A cheap tube amp for digital audio sources

This machine has only 6 components per channel, eliminating the power transformer, directly rectifying and filtering the mains, with sufficient power supply, and the filament is supplied by a small transformer. This step-up input transformer is passively coupled, with only voltage amplification and no power gain. Modern digital audio sources, whether CD, VCD, DVD, or MP3, do not directly output signals to the laser sensor, but after D/A conversion, analog amplification, and frequency correction in the machine, the analog signal of more than 1Vms is output through the emitter follower. Therefore, after the high-ratio transformer is stepped up, a power amplifier is added to meet the gain requirements. The input transformer also improves the digital sound. This machine does not have a potentiometer. If it is connected to a sound source without volume control, a potentiometer can be added to the input end.

There are two key points in production. One is the selection of input transformer. In order to ensure the performance of the whole machine, it is recommended to use finished products. The model used in this machine is TF1QX10YY, with an impedance of 600Ω:75kΩ, a frequency response of 300~3500Hz, and a transformation ratio of 0.02V~0.8V (40 times). Such a high change requires a very high transmission characteristic of the input transformer. The core is made of Permalloy, so that under the condition of ensuring the inductance, fewer turns are wound to reduce leakage inductance and capacitance, and obtain a better frequency response. The actual frequency response of this machine is better than the nominal. The second is grounding treatment. The direct rectification and filtering of the mains power has higher safety requirements. The ground after rectification is often called hot ground. The hot ground cannot be connected to the casing, also known as floating ground. The grounding terminals of the three transformers of input, output, and filament are directly connected to the casing, called cold ground. The hot ground and the cold ground are connected through a 0.01u high-voltage capacitor. Generally, two 0.02μF/1kV are commonly used in series to improve its safety and reliability. Under normal circumstances, as long as the capacitor and the primary and secondary of the transformer do not break down (the finished product is under pressure of more than 1kV), there will be no safety problems.

6P15 has good frequency characteristics, is cheap and easy to drive. It adopts triode connection, and the output impedance can be appropriately reduced, with an output power of about 2×1w. It is very simple to adjust the machine. Adjust the cathode resistance to make Ia 30mA. The function of the other resistor is to improve the frequency response of the transformer and prevent resonance. Its value can be selected between 270kΩ and 820kΩ. A small value will reduce the output. Readers who appreciate the charm of this machine may wish to give it a try.

6P16 tube amplifier circuit: