本帖最后由 djxu 于 16-4-28 13:58 编辑
今早我闲来无事上谷歌搜了搜,在普瑞声纳的官网中同样找到一篇文章叙述平衡与非平衡的转接
https://www.presonus.com/news/articles/balanced-unbalanced
以下是截取于文中。
Connecting an unbalanced output to a balanced input is straightforward: Simply connect the low (inverting) side of the input to ground. This puts makes one side of the differential always zero volts. Subtracting zero from the signal on the hot input doesn’t affect it in a good or bad way.
非平衡输出到平衡输入是相对简单的,只需要将输入的冷端接地即可。
Connecting a balanced output to an unbalanced input is a little trickier. You need to know something about the circuit that feeds the balanced output in order to do it properly. Although output connectors can come in various forms, the 3-pin male XLR and ¼” TRS jack are the most common.
平衡的输出到非平衡的输入,这里就有一些小技巧。你需要懂得一些平衡输出的电路以便选择正确的转接方式。
To unbalance a transformer output, it’s necessary to connect pins 1 and 3 together. Plugging a TS plug into a TRS jack usually will do this automatically. This does no harm to the hardware, the signal quality, or the output level, and the sonic characteristics of the transformer are maintained. The only compromise with this connection is that there’s no common mode noise rejection because by grounding one side of the transformer, the output becomes unbalanced.
变压器平衡转接为非平衡相对简单。只需要短接13即可。
Note that if you don’t ground the low (pin 3) side of an output transformer, but instead leave it floating, your receiving device will see a very low-level signal that is lacking in low frequencies. This is because there’s nothing but stray capacitance between the transformer windings and ground to complete the circuit. Capacitors are poor electrical conductors at low frequencies, a valuable characteristic when you’re building a filter, but not when what you really want is a piece of wire.
Electronically-balanced (or “active-balanced”) outputs come in a several flavors. Each of the two output legs carries an audio signal that’s referenced to ground, identical in voltage but opposite in polarity. When the voltage on pin 2 is +1 volt with respect to pin 1 (ground), the voltage on pin 3 will be -1 volt. If you measure between pins 2 and 3 with a voltmeter, you’ll see 2 volts, and that’s the voltage that a differential input sees.
Balanced output with inverting and non-inverting op-amps.
There are
two common (read: “cheapest”) circuits used for active-balanced outputs. One adds a unity-gain inverting operational amplifier (op-amp) after the non-inverted output to get the inverted output. In another configuration illustrated here, two op-amps—one configured as a non-inverting amplifier, the other wired in the inverting configuration—are fed the same signal. This is the type of output configuration used for the Main outputs of PreSonus® StudioLive™ mixers.
Op-amps sometimes can become “unhappy” when their outputs are shorted,
so simply tying pin 3 to ground, while electrically valid, isn’t a good way to connect this type of balanced output to an unbalanced input. It might work fine, but in the worst cases, the shorted-out op-amp could be damaged; the short circuit could be reflected back to the non-inverted output, causing the desired signal to distort; or the grounded output stage could start oscillating, usually at a frequency well above the audible range.
The proper way to connect this type of active-balanced output to an unbalanced input is to simply leave pin 3 disconnected. Signal voltage appears between pins 1 and 2, which is just what our unbalanced input wants. This is another instance where you’ll probably need to modify or custom-build a cable if the output is on an XLR connector.
电子平衡的转接就有几中不同的情形。在真平衡的功率输出端这里是忌讳短接13的,必需要悬空3脚。
文章很长,我英文也不是很好,也没有时间逐字逐句的去翻译。文章大意也是说明了某些时候需要短接,某些时候需要悬空的情形。至于一般是在什么情形下,还是我之前提到的,遵循“输入不悬空,输出不接地”的基本原则。在没有条件通过电路或变压器的情况下,还是要根据实际情况决定短接或悬空3脚,而非简简单单的短接13。
前面老师提及的13之间加一个缓冲电阻的做法不失为一个折中好办法,不过我今天想了一下还是同样存在问题,并不能说是完美的解决方案。负载输入端的阻值本身就是非固定的,不同的负载如何确定热端对地电阻阻值呢?实际使用中也不可能根据不同的设备换电阻,那样太麻烦了,所以也不可能保证冷热两端对称。选取100-600欧的电阻也只能是一个折中的解决方案。另外说说图5。这个图5上有一条备注,前面老师的回复中也提及,这种接驳方式,只有在输出设备设置到共地模式(set ground lift switch to grounded)下方可,为的是避免重复接地(Ground Loop)带进共地干扰(这里我不确定是不是应该这样理解)。在老师的图5上面的图3和图4,也明确有悬空3腿的接法,这种接法也不是我个人凭空捏造出来。除此之外还有图7 图8悬空地端的接法。可以说平衡转接非平衡远非短接13那么简单。