How to solve the current sound of the audio amplifier

Many of my friends are often troubled by the current sound of the amplifier. Most of my friends think that the current sound is caused by poor grounding, so they tried to ground the amplifier shell, and even shielded the entire amplifier with a metal cover, but the current sound remained the same. No countermeasure...

Everyone has different definitions of electric currents. First, find out which kind of electric currents you encounter.

In more than half of the cases, the grounding is not a problem. The sound of electric current is not caused by static electricity.

But looking at the descriptions of different friends, the "current sounds" are all:

● Continuous noise

● The loudness of the noise is quite stable

● Usually low frequency noise

Based on the experiences of friends from all sides, the "current sound" can be summarized into the following situations:

Ripple noise (Humming)

Pick-up noise includes body pick-up noise

Circuit board routing (Layout) or grounding topology (Grounding topology)

Ground noise and ground loop

Bad wiring and shielding strategy

Communication

The cause of the AC noise is improper design of the power filter or even cutting corners, causing a part of the AC signal of the mains power supply to slip into the amplifier. The characteristic of this kind of electric current sound is that the frequency is very low, and in most cases it is very faint. It can only be heard by putting the ear to the speaker, and even if the volume is adjusted to *, the noise still exists.

The filter capacitor (water pond), in addition to the larger the capacity, the better, the equivalent series resistance (ESR) is also as small as possible! This is the most common type of current sound. Under normal circumstances, some cheap knock-off amplifiers are more likely to encounter this situation, but DIY amplifiers are less likely to encounter this problem. Why? Because this current sound *commonly caused by insufficient materials for power supply filters and filter capacitors The capacity is not enough. Some cheap products are more prone to problems than DIY amplifiers because they are stingy with expensive large-capacity capacitors (commonly known as ponds) (because most people do DIY at no cost).

How to solve this kind of current noise? It depends on whether your amplifier uses an external or a built-in transformer (Accor). If you use a built-in transformer, the situation is more complicated. You need to replace the pond with a capacitor with a larger capacitance, or even Increase the number of capacitors. When several capacitors are connected in parallel, their capacitance values will add up. However, this method is limited by the space inside the machine and needs to be responsive.

Because the frequency of this noise is very low (100Hz), and usually very slight, if you encounter small-diameter speakers, these cheap amplifiers can often be confused, but if you want to detect power ripple noise, there is a very simple method, as long as When the amplifier is turned on, touch the diaphragm of the low-frequency unit with your hand. If you still feel the diaphragm vibration of the speaker when there is no signal or the volume is adjusted to *, this is an obvious evidence of communication.

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 with a switching power supply transformer (SMPS), the problem will be solved!

Surprised! First of all, the sound quality of the switching power supply is not bad, but to solve the AC noise, the switching power transformer is indeed effective. (Of course, the use of switching power supply may induce another problem, which will be discussed in detail later.)

principle:

The mains power we use is alternating current, but the amplifier is operated by direct current, so the power transformer (Accor) is not only responsible for stepping up/down the 220V voltage to a suitable voltage, but also taking the role of converting alternating current into direct current. This process It is called "rectification". The most common rectification method in the amplifier is "bridge rectification". Bridge rectification turns the AC of the mains into a variable DC. Technically speaking, this is a DC that contains an AC signal of 50Hz x 2 = 100Hz. The power supply, we call it "ripple", and if the 100Hz ripple is not cleared, it will slip to the speaker and cause noise. Since these ripples are very low in frequency, a large capacity is required. Filter capacitor (pond) to suck it out.

Why the switching power transformer can solve the AC noise? Because the operating frequency of the switching power supply is not 50Hz or 60Hz, but frequently over 100,000 Hz, the higher the frequency of the ripple, the easier it is to be absorbed by the pond (but too high it will attract Not to lose), and its operating frequency is far beyond the working range of ears and speakers, so the ripple noise of the power supply cannot be regenerated on the speakers (the frequency is too high), and secondly, it cannot be received by the ears and transmitted to the brain (both are The frequency is too high), so where does the noise come from?

Other causes:

1. Filter capacitor failure (commonly known as dry pond)

Due to the high cost and large volume of filter capacitors, high-temperature-resistant capacitors are generally not used. Therefore, in power amplifiers with high heat generation, the pond is often a part with a shorter life.

2. Output power tube bias current offset

The output power tube needs to operate under a certain bias current. If the current value is increased due to various reasons, the quiescent current of the amplifier will rise unusually, which will also increase the load on the pond, thereby enhancing the AC noise.

3. The signal ground and the earth (Earth) are connected. We will analyze this situation in detail in the Ground loop section.

4. The electromagnetic interference of the transformer, this situation will be analyzed in detail in the Wiring and shielding section.

Remarks:

The size of the filter increases with the amount of current consumed by the amplifier.

Class A amplifiers require a huge quiescent current, which makes the power supply ripple noise more easily detectable, so the scale of the filter is often very large.

The lower the equivalent series resistance (ESR) of a capacitor, the better its filtering performance.

In addition to connecting capacitors in parallel to increase the capacity, inductors can also be connected in series for LC or even Pi-shaped (π) filtering, but the cost is higher.