Analysis of headphone amplifier (including circuit diagram) without output transformer composed of 6N9C

Probe current voltage pin 420*4450 head diameter 5.0 over current current and voltage pin

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The circuit diagram of the headphone amplifier without output transformer is shown in the figure. The use of transformerless output is very beneficial for amateur production. This machine uses a small power pentode named 6N9C (6P9P), which uses four such tubes to form a WCF circuit for driving low-impedance headphones. The impedance of 30Ω is also no problem.
The characteristics of the pentode are delicate and round. After the pentode is connected to the triode, the linearity is better, the internal resistance is greatly reduced, and there is no big change in the transconductance. Although the output power is reduced, it is sufficient for the earphone. This machine uses 4 old Nanjing 6N9C, everyone knows that Nanjing is the cradle of China's modern electronics industry. Old Nanjing pipes are made in China. therefore. Using the old Nanjing 6N9C can get better sound quality than the ordinary 6P9P. The 6P9P is a wideband pentode. A wide frequency response can be obtained very conveniently.
The final stage of operation is WCF, a push-pull line that is very similar to SRPP, but it has no voltage gain when the screen resistance is optimized to the reciprocal of a tube transconductance. Its output impedance is essentially the reciprocal of twice the transconductance. therefore. Compared to SRPP and cathode output. WCF can achieve lower output impedance, which is not only good for driving low-impedance headphones, but also for better low-frequency effects for medium and high-impedance headphones. At the same time, compared to the cathode output. WCF is a push-pull line. It is easier to control noise.
In the output part, the 6P9P operating point is about 135V, the screen current is 40mA, and the cathode resistance is 44Ω. At this time, it is estimated that the transconductance should be above 11mA/V. In the case of open loop, the output impedance has dropped below 45Ω. Together with the large loop feedback, the output impedance should be as low as 20Ω. There is no problem in driving a 30Ω low-impedance earphone. .
The preamplifier is connected to the SRPP line by two 6N3s. Direct coupling 6P9PWCF. The reason why 6N3 is chosen is because 6P9P is a wide-band pentode, and 6N3 is a high-frequency amplifier tube, which is more advantageous for medium and high frequency, and is connected to SRPP amplification line, which has stronger control ability to WCF. At the end of the class, there is a Class A push-pull amplification that provides a very wide frequency response and excellent noise control. After the actual production, one welding was successful, and the sound field and frequency response were very wide, and it was not wide. Since it is a direct coupling, basically the working point of 6P9P has already determined the working point of 6N3. 6N3SRPP static working point: screen voltage of about 135V, cathode resistance is 1 kΩ, shed pressure is 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 non-label, unmarked 45W six-lamp power supply cattle provide powerful power. I applied this power transformer to make multiple dual-power headphone amplifiers. The results are very satisfying. Moreover, such transformers are currently priced at a lower price. The two units are less than 40 yuan, which is very value-for-money compared with the transformers that are short and short on the market. Since the six-lamp transformer uses tube rectification, the output voltage cannot meet the requirements. Therefore, the 1N4007 diode is used to rectify the rise. Pressing, then using 6Z4 as the secondary rectification and soft-acting, and then filtering through the capacitor choke, you can get the required voltage.
Design and production: I use a 2.5mm thick aluminum plate to make a split chassis. Power connection plug and socket withstand voltage 250V (AC), there is nothing wrong with the use process. The split cable is made of Teflon silver-plated wire and can be covered with a heat-shrinkable tube or a plastic sleeve. It is about 30cm long. Because the filament is powered by AC, the cable needs to be twisted.
Welding can start with the power box. The 6Z4 filament AC power supply requires double stranding and is close to the bottom plate and side plate wiring. The high voltage AC line and the 6N3/6P9 P filament are processed in the same way. The adverse effects of communication can be minimized. The AC high voltage is first rectified by 1N4007. Then straight into the 6Z4 screen for secondary rectification. And play the effect of soft start. In order to protect 6N3 and 6P9P., the filter is combined with a capacitor choke coil. Among them, the capacitor uses Philips blue hexagon 47uF/385V, two military products Moro 390uF/400V, large-capacity capacitor can effectively reduce power supply ripple and internal resistance. The choke uses Siemens antique 2H130mA (DC resistance is about 150-160Ω), don't look at the inductance, but the actual application feels very good. Use a little grounding in the power box. Note that the ground wires of the two chassis must be connected to produce sound. The filament of 6N3/6P9P uses a large current winding with a center tap. After the taps at both ends enter the power box, go straight to the power output socket and enter the amplification box through the external connection cable. The center tap is introduced into the power box and the filament is suspended from the ground with two resistors and one capacitor - this is because the maximum potential difference between the filament of the 6N3 and 6P9P and the cathode is plus or minus 100V. It must be suspended in the ground. In order to protect the tube.
The amplification box is more complicated. Because the 6P9P is a big eight-pin package, the four 6P9Ps use a lot of space. It must be well designed to fit the machine. The amplification box follows the signal direction and the layout of the circuit diagram, from the back to the front. The signal enters from behind, and 6N3 is next to the potentiometer. Then directly coupled to the upper 6P9P. The upper 6P9P is then cross-coupled by capacitors to the lower 6P9P, and then output. The potentiometer adopts Japanese ALPS double 100kΩ large blue plastic shell type 27. This machine is all welded by hand scaffolding. Be sure to pay attention to the layout of the high voltage line, the filament line and the signal line. 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 adopts the antique American CD big eight-pin plug-in twin capacitor 35uF×2/350V two. It should be noted that the negative end of the screen decoupling capacitor must be directly into the ground. Can not be connected to the 6N3 or 6P9P cathode ground and then grounded, because in this case, it may introduce annoying hum, the grounding point can be selected near 6N3. It should be pointed out that the coupling capacitor between 6P9P adopts the domestic military green oil immersion capacitor of 0.1uF/1000V, and the effect is very good. The output capacitor is electrolyzed with Speight 680uF/200V.
It needs to be pointed out. Since the third gate of the 6P9P is connected to the outer casing, the 6P9P is connected to the triode in this machine. At this time, the upper 6P9P casing will be charged, and the voltage is about 125V, because for the old Nanjing pipe, it must not work. Touch the upper 6P9P housing for the twilight. Although the outer casing is painted, it cannot be touched. It is better to insulate the upper 6P9P casing, which is safer. It is recommended to insulate the 6P9P housing.
After confirming the error, plug in the tube and power on the test. After power-on, use your three senses to confirm that there are no serious problems (such as short-circuits, etc.), which is "use the eyes to see (the same), use the nose to smell (odor), use the ear to listen (sound), to observe the components Is there any difference (resistance and capacitance change color, capacitor leakage, tube screen red, etc.), use the nose to smell if there is any smell, listen with the ear to see if there is any noise (at this time, don’t listen to the headphones, listen to the sound of the machine) , not the sound inside the headphones.) After confirming the error, use a multimeter to measure whether the voltage is normal.
Effect experience: In order to verify the performance of the various bands of this machine and the playback performance of music. Use a lot of software (CD record) to test. Listening to the CD includes both ethnic music, such as Marco Polo's Cross-Century Disc, Hugo Fever Disc 8, String Burning (1), etc., as well as Western Classics (Boutique Beethoven, Golden Foil Concerto, Boutique Bach, Red Devil). It includes popular songs (Best 8th, etc.), as well as New Century music, electronic music, etc., including DDD recordings, DSD (big movies, Great Western, etc.), HDCD and even XRCD (one listening to music, etc.). The overall feeling is that the sound of the golden stone is strong at the same time. It is soft and smooth, and it has a high resolution and dynamics. Then listen to the "Home Appliances Vision" original music version of the class owner "Chinese burning" ripped in the home appliances large field of vision commemorative CD, the second and last three, the clock is clear. Crisp and sweet. The last one, for the first time, heard the voice of the black man in the distance, clearly identifiable. It’s really dynamic and dynamic, it’s a charm, it’s clear and clear. The author believes that this is mainly due to two aspects. First, the choice of tubes, 6N3 and 6P9P are tubes with thicker bile; second, the choice of lines, SRPP and WCF give a large dynamic, extremely wide frequency response An important cornerstone.