Many audio engineering parents are often plagued by the "current sound" of the amplifier. Most people think that the current sound is caused by poor grounding, so they try to ground the outer casing of the amplifier and even shield the entire amplifier with a metal cover. But the current is still sound, and there is no countermeasure...
Everyone has different definitions of "current sound". First, find out which kind of "current sound" you encounter.
In more than half of the cases, there is no problem with grounding. "Current sound" is not caused by static electricity. But looking at the descriptions of different friends, "current sounds" are:
â— Continuous noise â— The loudness of the noise is quite stable â— It is usually the experience of all the friends of low-frequency noise. The "current sound" can be summarized in the following cases:
Ripple noise, ie Humming
Pick-up noise includes body pick-up noise (Body pick-up)
Layout or Grounding topology
Ground noise and Ground loop
Bad wiring and shielding strategy
The first lecture: exchange buzz
The cause of the AC hum is the improper design of the power filter, or even cut corners, causing some of the AC signal from the mains to slip into the amplifier. This type of current sound is characterized by a very low frequency, which is very weak in most cases. It is only heard when the ear is attached to the speaker, and even if the volume is set to a minimum, the noise still exists.
Filter capacitor (tank), in addition to the larger the better, its equivalent series resistance (ESR) is the smaller the better!
This is the most common type of current sound. Under normal circumstances, some cheap cottage amplifiers are more likely to encounter this situation, but DIY amplifiers rarely encounter this problem, why? Because the most common cause of this current sound is insufficient power supply filter, the capacity of the filter capacitor is insufficient, and some cheap products are more prone to problems than DIY amplifiers because they are not suitable for expensive large-capacity capacitors (commonly known as reservoirs). The average person DIY will not hesitate to work.)
How to solve this current sound? Then look at whether your amplifier uses an external or built-in transformer (Fire Bull). 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 the capacitors are connected in parallel, their capacitances will be added, but this method will be limited by the space inside the machine and requires random strain.
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!
Very surprised! First of all, the sound quality of the switching power supply is not very bad, but the AC hum is solved in a single way, and the switching source transformer does have an immediate effect. (Of course, using a switching power supply may induce another problem, and I will have a chance to discuss it in detail later.)
Principle The mains electricity we use is AC, but the amplifier must be operated with DC power. Therefore, the power transformer (Fire Bull) is responsible for converting the AC voltage into DC power in addition to raising/lowering the voltage of 220V. This process is called "rectification." The most common rectification method in an amplifier is "bridge rectification". Bridge rectification turns the mains AC into a variable DC. Technically, this is a DC with 50Hz x 2 = 100Hz AC signal. The power supply, which we call "chopper", and if the 100Hz chopping wave is not removed, it will let it slip to the speaker, creating noise. Because these chopping frequencies are very low, it requires a lot of capacity. Filter capacitor (tank) to suck it up.
Why does a switched-mode power transformer 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 frequency of the chopping wave is more easily absorbed by the pond (but too high and can not be sucked), and its operating frequency is far more than In the working range of the ear and the speaker, 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 (all frequencies are too high), then what is the noise?
Other causes:
Filter capacitor failure (commonly known as dry pond)
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.
The output power tube bias current offset 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 strengthening The buzz of communication.
Signal ground is connected to Earth, which is the case in the Ground loop section.
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 large quiescent current, making the power supply chopping noise more noticeable, so the size of the filter is often large.
The lower the equivalent series resistance (ESR) of the capacitor, the better the filtering performance.
In addition to shunt capacitors to increase power capacity, series inductors can also be used for LC or even Pi (Ï€) filtering, but at a higher cost.
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