[Pictures and Text] Production of a bile duct integrated circuit hybrid power amplifier

This article introduces a "biliary tube + integrated circuit" power amplifier, in which the bile tube is used for voltage amplification and pre-stage electronic frequency division, and the integrated circuit is used for power amplification.
　　
   1? The sound source is amplified by the 6N11 pre-stage voltage, the 6N11 pre-stage electronic frequency division, and then the LM1875 power amplification, and the output power can reach 2×50W. This circuit gives full play to the respective advantages of the electron tube and the integrated circuit. It has both the fast speed and good frequency response of the transistor, and the good transient and warm and transparent sound quality characteristics of the electron tube.
　　
   2? The voltage amplification and electronic frequency division circuits are designed as cathode output circuits. Its output end is taken from the cathode of the electron tube, and the output phase and amplitude also follow the changes of the input gate. It has a series of advantages such as high input impedance, low output impedance, small nonlinear distortion and high stability. This circuit has a good connection effect with the pre-stage sound source and the post-stage LM1875 power amplifier circuit.
　　
   3?The two-channel frequency division of the front stage can overcome some defects of the ordinary final stage power LC frequency division network. The bass frequency and the mid-high frequency go their separate ways, which greatly overcomes the intermodulation distortion in the amplifier, reduces the internal resistance of the power amplifier, increases the damping of the power amplifier, and makes the impedance matching between the amplifier and the speaker better.
　　
   The frequency division circuit adopts the RC second-order active filter circuit. In order to overcome the defect of the RC passive network that the amplitude drops too much (up to 6dB) in the passband near the cutoff frequency point, the cathode output device is used in this circuit, which significantly improves the gain in the frequency band near the crossover point, improves the filtering performance, and makes the frequency curve of the synthesized output relatively flat.
　　
   4?The LM1875 power amplifier circuit has made two improvements compared with the traditional circuit: one is to cancel the negative feedback grounding capacitor, so that the power amplifier becomes a DC amplifier (because the output offset voltage of the LM1875 is extremely low, only 2μV); the second is to increase the resistors R17 and R18, of which R17 is set to prevent no-load self-excitation, and R18 is the current negative feedback sampling resistor. This amplifier is a current negative feedback DC amplifier, which can overcome the defects of traditional amplifiers, such as stiffness and dryness. Its bass dives, and the resolution of mid-high frequency is enhanced. In particular, the current negative feedback circuit makes the amplifier have the charm of electron tubes, which is icing on the cake when combined with the front-stage bile tube.
　　
   5? This amplifier uses two transformers, one of which is dedicated to the bile tube; the other is dedicated to the LM1875 circuit, and the secondary has two 2×25V DC outputs, which are respectively for the left and right channels, so as to improve the separation of the left and right channels.
　　
   After the machine is completed, the debugging is very simple. First connect 4 low-end speakers (or dummy loads) to the output, do not plug in the bile tube first, turn on the power supply, and see if there is any ignition or smoke. Touch the input capacitors C8 and C′8 of the rear-stage amplifier with a screwdriver, and the speaker should make a sound. Turn off the power supply, plug in the electron tube, turn on the power supply, connect the sound source to listen, and if there is no abnormality, connect the speaker to listen normally.
　　
   After connecting the VCD, you will be surprised to find that the sound has changed from the stiff and dry "digital taste" of the past. The bass is sonorous and powerful, and the mid-high notes are bright and delicate. The sound quality is truly a combination of hardness and softness, and the sound is complementary and pleasant to the ears.