6N5P Class A push-pull 20W amplifier

I love audio, and have built more than 100 amplifiers in the past 40 years. After decades of listening and comparison, the 6N5P Class A push-pull non-feedback amplifier is the most satisfactory and cost-effective one for me. It has excellent technical performance and a great listening experience, so I need to introduce it to audiophiles.

1. Circuit structure

The principle of the power amplifier circuit is shown in Figure 1. The figure shows the circuit of one channel, and the other channel is exactly the same. V1 constitutes the input voltage amplifier stage, V2 constitutes the inverted driving stage, and V3 and V4 constitute the push-pull output stage.

Since the pre-input eight-voltage amplifier stage plays a decisive role in the signal-to-noise ratio of the whole machine, the quality of V1 is very important. For this reason, the 6N11 audiophile tube is used for this task. This stage adopts the classic triode amplification form, which is reliable, simple in circuit, and more acoustic.

The driver stage uses a 6N6 cathode split phase circuit: under amateur conditions, the common cathode split phase circuit can easily obtain positive and negative signals with equal amplitudes. This stage is crucial to the connection between the upper and lower stages, laying the foundation for the power amplifier stage to work in a low distortion and high efficiency state.

The output stage is 6N5P, which is a low internal resistance, high current dual triode. Practice has proved that it is very good for audio power output. The output stage of the power amplifier adopts a dual bias circuit structure, which can not only improve the stability of the circuit, but also adjust the working state of the final power amplifier at will (the discreteness of this tube is large, and it is difficult to achieve the best state with self-bias)

2. Welding debugging

After the whole machine is welded, do not insert the power amplifier tube after careful inspection. Turn on the power and adjust the negative voltage to 90V, turn off the machine, and then plug in all the tubes for power-on detection. The voltage of the 6N5P cathode resistor to the ground should be 14v. At this time, the current per screen is 46mA, and each tube is about 92mA, which is considered normal. When the screen voltage is 240v, the power dissipation of each tube is 22w (maximum 26w), which is close to the limit value. Practice has proved that this state is safe and reliable, and the linearity is good. Since my power amplifier was successfully built in 1995, I have listened to it for about two hours every day, and there has been no malfunction so far, and the sound effect is becoming more and more perfect.

The power supply circuit is shown in Figure 2. The power of the power transformer must be sufficient, and the secondary high voltage current must not be less than 600mA. The high voltage filter resistor uses an aluminum shell resistor, 50Ω/50w×4, and the total amount of filter capacitors used is more than 1000μF, which is better than the filter of transistor machines.

In order to ensure the quality of the whole machine, it is best to buy the output cattle from a professional manufacturer. I ordered it by mail from Hebei, and the cattle are genuine and of high quality.

3. Listening Evaluation

Equipment: CD player: CE CZL100, audio decoder: Xindak DAC-3, pre-amplifier: self-made 3A5 tube pre-amplifier and 6V6 pre-amplifier. Speakers: Denmark Jamo 307 and Hivi D6B. Signal cables and speaker cables are all from Longyu.

After turning on, the noise is very low. When you put your ear to the speaker, you can hear a slight "rustling" sound (from the preamplifier. It is the same when the preamplifier is disconnected and not turned on). When you play a familiar piece of music, it is very similar to the "Guifeng" machine. Interested audiophiles may try it. It will never let you down. After comparing it with various famous machines at home and abroad, you will know its charm.