关于前级的疑惑

zfl1211 · 发表于 2010-7-23 16:02

像音源切换、音调控制、音量控制、磁头放大、MIC放大、唱头放大、响度控制等都是前级，过去音源用磁带、LP等，输出的信号不强，用前级放大或其他控制是很常见的。而现在，CD机或DVD的输出已高达2Vrms，完全能满足后级，如果要用调音调、要进行阻抗匹配，那么前级放大是必要的；但如果没有这些，前级放大就没什么必要了。但很多人喜欢花重金重料做一个单独的前级放大器，不明白能给音质带来多大的好处。强信号衰减——前级又放大——再衰减——后级，这不是多些一举吗？近来也看到坛中高手做很专业的前级，如果将电路的元件和参数稍做变换，这前级就是一个很好的后级了。如果孤立地去做只有放大功能的前级，不考虑后级的话，这样的前级适用面是很窄的。前后级做一体的设计不好吗？为什么还要去浪费元件浪费钱呢？在此倡导简洁发烧、理性放烧，不为有着商业目的或其他目的的引导所左右。此帖不针对任何人。

smg001 · 发表于 2010-7-23 16:20

Why use a preamplifier?.....
Difficulties encountered with the amplification of analog signals can often be traced to the interface between the signal source and the amplifier system, especially if insufficient effort has been devoted to this connection.

Consider some of the problems:

The source output impedance may be incompatible with the amplifier input impedance.
The sensitivity of the amplifier may be too low to match the signal of interest.
There is some distance between the source and the amplifier resulting in long cable runs which cause either degradation of the signal itself or degradation of the signal to noise ratio through pick-up.
In these cases and many others the use of an appropriate preamplifier can alleviate the problems and yield significant improvements in signal measurement capabilities.

For example:

Input impedances can be matched to the source to achieve the most appropriate loading.
Differential input preamplifiers have the ability to reject pick-up noise, offer a high degree of isolation to floating sources and avoid ground loops.
Conditioning filters within a preamplifier can restrict the bandwidth of the signal path allowing gain to be applied only to the frequencies of interest thus improving signal to noise ratios.
A preamplifier allows the user to apply signal gain at the most appropriate point in a system, for example close to the source if long cable runs are necessary.

smg001 · 发表于 2010-7-23 16:45

要考虑下列问题：1，信号源的输出阻抗可能不符合放大器的输入阻抗
               2，放大器灵敏度的可能会太低，信号匹配的关系
               3，如果信号源与放大器之间的距离过大，用长距离电缆会造成信号小或信噪比下降
         例如：输入信号的可选择的匹配，以符合最适当的负载。
               差分输入预放大器，有能力拒绝接收噪音，提供一个高度隔离，避免浮动源和接地回路。
               前置放大器中的条件滤波器可以滤除信号通道内的无用信号从而提高信噪比。
               前置放大器允许用户应用增益在系统的最适合的位置。如如果长电缆必须的话靠近信号源的位置。    英文太差了，还是请高手翻译一下吧。