulse current has been greatly reduced. It flows through the ground bus in the C1, C2 between a negative, so it can not access a bus amplifier. C2 is basically non-existent right 100Hz pulse current pollution, but by the C2 is extreme, consumed by the power amplifier tube has to a large current from the power tube audio V4, V5 cathode resistance grounding bus people back into the C2 negative. C2 cathode resistor into the negative pole location to place a bus which became the audio sources. The audio drop directly through the C3, C4 feedback back to the previous level, ranging from generating the waveform distortion is caused by self-oscillating weight, great harm. Similarly V3 C3 level flowing through the promotion of the audio current in the C3 and C2 negative extreme negative extreme indirectly generate audio bus voltage drop, through the C4 pollution to the V1. While the output transformer secondary site through a negative feedback signal has a large dynamic audio output stage current in the bus on a drop of pollution, given away by the high sensitivity of the V1 Rf cathode, but also to the negative feedback signal disturbance, destruction of sound quality. Through the above analysis shows that, although grounding bus, but an amplifier is not good.
Figure 2 Schematic diagram of star grounding connection
Countermeasures against these problems as shown in Figure 2, eliminating the ground bus, all the decoupling capacitors filter centralized allocation summary of all the ground that E. Negative rectifier with a line single access point, amplifier tube V4, V5 cathode resistor ground point with a line E1 access it. If it is a fixed bias is negative rectifier filter circuit ground point should also be remitted E1, access it. Similarly, the cathode resistor to promote the class through V3 E2 site is also a separate access point with the line. The V2 is inverted relative to the load plate negative points, because it put a lot of no, so I can share, and a ground input stage V1 E3, access punctuality, with lines only 0.5-0.75mm2. All the ground spread out from the E point star-shaped, so named. Negative feedback for the end of the current pollution from power level, the outer shield can be used to output "O" E3 terminal end and connected to the negative feedback signal transmission wires. As was extreme from the point of the B + decoupling filter resistor isolation, simply select the adequate R, C time constant. In line with the high signal to noise ratio, the filament input stage DC power supply should be used. Left and right channels can not generally use the line were ground and then B +, which has does not affect SNR. But to a higher degree of left and right channel separation, the left and right channels were also used lines.
I recently fitted with a star grounding a tube amp, the data has been marked in Figure 2, the input stage SRPP circuit with induced noise can be reduced, and the V1, V2 are the DC filament supply. Down phase, to promote the power three circuits are used Williamson, 3800 ~ t when the load RMS power of 2x25W. The original sound quality of the circuit, together with the application of star grounding themselves, extremely pure sound quality. This machine not only job stability, signal to noise ratio is also unusually high, no matter on what the volume knob position, as long as the stop signal, the ear in the middle of the night he was not able perceptible attached speaker is turned on, is really more with less in terms of signal to noise ratio . I am afraid that such a low background noise is also more difficult to achieve the transistor drive, do not believe you can try.