[AV] Single-End RCA 單端 / Balanced XLR 平衡接頭類型

[Raymond Wan]

單端 (Single-End) 和平衡 (Balanced) 操作之間的差異是否有一個簡單的解釋？

“單端”和“平衡”描述了器材之間的接頭類型，例如前置放大器(前級)和功率放大器(後級)之間的連接。

單端接口作為信號返回路徑中使用“共同”的導體（屏蔽，接地或儀器機箱）。另一方面，平衡線，使用兩條專用的導線，提供前向和返回的信號路徑。接地連接的方式是通過在平衡線內配置的第三條專用接地導線。

系統中的任何兩件器材，他們均有可能與機箱之間有一定可量度出的的電壓差。

當用單端接線連接這兩件器材之間時，這條連線的電壓差將出現自共同導體及屏蔽之間的。因此，屏蔽將內會包含兩個機箱之間接地的寄生電流噪聲。由於屏蔽是在信號傳輸的直接路徑內，沿接地導線的壓降將會與互連的信號結合。其結果是直接引入及增加噪聲和失真在信號路徑內。

在平衡的系統由，因用一條單獨的屏蔽或接地導線將用於兩個機箱連接在一起，減少它們之間的電壓差。故屏蔽的電壓下降，也不會添加到信號中，因為這第三條導體不攜帶信號。通過平衡連接是得到乾淨的信號，信號是可有效地在加多的接地線中之電流噪聲分隔。平衡電路本質上是對稱的亦是有額外的好處。內部電路的平衡性，大大降低了對組件電源的瞬態需求，進一步提高了信號的完整性和抗噪能力。

In this context, the terms "single-ended" and "balanced" describe the type of electrical interface between components: i.e. preamplifiers and power amplifiers.

Single-ended interfaces use a "common" conductor (shield, ground or instrument chassis) as a signal return path. Balanced lines, on the other hand, use two dedicated conductors to provide forward and return paths for signal. The ground connection in balanced configurations is accomplished by means of a third, dedicated, ground conductor.

Any two components in your system will, most likely, have a measurable voltage difference between their chassis.

When a single-ended cable is connected between these two components, this voltage difference will appear along the common conductor (shield) of the interconnect. As a result, the shield will now carry the parasitic ground noise current between the two chassis. Since the shield is directly in the signal path, the voltage drop along the ground conductor will be combined with the signal that the interconnect carries. The result will be added noise and distortion introduced directly into the signal path.

In a balanced system, a separate shield or ground conductor will be used to connect the two chassis together, reducing the voltage difference between them. But the voltage drop across the shield will not add to the signal, because this third conductor does not carry the signal. What flows through the balanced interconnect is a clean signal, separated from extraneous ground current and noise. Additional benefits are derived from the fact that balanced circuits are inherently symmetrical. The balanced nature of the internal circuit greatly reduces transient demand on the component power supply, further improving signal integrity and noise immunity.

[Raymond Wan]

基本技術資料分享



What makes the balanced interface superior to a single-ended interface?

The superiority of the balanced interface comes from at least four areas:

1. Connector Quality

This one is probably the easiest to understand. Balanced XLR connectors use large diameter signal pins. They also are superior to the common RCA in that they provide a positive locking action. They incorporate properly designed strain relief as a feature. In the case of an RCA connector, the signal-carrying ground conductor also works as the strain relief - a situation far removed from ideal. Many of us have experienced broken RCA connectors when subjected to lateral forces - including the weight of some high-end interconnects. The high degree of mechanical and electrical integrity makes balanced XLR connectors the natural choice when signal integrity counts.



2. Balanced Interface Noise Immunity

When discussing different signal interfaces, it is important to evaluate their immunity to various noise sources. One group of these sources is represented by noise currents that flow between different chassis in a system (and between different parts of the same chassis, in fact). We shall call them ground noise sources. The second group includes various external sources that do not have a direct electrical connection to our system, but can affect it through their electromagnetic fields. Items commonly included in this category are various RF sources (radio stations, RF remote control transmitters, etc), magnetic fields (fields commonly found around large power transformers, power lines and home appliances), and electrostatic discharge events. Because of if its three-wire configuration, a balanced interface is substantially more immune to all of the above interference sources. By using balanced interconnects throughout our system, we, therefore, noticeably improve our signal integrity - our music signal is much less affected by extraneous noise.



3. Internal Power Supply-Gain Stage Interaction

It has been stated thousands of times before that the power supply has an enormous influence over the resulting sound of a product. With this in mind, we can take two different design approaches. One would be to put very high requirements on the power supply and then hope that it is up to the task. A second approach would be to simply minimize the demand on power supply performance from the start. Single-ended circuits put very high demands on the power supply's ability to keep up with signal-induced current fluctuations. Single-ended signals produce changes in gain stage current that must be accomodated in the power supply. Unless the power supply is capable of coping with large and fast current changes, it will constantly fall behind and the resulting sound will be degraded. Balanced circuits interface with their power supply in a "balanced" fashion. There are two gain stage currents present in the balanced circuit at any time. By the very nature of a balanced circuit, when signal appears at the input, these two currents will change in unison. One will increase, and the other one drop by the same amount. The resulting gain stage current change can be made almost infinitesimally small. In the ideal case, the power supply will not see ANY current fluctuation at all, and its job will become quite easy. This reduced demand not only makes power supply design far more efficient, it improves the performance of the power supply substantially.



4. Symmetrical is Good

Balanced gain stages are inherently symmetrical. Little wonder then that many famous designers reached for balanced circuits long before the word "balanced" was used in high-end audio. As long as forty to fifty years ago, when all stereo components contained nothing but RCA connectors, many now classic designs were already completely balanced internally. To work with the RCA interface (the only one available at the time), one input of the balanced circuit was simply grounded. Why would designers use fully balanced internal circuits with RCA jacks for interfaces? The answer is that, even then, many designers believed in the inherent superiority of balanced designs.



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[Raymond Wan]

Is there going to be any performance degradation in using a mixed balanced/single-ended system?

No. All electronics will work perfectly with any mix of components of RCA / XLR.

However, moving to a high performance good designed fully balanced system will bring additional musical enjoyment.



Not all units that have XLR jacks are truly balanced components?

Yes. There are many products on the market that add XLR connectors to an internal single-ended circuit. Such designs, while sporting XLR connectors, don't process the signal in balanced form. It is fair to call such units "pseudo-balanced". Unfortunately, this fact is usually not stated accurately in the company literature for these products.



What differences are heard when moving to a balanced system from a single-ended system?

Someone had conducted extensive comparisons of identical systems operating in both balanced and single-ended modes. The same external components were used as well as the same manufacturer's cables. Consistently, the balanced connection yielded a superior sense of air, more three-dimensional images, and a more clearly defined soundstage. The music was infused with greater life and energy.

However, someone may still like to stick with the RCA connections due to the personal preference on musical or audio quality based as well as based on individual taste.

At the end, the choice is always up to an individual's choice and selection.

[aw11onfire]