Adopting direct heat pipe 2P3 tube pre-stage circuit design

The circuit of this machine is shown in the figure. The other channel is exactly the same (omitted). The front stage uses a direct-heated tube 2P3 and a triode connection method.

.
Usually, when listening to multimedia music on computers, the volume is not loud, mainly for listening by oneself. Because the volume is turned down when listening, the low frequency, strength and dynamics of the music are often insufficient. In view of the low sound pressure and insufficient high and low bass when playing at a low volume, this machine has installed an equal loudness compensation circuit on the volume potentiometer. So that the listening experience remains the same at a low volume.
The power supply part adopts the 6Z4 tube rectifier and the inductor filter output. The 2P3 filament is powered by a DC regulated power supply to improve the signal-to-noise ratio and working stability.
The high-power resistor uses a porcelain tube wire wound resistor, and the low-power one uses a carbon resistor or AB carbon resistor.
The power of the power transformer can be around 100VA.
The choke is 10H50mA. The chassis is selected or made according to the size of the components. After the whole machine is constructed according to the drawing and checked correctly, the working point is initially adjusted. The power output of the power supply B is 150V. By adjusting the 2P3 cathode resistor, the 2P3 grid bias is -7.5V. The screen current is around 6mA.

The system configuration should not be too low. Otherwise, no matter how good the preamplifier is, it will not be able to express its unique charm. The author used a self-made monitor-grade fully balanced 6L6 push-pull machine and a self-made coaxial audio signal line. The speaker uses a 90dB five-inch full-range speaker, and the sound source is output by a computer sound card. Turn on the power. There is no noise at all, play music, and adjust the value of the resistor and capacitor components of the loudness compensation circuit appropriately. This can balance the three terminals of the high, medium and low frequencies of the sound.