Installing a tube preamplifier in the power amplifier

Using the free space in the power amplifier to add a tube preamplifier can improve the sound quality and grade of the power amplifier. Choosing the small seven-pin electron tube 3A5 as the preamplifier is very suitable for installation in the machine. It is small and has very low power consumption. The dynamic range is large and the sound is extremely bright and mellow, but it will encounter some troubles in practice, mainly because of the loud AC noise, which many people are afraid of.

The author has designed a pre-stage board circuit (as shown in the figure). As long as you pay a little attention to the following key points, you can get quite satisfactory results.

1. The filament needs a separate DC power supply, and the current should be clean and smooth. The filter capacitor used should have a large capacity and good quality. I use ruby ​​electrolytic capacitors. C7 and C8 should be installed near the power output end and the filament part respectively, and connected with a double-core shielded wire in the middle, with one end of the shielded wire grounded to avoid high-frequency radiation interference.

2. In the high-voltage part, in order to save space and reduce heat, the tube rectifier and choke filter are abandoned. But the sound quality is still satisfactory. C3 and C4 are Philips blue hexagonal electrolytic capacitors. C5 is an EC metal film capacitor. The output capacitor Cl can use MKT, MKP capacitors, and C2 is a CBB capacitor. The above capacitors have a great influence on the sound quality. In order to protect the electron tube, a high-voltage delay circuit is added, R7 is a bleeder resistor, and R8 is a large current buffer resistor.

3. Make sure that the ground wire of a unit is centrally grounded, and avoid scattered, nearby, and series grounding. Try to avoid using jumpers for the entire machine. If necessary, use shielded wires.

4.3A5 tubes are very sensitive, and the speaker will echo with the slightest vibration (microphone effect - editor's note). The tiny AC oscillations inherent in the iron cores of each transformer in the machine.

All of these will cause 3A5 to produce buzzing sound (for this, the manufacturer provides special shielding cover and vibration-damping rubber ring). I enlarge the fixing hole on the board to φ6mm, pad it with rubber on both sides, and then fix it to the bottom plate of the case with M3 screws. When connecting to the power amplifier, it is better to connect the original volume potentiometer to the output end. After a few burn-in times, you can play songs.