6N1 Homemade OTL Headphone Amplifier Circuit

Its impedance is 300Ω, which is considered a high-impedance headphone. When it is driven by the headphone amplifier output interface of the CD player, although the sound pressure reaches a certain level, the driving power is too small. When the volume is turned up, the distortion is large and the sound is unbearable. The high sound quality characteristics of HD600 cannot be brought into play, so I decided to make a headphone amplifier myself
. In the past few years, I have also made several different tube amplifiers. From the listening experience alone, I think the sound of tube amplifiers is more pleasant than that of transistor amplifiers, so I also plan to make the headphone amplifier with tubes. After reading the production experience of some headphone enthusiasts on the Internet and studying many different types of tube headphone amplifier circuits, and considering my own tube inventory, I decided to use the tube headphone amplifier designed by Morgan Jones.
Circuit Principle
The circuit schematic is shown in Figure 1. It is an output transformer-free (OTL) circuit without loop feedback. The circuit is very simple and very suitable for primary headphone enthusiasts to imitate. This headphone amplifier uses only one type of 6N1 dual triode tube. The left and right channels share three 6N1 tubes. 6N1 has a good parameter curve, a large social inventory, and a low price, which is conducive to reducing costs. Although the sound characteristics and features may be different, 6N1 is compatible and interchangeable with 6N11 (6922, 6DJ8, ECC82, E88CC) in principle. Of course, if 6N11 is used, the relevant components and screen voltage of the circuit must be changed accordingly. Figure 2 shows the headphone amplifier made with 6N11 tubes for reference by interested friends.

Here I use the 6N1T (special grade) tube produced by Beijing Electron Tube Factory. The biggest feature of this 6N1 OTL amplifier circuit is the asymmetric output. It is actually similar to the circuit of the Hetian Maoshi amplifier that was very popular some time ago. It removes the line input stage of the Hetian Maoshi amplifier. The signal is input to the gate of V1 after being controlled by the 100k volume control potentiometer. Its screen-shaded output allows various impedances, especially high-impedance headphones, to have a more abundant volume output. However, the performance of the 6N1 OTL output in driving low-impedance headphones may not be as good as that of driving high-impedance headphones. Because its final stage uses an asymmetric output like SRPP, it requires a higher voltage power supply. Figure 3 shows the power supply part.

In my experience, a simple circuit needs to have good sound effects, and the power supply part needs a lot of effort. The power supply of the headphone amplifier made by the author is not complicated, but the filtering effect is very good. The high voltage part adopts the classic π-type LC line filtering, and then passes through two resistors for RC filtering, which has good noise reduction characteristics. The high voltage of this circuit is 350V, and the voltage between the cathode and the filament exceeds the requirement of no more than 100V in the 6N1 specification. Here I use a 0.33uF/400V CBB capacitor to suspend the filament power supply and the ground, so that the connection between the entire circuit and the ground is through this grounding capacitor. The output part of the high voltage power supply has a 1.5kΩ/3W and a 2kΩ/5w variable resistor in series. When using it, the output voltage can be adjusted by adjusting the resistance value of the 2kΩ/5w variable resistor. The filament power supply here is a 7805K metal shell three-terminal voltage regulator for DC regulated power supply after rectification; at the same time, the 9th pin of 6N1 is the shielding layer of the two vacuum triodes in the electron tube. It should be grounded when in use, which can not only reduce AC noise, but also help to reduce the mutual interference between the two vacuum triodes.

Production adjustment
The chassis uses an aluminum chassis, the transformer and the electron tube are installed on the upper part of the chassis, and the chassis is wired internally. The AC power socket and audio input socket are placed behind the bottom plate, and the power switch, headphone socket, and volume control components are installed in front of the bottom plate. Among them, the input socket and volume control are as far away from the power supply and power switch as possible. In order to achieve good sound effects, the resistors used in the audio circuit of this machine are 1/2W American DALE resistors, high-power resistors are high-quality metal film resistors, the input volume control uses Japanese ALPS 100K blue plastic shell varieties, C1 and C3 are Japanese ELNA high-speed electrolytics, C4 uses German WlMA polypropylene capacitors for clear and bright sound, and French SOLEN is softer, and the output capacitor here is Philips 330uF/400V electrolytic and 68uF/400V SOLEN polypropylene capacitors in parallel. Facts have proved that this connection method can effectively improve the sound quality without greatly increasing the cost.
The transformer uses a 60W R-type transformer with low magnetic leakage and high efficiency, which is conducive to improving the sound quality. The voltage of the secondary high-voltage coil of the transformer should be between 265V and 280V, the current should be greater than 50mA, and the voltage of the low-voltage coil should not be less than 10V. Since the current of each 6N1 filament is 0.6A, it is necessary to supply electricity to three 6N1 filaments, and the current capacity of the low-voltage coil should be greater than 2A. The line adopts the scaffolding method, and each grounding point connects the total ground wire (12AWG solid copper wire) to the fixed bottom plate in a star-shaped grounding. The bottom plate is polished with sandpaper to give it a metallic luster to ensure reliable grounding. If conditions permit, the R-type transformer and the 15H choke inductor are best encapsulated in a grounded metal box to reduce interference with surrounding lines.
Before powering on, test whether the power supply of the amplifier is normal. If there is no load, the power supply voltage will increase a lot (about 400V). Therefore, it is best to plug in the electron tube when testing the power supply voltage. Before plugging in the electron tube, check whether the welding is correct and whether there is any cold welding. When adjusting the high voltage power supply voltage, the resistance of the parallel resistor 2k/5W can be changed to adjust the voltage to about 350V. When testing the filament voltage, the tube is also required to be installed in the socket. If it is unloaded, the voltage at point A should be between 6.5 and 7V. After the tube is installed in the socket, the voltage at point A should be between 6.1 and 6.5V.
Whether the circuit is normal should check the voltage values ​​of each point on the circuit diagram 1. For the 6N1 circuit, I measured the voltage at the junction of R3 and V1 to be 173V, the voltage at the junction of R6 and V2 to be 347.5V, the voltage at the positive pole of the output capacitor to be 176V, and the voltage of 1.2V at the cathode of V1. In different usage environments, the voltage measurement values ​​of these points will be slightly different, and there may also be slight differences between the L and R channels, which are all allowed. If these checks are normal, you can prepare to turn on the machine and listen. Audition and calibration

This
carefully adjusted headphone amplifier has a very solid sound quality when playing CDs, and the signal-to-noise ratio is very high, and the background is quiet. When listening to Chopin's Piano Concerto No. 2, the stereoscopic sound and clear sound effects of the orchestra seemed very realistic through the playback of HD600. I have listened to this CD many times, but this headset still gave me an extraordinary feeling. After being "burned in" for nearly a week, the sound is soft and distinct, the high and low frequencies extend smoothly, the tone is ups and downs, the changes are rich, and the low frequency is full of subtle texture changes. In particular, the touch texture of the piano is both beautiful and rich and full of vitality. The sound is not thin like ordinary transistor headphone amplifiers, but full of energy and dynamics, and the stereoscopic performance is also excellent.
The capacitors used in this machine have a great influence on the sound. If conditions permit, please use high-quality products. The output electrolytic capacitor C4 uses German ROE supplement electrolytic capacitors, and the sound is unique compared to Philips electrolytic capacitors. Of course, if conditions permit, it is best to use the components of the left and right channels, especially the electron tubes, in pairs, and try to reduce the difference in the voltage measurement values ​​of each point of the left and right channels. In this way, the stereoscopic sense of the sound effect and the sound image positioning will be more accurate, the sound will be smoother, and the rhythm will be stronger.
In general, even if the volume is turned up very high, the sound of this amp is very mellow. Its sound field is slightly backward, but very grand and powerful. It is not at all stretched to drive the HD600. Not only is the mid-frequency sound smooth, but the bass is full and soft, which is quite pleasant to listen to. The ears will not feel tired even if you listen for a long time. When using it to drive low-impedance headphones, the sound is also just right. Friends who are interested in it may wish to follow suit.