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Getting started with speakers and headphones

2021-04-25 351
In audio equipment, what is a balanced output/input and what is an unbalanced output/input? I am afraid that this concept has never been exposed to most users. Although we often see some devices with such functions in the evaluation For example, we use the test platform E-MU 1616m audio interface, E-MU PM5 speakers, etc., but we do not usually use a balanced interface, after all, our article should serve ordinary users.

It is not difficult to see that this concept tends to appear more in professional monitoring equipment. And from signal balanced input, balanced power amplification to balanced output, what advantages will it bring? For some high-quality fever equipment, what does it have? No use? How to use it on headphones and amps?

What is unbalanced output/input?

Why don't we usually use balanced output, but unbalanced output? It seems that there is no reason to say. For a stereo audio system, according to the standard, its left and right channels have two wires each to form a signal loop. One is responsible To transmit the signal, the other ground leads to noise. For devices such as headphones, their ground wires can also be combined into one, and the two channels use one ground wire. Because the 3.5mm plug is defined in this way, it doesn't make sense Speakable? So the vast majority of what we usually see are unbalanced input and output.

This is the same as the 3.5mm [6.25mm] interface used by our most common stereo. Their plugs are divided into three sections, corresponding to three wires. They have standard definitions. All audio equipment uses 3.5mm stereo input and output. , Are connected according to this specification. As shown in the figure, the root section is used to connect the ground wire to derive system noise. The top is the right channel signal, and the next is the left channel signal. The black part is the insulating ring.

Another type of interface, the RCA interface commonly used on our sound cards, is also an interface for unbalanced signal input and output. The general standard defines RCA interfaces as red and white. Red is the right channel and white is the left channel. The plug and The outer side of the socket is grounded, and the center is the signal.

The unbalanced output interface obviously occupies the mainstream of most audio equipment. As for why? The specification is passed down in this way. To use a balanced interface, it is necessary to provide a balanced amplifier circuit for power amplifiers or audio source equipment. And "balanced" It is more used in the professional field because of the advantages of balanced power amplifier circuits. Strictly speaking, what you need to talk about "balance" is its "balanced power amplifier circuit".

What is balanced output/input?

The definition of unbalance comes from relative balance. Balance has not been promoted in civilian use, and it has a certain relationship with power amplifiers, cost and overall standard setting. There is no need to entangle why. Instead, use balanced input and output and take advantage of its advantages. , The core is the power amplifier. Because of its different output forms, the interfaces are naturally different.

From the point of view of the interface, each channel of balanced input and output needs to provide two sources of input signals [wiring]. As for why you want to do this? We talk about the working principle of balanced power amplifiers below. Therefore, we are the most common There are about three types of plugs and sockets for balanced input and output. One, 6.25mm three-section [three-pin] plug; second, XLR three-pin Canon plug; third, XLR four-pin Canon plug.

6.25mm three-pin plug: Compared with XLR, it can save a lot of volume, so it can be seen on many audio interfaces. Even some more entry-level USB sound cards, such as E-MU 0204USB, etc. 6.25mm plug and 3.5 The mm plug is actually no different, just thicker. There are probably two reasons to use it for professional applications. One, it looks professional; second, the size is large, the contact surface is large, and the plugging ratio is 3.5mm. More stable. Third, due to standards. When a 6.25mm three-pin plug is used for balanced signal access, its root is still defined as connecting to the ground, and the middle is the signal positive, and the top is the signal negative. One plug is only responsible for One channel.

XLR Canon plug: Compared with the 6.25mm plug, the XLR plug has better insertion and removal stability and reliability. As shown in the figure, it is a schematic diagram of the four-pin XLR Canon socket and the three-pin XLR Canon socket. The black part is the unique "clamp" locking design of Canon plug.

Our common XLR interface is a three-core standard. Its plugs and sockets have no definition of "male" or "female". The numbers 1, 2, and 3 are marked on the plug or socket. According to the specification, a single balanced signal input and output uses one The three-core XLR Canon plug, 1 port is grounded. According to different specifications, 2 and 3 are positive and negative. It is similar to the 6.25mm three-core balanced plug.

Getting started with speakers and headphones

How is the four-core XLR defined in the balanced signal connection? As the output of the balanced power amplifier, it does not need the ground wire to form a loop on the load (such as speakers, headphones), so we can indeed omit the ground wire connection [if we wish, The shell can be grounded], so there is a four-core XLR. Its convenience is that one interface has four connections, and each channel corresponds to two, which can realize stereo input and output. The definition of the 1234 interface is left﹢left﹣right. ﹢right﹣.

In fact, the Canon interface is just a form of interface, it is mostly used in professional applications, and there are also five-core standard Canon interfaces.

Why use balanced input and output?

Although the unbalanced interface is our most common interface, and the balanced interface is more used in professional equipment, the above dazzling introduction is estimated to make people dizzy? But this is just a superficial phenomenon, why should we use a balanced interface? Strictly In other words, the existence of the balanced interface is to correspond to the output of the balanced power amplifier [the three-core microphone XLR interface has different functions]. If there is no balanced power amplifier, the use of the balanced interface is theoretically meaningless. So what is a balanced power amplifier?

As shown in the figure, this is a balanced bridge connection input and output diagram. As a balanced power amplifier, whether its input signal is balanced or unbalanced, it can be divided into phase difference by circuit & pi; [180°] two signals. As shown in the figure, the left and right sides are a pair of signals with opposite phases [Note: If used for stereo amplification, this is a schematic diagram of one channel]. The two signals are the same in theory, but The phase polarity is opposite. Each group of amplifiers is a differential double-ended input mode [note that the input signal phase polarity is opposite here].

The R load in the figure can be regarded as a speaker or earphone. After being amplified by a power amplifier, the signal at the output end is also a signal of opposite phase and polarity, and other signals of exactly the same nature. Connect the output end to the positive and Negative, the signal polarity is reversed, but the polarity of the common mode noise generated in the power amplifier circuit is therefore changed. Therefore, the two signal strengths on the load (such as headphones) are doubled, and the noise is opposite to each other. The phase is cancelled out.

The output form of balanced amplification is the BTL output form. Many people have asked BTL [Balanced Transformer Less] what is the difference between balanced bridge load output and balanced output? The answer is obvious. BTL refers to the output circuit working form, and we are used to it. The "balanced output" seems to include its input form, connection method and working method of the power amplifier. Of course, it is often understood one-sidedly. For example, when you see an XLR interface, everyone will say that it is a balanced interface. Regardless of how its power amplifier works, etc.

Obviously, the advantage of balanced output must be realized by a balanced power amplifier. It does not care whether the system input signal is balanced (of course, the balanced signal input is more refined), but is forced to divide the signal into a balanced signal with opposite phase polarity at the input of the power amplifier circuit. These two sets of signals are amplified by a set of circuits, and the output ends are connected to two load segments with opposite polarities. At this time, the useful signal polarity is reversed twice to form a superposition, and the output power is doubled. The noise only undergoes one phase reversal at the output, so it is coupled to the load and the common mode noise is canceled.

This working method of a balanced power amplifier may be considered as "space" for "quality". In a single-ended unbalanced power amplifier, the general way we want to increase power is to increase the voltage or current of the power amplifier. And the way of balanced power amplifiers It divides the input end into two, adds a set of amplifier circuits, and finally couples and stacks at the output end. In the case of a certain supply voltage, a balanced power amplifier can achieve greater output power.

How to use balanced power amplifier and headphone change line?

We have briefly introduced the working methods and advantages of balanced power amplifiers. So, how do we use them? First of all, traditional three-core XLR cannon interface speakers are naturally uncommon. If you have a matching balanced output sound source and power amplifier, then Can consider it.

Why do many audiophiles change the line and let the headset have a four-core XLR interface for use?

You should already understand from the above introduction. If you don't consider grounding, the interface corresponding to the balanced output only needs 2 wires per channel, so the 3.5mm four-pin plug can meet this requirement. The original 3.5mm four-pin plug is It is used for a stereo plug with a microphone. The two at the top are defined as left (right), and the two at the root are different according to the standard, one is grounding and the other is the microphone interface. On the balanced interface of HiFiMan, it will These four cores are exactly defined as left﹢right﹢left﹣right﹣. Therefore, such a plug cannot be used on other 3.5mm players. Similarly, without such a plug, you cannot experience the effect of a balanced power amplifier, and it will Because the ground wire is connected to the negative phase signal, the sound is chaotic.
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