Author: Zhong Wenxiang
Direct thermal small power pentode 2P29-based small seven legs, mainly for transmitting high-frequency oscillation circuit and a low power amplifier, this tube connected to an audio pre-transistor voltage amplifier circuit, linearity, beautiful tone, transparency high, I use this tube production before the class. And achieved satisfactory sound.
First, the principle of circuit principle This machine circuit is shown in the figure, and the classic common yin amplifier can be used. Its main features are:
1. In order to avoid the common transmission distortion volume potentiometer. This machine uses a very low noise V-MOS field effect tube to control the audio control. With respect to the keying circuit and reduces the number of volume elements, and be shielded, so that the noise figure of the volume control portion reaches ldB the following (V-MOS FET noise figure of about 0.5dB), can compete with high-grade vacuum stepper potentiometer .
V-MOS FET of high resistance, is a voltage controlled device. A charge capacitance is received between the gate and the source. Since the gate leakage current is small, the capacitor voltage can remain substantially unchanged for a long period of time, and when the tube is operating in the adjustable resistance region.
Its drain-source resistance will be the gate-source voltage of the control voltage of the capacitor that is, when the tube voltage-controlled variable resistance equivalent, when referring to the touch (by finger resistance of the conductive) switch Sl is closed, namely charging the capacitor, when referring to touch switch S2 is closed. About to discharge the capacitor. So as to achieve the purpose of voltage control of the drain-source resistance. Access to audio equipment, to adjust the size of the volume. Sl and S2 available thin silver sheet or thin copper sheet production, the pitch of about 1mm. After the debug is determined, the volume increase or decrease is set to + 2dB. I mounted on the top of the sheet knob.
2.2P29 using triode connected to reduce the internal resistance. Improve linear.
3.2P29 using self-gate bias, self-gate bias over a fixed gate bias soft playback.
4.2P29 base resistor selection screen, with a direct impact on the size and level of noise before the gain, frequency response and distortion. Noise resistance. Sound in two categories. A method of producing a resistor by the thermal motion of free electrons, called thermal noise. The other is excessive noise resistance.
It is related to the voltage across the resistor, its value is much greater than the thermal noise, both ends of the tube plate load resistance generally have a higher DC voltage, a very large resistance value of the pressure drop is large screen. The resulting excess noise is large. Optimum anode load resistance value is usually 3 to 5 times the internal resistance. After 2P29 transistor connected to the internal resistance is about 3kΩ.
To ensure good low frequency response, i.e., calculated by taking about five times to obtain a better signal to noise ratio 15kΩ and resistor classical player reference frequency equalizer circuit (McIntosh-C22) from about l: lO divided into two parts, i.e., labeled with 1.2k nominal value (high frequency emphasis) + 12kΩ (low-frequency emphasis) as two resistors in series 2P29 plate load resistance.
Such voltage anode load borne by two resistors respectively, so that the excess noise has been reduced. But also further improve the frequency characteristics of the front stage of the present. Two resistors can be selected low noise resistance.
The power rectifying part is 6Z4. Ratulating WY4P regulated. Bile duct using the rectification and regulation, there is no avalanche transistor switch distortion, and the distortion of the tunnel. Since the operating current of the preceding stage 2P29 not directly heat cold start without power loss bile regulator regulator, there is less than a thermal noise voltage regulator circuit bile thermal power loss, while also avoiding an increase in power resistance. In addition, the stabilization is connected in parallel with the power supply output. The equivalent of a school sound capacitor, to have a sound landscaping.
6.2p29 is a straight heat pipe. Filament voltage changes significantly affect the heat pipe side than the electron emission conditions, so the filament used in the present machine 2P29 2.2V DC power supply in order to improve signal to noise ratio. Power supply module LM-317 as the voltage output, the maximum output current of 2.2A, AC ripple less than 0.4mV.
Second, the configuration element selected sound type FET. VMOS or NMOS small power tube. The IRFDll3 (60V / Vds, 0.8A / Ids, lW / Pd), BSl70 (60V / Vds, 0.3Mid, 0.63W / Pd) and the like. Two channels shared a set of fingers.
Output coupling capacitor capacitance Teflon is preferably used, the silver film capacitor oil may also be used. The silver film capacitor Sprague VQ oil. West oil capacitors and so on. Such capacitance difference frequency characteristics of a good conductivity, low high-frequency loss, mellow tone sounds.
2P29 cathode capacitance, electrolytic capacitors use of domestic day cards, card winds, Jianghai cards, and other cards Wuxi bulky aluminum electrolytic capacitors, may also be used reaction speed camera flash electrolytic capacitor, such as CDl7H like. Power supply transformer of 100VA, choke coil specifications 15H / 20mA, or with a small leakage magnetic radiation R type power transformer. Filament transformer is 220W6V, power is 3W.
The machine 2P29 filament power supply scaffolding welding portion should not be used, because the exchange of radiation of induction, good shielding effect, the circuit board in the restructuring of the available energy saving light bulbs, making convenient, but also help to improve the exchange ratio.
Third, assembly debug selection suitable floor reasonable layout. Especially supply portion away from the front stage portion, and the isolation barrier metal plate. Considering the skin effect single-core silver plated copper wire of 1.5mm diameter is welded by a circuit diagram, for machine input signal lines, output lines remain coaxial television, good vibration isolation and shielding measures. Silver plated products are also used in the connector. The 2P29 cathode voltage was adjusted to 2V, and the corresponding current was 2 mA.
Fourth, the unit access sound effect system, a computer sound card audio, Dolby AC-3 surround virtual software decoding, self 2x50W V-MOS FET amplifier, a signal line is made using coaxial TV, a speaker 8 91dB-inch full-range speakers, turn on the power, little noise are not, click on a "passionate Dongjiang" from the MP3; then went on to put "Hugo Audiophile CD 8" and full-range disc and pop music with a DVD try, out loud whom Yizheng, comprehensive high school low-frequency performance, vocal music emotion beautiful, rich taste of bile, especially the sense of permeability tone, fidelity, clear bottom, it is difficult to explain in words.
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