功放接地原理分析

电子电路要引导或要隔离的成分无非是两种，DC与VC。音响功放的机壳与市电之地线(大地点）接通，仍使用安全的最基本要求。机壳万一带有电能，就必须即时引入大地化零。

对人生有生命危险的电能也不外乎DC与VC。

对人生有生命危险的VC何来？市电。所以音响功放的机壳与市电之地线（大地点）直接相通。

对人生有生命危险的DC何来? 音响放大器。如果将音响功放的零电位点（通称地端）与机壳直接地相通，也就解除了对人生有生命危险的电能。但是，那些对人生无害，但是却可能会引噪音的外界微小杂波VC，也会顺此通道溜入到音响功放内。所以，音响功放的零电位点（通称地端）与机壳用单向整流桥接通，泄引内在的万一漏到音响功放零电位点的DC进入大地化零。又因为单向整流桥的保护，巿电中的噪音杂波VC则被拒于音响功放的零电位点之外。

那么音响功放的零电位点自身也有可能染有微小杂波VC，它们可能源于整流电路的残留纹波，整流噪声，机内寄生振荡，等等。这些微小杂波VC也需要一条低阻通道引到机壳-市电大地作零化处处理。那个小电容，就是为他们专门设置的快速通道。

至于那个小电阻的功用，解释开来可能要悬乎得很多。简单地理解，它是为解决地环回路的电势所设。


现也找到原图作者相关的注解，出处：http://sound.westhost.com/earthing.htm。逐步来理解消化吧。

注：文中的loop breaker就是指小电阻和电容的组合体。本人把它试译为：地回环破解器。文中又特别注明，必须按照标记正确接上整流桥，否则有危险。

The loop breaker works by adding a resistance in the earth return circuit. This reduces circulating loop currents to a very small value, and thus breaks the loop. The capacitor in parallel ensures that the electronics are connected to the chassis for radio frequency signals, and helps to prevent radio frequency interference. Finally, the diode bridge provides the path for fault currents. The use of a large chassis mounting (35A) type is suggested, since this will be able to handle the possibly very high fault currents that may occur without becoming open circuit. Note the way the bridge is wired, with the two AC terminals shorted, and the two DC terminals shorted. Other connection possibilities are dangerous, and must be avoided.

In the event of a major fault, one (or more) of the diodes in the bridge will possibly fail. Semiconductors (nearly) always fail as short circuit, and only become open circuited if the fault current continues and 'blows' the interconnecting wires. High current bridge rectifiers have very solid conductors throughout, and open circuit diodes are very rare (I have never seen a high power bridge go open circuit - so far at least). Use of the bridge means that there are two diodes in parallel for fault current of either polarity, so the likelihood of failure (to protect) is very small indeed.

When a loop breaker is used, it is vitally important that all input and output connectors are insulated from the case. If not, they will instantly defeat the loop breaker by providing a direct connection from the zero Volt point to chassis, and no benefit is obtained. (Electricity has an annoying - but perfectly logical - tendency to travel along the path of least resistance, and a direct short circuit will always have less resistance than the loop breaker.)

It is not uncommon to have a voltage of 1V RMS between the earth connections of power outlets that are wired separately back to the switchboard. This small voltage, with a total resistance of perhaps 0.2-0.5 Ohm, will cause a loop current of 2 to 5 Amps, all of which flows in the shield of the interconnect. This is sufficient to cause a voltage difference across the interconnect, which the amplifier cannot differentiate from the wanted signal. By breaking the loop with the 10 Ohm resistor, the current is now less than 200mA, and the voltage across the interconnect will be very much smaller, reducing the hum to the point where it should no longer be audible.

Never route an earth wire to the main (star) earthing point on a chassis in such a way that it forms a partial (or full) turn around a transformer. It is better to relocate either the star earth point or the transformer to ensure that no earth conductor can create a partial turn. There may often be conflicting requirements, but there is usually no reason that proper earthing for minimum hum and maximum safety should be mutually exclusive. Both are important, and both must be accommodated in the final design.


「地环破解器的工作原理是在地环中增加了阻抗。减少了环路的电流，进而破解环路。与电阻并联的电容将射频信号引接到机箱，这有助于防止射频干扰。最后，二极管桥堆是为事故电流提供通道。建议使用一个大规格，（35A）型的机箱，因为这将能够应付可能发生非常高的故障电流，而不会变成开路状况。留意桥堆的接线方法，两个AC端子合接一起，两个直流端子也合接一起。其他连接的方式会是危险的，切不可这样做。

在重大事故中，二极管桥堆中的一个（或多个）二极管可能会失效。失效的半导体（几乎）总是处于短路状态。只有当事故电流继续存在或者是连接导线被烧断，失效的半导体才会处开路状态。大电流的桥堆具有非常坚实的导体贯通性能，被击断变成开路的桥堆是非常罕见（至目前为止，我从没遇到过被击断变成开路的大功率桥堆）。桥堆的使用意味着正反两个极向都有两个并联的二极管，所以失效的可能性是非常小的。

如果使用地环破解器，將所有的音响输入和输出接口与机箱绝缘是至关重要。如果不是这样做的话，就会立马毁掉这个地环破解器，零电位直接地与机箱相通，没有获得任何好处。 （电有一个恼人的 - 但却又完全合乎逻辑的倾向，即永远都是寻找一条阻力小于地环破解器的捷径）。

通常在机箱电源插座的地线点与分电闸之间有一个1V RMS噪声电压。这个小电压，也许有0.2-0.5欧姆总阻抗，即会引至2～5安培回路电流，所有这些电流是流通于过机信号线的屏蔽层上。这足以造成的一种和普通音信非常相似的电压差，被放大器放大。如果此电流是通过地环破解器10欧姆的电阻，它就会被降至小于200mA，在过机信号线上造成的电压差也就非常小，不再引起可闻的交流声。

切不要將巿电的地线间接或绕行到机箱的集中（星状）接地点上。这样做的话，将会和机内的变压器抅成一个半环或全环回路。应尽可能选择合理的机箱集中接地点或变压器的摆位，以确保避免造成这种环路。这样做可能会相互矛盾，但是正确的接地方式，确保最大限度地减小交流声和最大程度的安全保护，通常应该不会是相互排斥的。两者都重要，而且两者都必须被兼顾于最终设计理念中。

地环破解器通常可以防止50Hz或60Hz的交流声窜入到音信中，如果听到的是100Hz或120Hz的交流声（这种交流声通常是较为清脆），那就是接错了电源线，是变压器感应形成的错频电流。大尾鱼拙译于2013年夏」