How to solve the sound current of audio amplifier? (II)

The filter capacitor (tank), in addition to the larger the better, the smaller the equivalent series resistance (ESR), the better! This is the most common type of current sound.

Then look at whether your amplifier uses an external or built-in transformer. If you use a built-in transformer, the situation is more complicated. You need to replace the tank with a capacitor with a larger value, or even increase the number of capacitors. When several capacitors are connected in parallel, The values will add up, but this method will be limited by the space inside the machine and needs to be strained.

Because the frequency of this noise is very low (100Hz), and usually very slight, these low-cost amplifiers can often get rid of the small-caliber speakers, but to detect the noise of the power supply, there is a very simple method, as long as When the amplifier is turned on, touch the diaphragm of the low-frequency unit by hand. If the vibration of the diaphragm of the speaker is still felt when there is no signal or the volume is adjusted to the lowest, this is a very obvious evidence of the AC hum.

If you are using an external transformer (such as a general mini amplifier), of course, the above methods are applicable, but there is another simpler method - replace it with a switching power transformer (SMPS), the problem will be solved!

I am very surprised! First of all, the sound quality of the switching power supply is not very bad, but the AC hum is sounded, and the switching source transformer does have immediate effect.

principle:

The utility power we use is AC, but the amplifier must be operated with DC power. Therefore, in addition to being responsible for raising/lowering the voltage of 220V to a suitable voltage, the power transformer is also responsible for converting AC power into DC power. This process is called “rectification”. "." The most common rectification method in the amplifier is "bridge rectification". Bridge rectification turns the AC power of the mains into a variable DC power. Technically, this is a DC power supply containing 50Hz x 2 = 100Hz AC signal. We call it "chopper", and if this 100 Hz chop is not removed, it will slip it to the speaker, creating noise. Because these chopping frequencies are very low, a large amount of filtering is required. Capacitor (tank) to suck it up.

Why is the switching power supply transformer able to solve the AC hum? Because the operating frequency of the switching power supply is not 50Hz or 60Hz, but it is more than 100,000 Hz. The higher the frequency, the more easily the chops are absorbed by the pool (but too high and suck It does not fall off, and its operating frequency far exceeds the working range of the ear and the speaker, so the chopping noise of the power supply cannot be regenerated on the speaker (the frequency is too high), and the second can not be received by the ear and then transmitted to the brain (both The frequency is too high), then where is the noise?

Other causes:

Filter capacitor failure:

Due to the high cost and large size of the filter capacitor, high-temperature capacitors are not generally used. Therefore, in power amplifiers with high heat generation, the reservoirs tend to be short-lived parts.

Output power tube bias current offset:

The output power tube needs to operate under a certain bias current. If the current value increases for various reasons, the quiescent current of the amplifier rises unusually, which also increases the load of the reservoir, thereby enhancing the AC hum.