Headphone amplifier made with 6N9C

The circuit diagram of the headphone amplifier without output transformer is shown in the figure. The whole circuit is relatively simple, and the push-pull circuit is very convenient to install and debug, and it can basically succeed at one time. The following is an introduction to the author's production process.

The use of transformerless output is very beneficial for amateur production. Many high-quality domestic tube headphone amplifiers now also use no output transformer, which shows that capacitor output can achieve better results. In fact, using capacitor AC output can easily achieve a wide frequency response, which is also beneficial for impedance matching.

Although there are many types of electron tubes, there are not many electron tubes that can be used to drive headphones, have good sound quality and are low in cost, because the output tube requires a lower internal resistance, higher transconductance and larger screen current. This machine uses a low-power pentode named 6N9C (6P9P). Four of these tubes form a WCF circuit to drive low-impedance headphones. There is no problem with the impedance of 30Ω.

The characteristics of the pentode are delicate and round. After the pentode is connected to a triode, the linearity is better, the internal resistance is greatly reduced, and the transconductance does not change much. Although the output power is reduced, it is enough for headphones. This machine uses 4 old Nanjing 6N9C. As we all know, Nanjing is the cradle of my country's modern electronic industry. The tubes of old Nanjing are even more domestically made. Therefore. Using the old Nanjing 6N9C can obtain better sound quality and tone than the ordinary 6P9P. 6P9P is a wide-band pentode. It is very convenient to obtain a wider frequency response.

The final stage is WCF, which is a push-pull circuit, very similar to SRPP, but it has no voltage gain. When the screen resistance is optimized to the inverse of the transconductance of a tube, its output impedance is basically the inverse of twice the transconductance. Therefore, compared with SRPP and cathode output, WCF can obtain lower output impedance, which is not only conducive to driving low-impedance headphones, but also for medium and high-impedance headphones, it can obtain better low-frequency effects. At the same time, compared with cathode output, WCF is easier to control noise because it is a push-pull circuit.

In the WCF output part, the 6P9P working point is about 135V screen voltage, 40mA screen current, and 44Ω cathode resistance. At this time, the estimated transconductance should be above 11mA/V. In the open loop, the output impedance has dropped below 45Ω. With the large loop feedback, the output impedance should be as low as 20Ω, and there is no problem driving 30Ω low-impedance headphones.

The preamplifier is connected by two 6N3s in SRPP circuit. Direct coupling 6P9PWCF. The reason for choosing 6N3 is that 6P9P is a wide-band pentode, while 6N3 is a high-frequency amplifier tube, which is more beneficial for medium and high frequencies, and connected in SRPP amplification circuit, it has stronger control over WCF. It is a Class A push-pull amplifier from beginning to end, which can obtain a very wide frequency response and excellent noise control ability. After the actual production, the welding was successful at one time, and the sound field and frequency response felt very wide, and not just a little wider. Because it is directly coupled, the working point of 6P9P has basically determined the working point of 6N3. The static working point of 6N3SRPP is: screen voltage of about 135V, cathode resistance of 1kΩ, and shelf voltage of about 2.6V, so the screen current is about 2.6mA, which is enough to drive 6P9P.

The power supply adopts split dual power supply. Two previously produced unbranded and unmarked 45W six-lamp power transformers provide strong power. The author uses this power transformer to make several dual-power headphone amplifiers. The effect is very satisfactory. Moreover, this transformer is currently sold at a low price. Two units cost less than 40 yuan, which is really a great value compared with the "short and short" transformers on the market. Since the six-lamp transformer uses electron tube rectification, the output voltage cannot meet the requirements. Therefore, 1N4007 diode rectification and boost are used first, and then 6Z4 is used as secondary rectification and soft start, and then filtered by capacitor choke coil to get the required voltage.

Design and production

The author made a split case with 2.5mm thick aluminum plate. It is very convenient to drill holes with a hand drill. The power plug and socket withstand 250V (AC), and there is no problem during use. The split connection wire is made of Teflon silver-plated wire, and the outside is covered with heat shrink tubing or plastic sleeve, about 30cm long. Because the filament is powered by AC, the connection wire needs to be twisted in two strands.

Welding can start from the power supply box. The 6Z4 filament AC power supply needs to be twisted in two strands and routed close to the bottom plate and side plate. The high-voltage AC line and the 6N3/6P9P filament are the same. The adverse effects of AC can be minimized. The AC high voltage first passes through the 1N4007 full-wave rectifier. Then it goes directly into the 6Z4 screen for secondary rectification. It also has the effect of soft start. To protect 6N3 and 6P9P. The filter uses a capacitor choke combination filter. Among them, the capacitor uses Philips Blue Hexagon 47uF/385V, 2 military Moroli 390uF/400V, and the large-volume capacitor can effectively reduce the power supply ripple and internal resistance. The choke uses Siemens antique 2H130mA (DC resistance is about 150-160Ω). Although the inductance is not large, it feels very good in actual application. One-point grounding is used in the power supply box. Note that the ground wires of the two chassis must be connected to make a sound. The filament of 6N3/6P9P uses a high current winding with a center tap. The taps at both ends go directly to the power output socket after entering the power box, and enter the amplifier box through the external connection line. The middle tap is introduced into the power box, and two resistors and a capacitor are used to form a filament suspended to the ground - this is because the maximum potential difference between the filament and the cathode of 6N3 and 6P9P is plus or minus 100V. It must be suspended to the ground to protect the electron tube.

The amplifier box is relatively complicated. Because 6P9P is a large eight-pin package, four 6P9Ps take up a lot of space. It must be well designed to install the machine. The amplifier box is laid out from the back to the front according to the signal direction and the circuit diagram. The signal enters from the back, and 6N3 is next to the potentiometer. Then it is directly coupled to the 6P9P above. The 6P9P above is then cross-coupled to the 6P9P below through a capacitor, and then output. The potentiometer uses Japan ALPS double 100kΩ large blue plastic shell 27 type. This machine is all hand-made scaffolding welding. Be sure to pay attention to the layout of high-voltage wires, filament wires and signal wires. Signal channel. Whether it is input or output. Try to use shielded wires. Grounding uses a combination of one-point grounding and common grounding. The screen decoupling capacitor uses two antique American CD large eight-pin plug-in twin capacitors 35uF×2/350V. It should be noted that the negative end of the screen decoupling capacitor must be directly connected to the ground. It cannot be connected to the cathode ground of 6N3 or 6P9P before grounding, because in this case, it may introduce annoying AC noise. The grounding point can be selected near 6N3. It should be pointed out that the coupling capacitor between 6P9P uses the domestic military green oil-immersed capacitor 0.1uF/1000V variety, which has a very good effect. The output capacitor uses the electrolytic 680uF/200V of Spi. The sound is still very good, and the larger capacity can obtain better low-frequency response.

It should be pointed out that since the third grid of 6P9P is connected to the shell, and in this machine, 6P9P is connected to a triode, the upper shell of 6P9P will be charged at this time, and the voltage is about 125V. Because, for the old Nanjing tube, the upper shell of 6P9P must not be touched during operation, and for Shuguang, although its shell is painted, it cannot be touched. It is best to insulate the upper shell of 6P9P, which is safer. Because, even in the power-off state, since 6P9P is connected to the output capacitor, and high-quality output capacitors can often maintain a higher voltage for a long time, once touched, although there is no major hindrance, the numb feeling is really uncomfortable, so it is recommended to insulate the shell of 6P9P.

After the production is completed, first carefully check everywhere to see if there are short circuits and open circuits. After confirming that everything is correct, plug in the electron tube and test it with power. After powering on, use your three senses to confirm that there are no serious problems (such as short circuits, etc.), which is "look with your eyes (strangeness), smell with your nose (strange smell), and listen with your ears (strange sound). Observe whether there are any abnormalities in each component (resistors and capacitors change color, capacitors leak, the electron tube screen turns red, etc.), smell with your nose to see if there is any peculiar smell, and listen with your ears to see if there is any abnormal sound (don't use headphones at this time, you should listen to the sound of the machine, not the sound in the headphones). After confirming that everything is correct. Use a multimeter to measure whether the voltage at each location is normal. After confirming that it is normal, you can plug in a cheap small headset to listen, and finally plug in your favorite headphones to enjoy the music.