How to use a switching power supply to replace the amplifier transformer
The ring transformer of a certain power amplifier has been burned out due to overheating, and the nominal voltage cannot be seen clearly. There is no circuit diagram, so all parameters can only be judged based on observation. The secondary of the transformer has a total of 6 output lines, three of which are connected to the power amplifier board. After rectification, they provide positive and negative voltages to the power amplifier circuit. Observe that there are two 10000 μ F/50V electrolytic capacitors on the power amplifier board serving as filters. The estimated supply voltage will not exceed ± 40V. The other three wires are connected to the front panel, one of which is connected to the filament end of the display screen to supply power to the filament (estimated voltage is about 6V), the other wire is half wave rectified and filtered by a 100 μ F/35V electrolytic capacitor to supply power to the display screen and electronic switch circuit (estimated voltage is about+30V, which is different from the common -30V power supply), and the remaining wire is a common grounding wire. The author chose the "Huazheng brand audio dedicated 150W switch power board" for replacement. The main voltage output of this power supply can be adjusted from ± 22V to ± 32V, and there is also a backup voltage of 8-12V. When replacing, first remove the original transformer and connect the main voltage output terminal on the switching power supply to the power supply terminal of the amplifier board. Then connect a lead from the positive terminal of the main voltage output of the switching power supply to the power supply terminal of the panel display screen and electronic switch circuit. Finally, the secondary voltage output terminal of the switching power supply is stabilized by a 5V regulator block 7805 and connected to the filament power supply terminal of the display screen (see attached figure). Due to the ground wire connected to the front circuit on the original machine panel, there is no need for a ground wire. To prevent self excitation.
After connecting all the wires, turn the voltage adjustment potentiometer on the switch power board to the middle position before starting up.
To prevent damage to the amplifier circuit caused by high voltage. Then turn on the device. Observe that the main voltage output of the power supply is ± 25V At the same time, I heard the sound of the relay on the amplifier board closing. Looking at the display screen in front, it was found that the filament power supply of the display screen was stable at 5V Although slightly lower than the rated value of 6V
But the screen brightness is still acceptable. Press the micro switch on the panel, and the electronic switch circuit can switch normally.
Connect the speaker and plug in the signal source for testing. A beautiful and melodious song came from the speaker. At the end of the song, there is very little static noise in the speaker, which feels better than a regular transformer power supply. Try gradually increasing the volume of the amplifier. When the volume is turned up to 60%, I feel that the singing is a bit rough. It is estimated that the output voltage of the power supply is too low. When the voltage is adjusted to ± 32V for testing, the fuzz phenomenon disappears and the bass is rich. Clear high notes. After two hours of continuous testing at high volume, turn off the machine. Touching the heat sink of the power tube on the power board only heats up slightly, proving that the power supply is working stably. Finally, fix the power board properly at the original ring transformer, and the replacement work is completed.
